So I have a mint-condition Agilent 3458A ... - Page 1
Forum section: electronics / general

author: Zondar
author_rank: Regular Contributor
timestamp: May 06, 2025, 06:52:38 am
content: The good news is that I have an Agilent 3458A from about2001 or 20022004, with option 001, in stunning, practically brand-new condition.The bad news are the error messages. Upon turning it on:Acal all required.Scal all required.Secure required.After-startup error messages:110, "calibration required -- scal"114, "system error -- nonvolatile RAM checksum"After running Auto-Cal:204, "hardware failure -- flatness dac convergence: 197"210, "hardware failure -- calibration required"101, "calibration error"I did quick Volt and Ohms measurements. Both work, but both are way out of calibration. I haven't tried Amps yet, but I presume the result will be similar.I'm an experienced engineer, but this is a highly specialized product, and I believe there are some experts on this instrument here. Obviously the original calibration is gone, and I've seen a video in which the flatless DAC was a problem. But can I get some feedback from the experts about whether the above is or is not indicative of some impossible-to-get part going bad, or what the prognosis may be?I don't really have a proper use for this instrument, except for bragging rights and looking cool on my bench (if it worked properly, anyway), but if it's repairable, that would be an interesting project, and then I can sell my soul trying for the perfect 10V reference, etc.Thank you for any advice or commentary.

author: coromonadalix
author_rank: Super Contributor
timestamp: May 06, 2025, 09:13:38 am
content: you should do search here there is many threads about the 3458 and problems who goes with itmermory devices who can/will fail in itthe famous a3 board problem / drifting of an ic, you're lucky if you don't have this defectyou can try to partially calibrate it with a very good 10v source and a very good 10k resistor, but be warned the "real certifications" for this meter are very expensivesome linksHP3458A FLATNESS DAC CONVERGENCEhttps://www.eevblog.com/forum/metrology/hp3458a-flatness-dac-convergence-195-error/3458 errorshttps://www.eevblog.com/forum/repair/3458a-repair/https://www.eevblog.com/forum/metrology/(3458a)-yet-another-3458-repair-thread/https://www.eevblog.com/forum/metrology/agilent-3458a-i-joined-the-club/even if your meter look Mint, be prepared ... this beast is one of a kind, but can or could very expensive to put back into workread everything about theses, you can register on Xdev's for free, get all SV manuals and more, some repairs too on Xdev's site ... some FW dumps etc ....

author: Kleinstein
author_rank: Super Contributor
timestamp: May 06, 2025, 09:18:22 am
content: The lost calibration and NV RAM checksum error suggest that the battery backuped RAMs need to be replaced (new of the same or alternatively an FRAM conversion). Given the age it makes sense that the batteries fail, though often they did last longer.The flatness DAC error indicates some HW problem, that effects the AC ranges, but should have no effect on DC. The part could get away without very special parts.There is also a build in self test that can be run. This may do a few more tests than just ACAL and the power on self test.A point to check is also the ADC gain drift as described in SN18. This need some time (like 2 week - maybe a bit more if the meter was not used for a longer time) of recording the temperature and cal constant CAL72 after an ACAL run.

author: Zondar
author_rank: Regular Contributor
timestamp: May 06, 2025, 03:46:24 pm
content: Thank you for the comments.I'm not going to open this unless and until I have a much better understanding of what I'm facing, so I've got some research ahead.I'm sure the NVRAM is toast, but I'm not worried about replacing those chips.If I correctly understand the comment that Kleinstein made, the flatness error may be repairable without requiring an impossible-to-find part. And also, I need to figure out how to run the internal self-test.Of course, there may be other problems too. This unit has not been abused, but what is the root cause of the "dreaded A3 drift problem" - just aging?Trying a "very good" 10k resistor and a 10V reference for a pseudo-calibration to see how that goes would be interesting. I can get a 10k 0.005% resistor without too much trouble or expense, and with a quick look on Mouser, the Ampex 0.01% 10V reference might be a starting point. Is there something obviously better that isn't crazy-expensive? I wouldn't mind attempting one of the reference circuits I see on Xdevs, for example.I'm aware that a proper calibration is very expensive. Since I don't have a real use case for the device, that's not going to happen. Instead, this would likely never be more than a hobby project and museum-grade dust-collector.Thanks again.

author: TERRA Operative
author_rank: Super Contributor
timestamp: May 06, 2025, 04:30:04 pm
content: I'd first try to pull the cal data out via GPIB, you miigghhhttt get lucky.If not, pull the NVRAM and see if you can read it out in a chip programmer.FRAM replacements are definitely the go here, check out my thread on my 3458A to get a reasonable rundown on restoring these meters.

author: ivo
author_rank: Regular Contributor
timestamp: May 17, 2025, 01:35:53 pm
content: FRAM:High-grade, 'hobby' 10V references that aren't designed to destroy your bank account:TEN399 by CircuitValley, I think they post around these parts as wellTindie,eBay10V 0.001% Reference by VoltageStandardDescription,Shop link(can also get their DMMCheckPlus as well)LM399 Box by emeco onAliexpressAll three I've seen good reviews about.Any of these should do to an unoffficial 7.5ish standard.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2025, 03:21:06 pm
content: Before doing a calibraion I would definitely read the old cal constants - they may still be valid at the critical points (10 V --> internal 7.x V ref. and 10 K --> internal 40 K resistor ref.). Many of the other cal constants are less critical and could be just ACAL.Much of the memory may still be OK.For a reference another DMM may be alternative for a ref. value.Especially with the voltage ref. chips keep in mind that soldering the chip to a PCB may already change the value a bit.For the voltage there may also be some measured voltage ref. modules (e.g. around an LM399). It is still a gamble: some seem to be legit and some provide fake numbers. One would be able to see if the numbers are fake: They usually provide more than 1 voltage and the 3458 is good in checking the ratio even if the calibration is way off.Quote from: Zondar on May 06, 2025, 03:46:24 pmThe details one the A3 drift issue are still not public known. It definitely seem to be not abuse, but more aging or temperature cycles and just bad luck. Some older series seem to have been more susceptible but it looks like the issue was never really solved and newer versions can still fail.It is not just normal slow aging and drift, but more like everything is fine for a long time and than things go downhill relatively fast (like 1 year), a bit like maybe failure of the hermetic seal or a crack in the die attachment. One rarely sees borderline cases: either good or quite bad.

author: Zondar
author_rank: Regular Contributor
timestamp: May 17, 2025, 04:11:04 pm
content: Thank you all for the links and tips. I did see the VREF10-001 earlier, and that looks good for a non-critical calibration if I want to attempt that.I figure a "first do no harm" policy should be in effect here. But I'll open it soon to look for leaking capacitors and things, and see what the memory section looks like.I am slightly surprised at how expensive GPIB to USB adapters still are. But one would be useful for operating the machine anyway. I've also heard that there are programs out there that assist in extraction via GPIB for this DVM (anyone have links?). I guess a PROM programmer could do the job too.Thanks again.

author: Grandchuck
author_rank: Super Contributor
timestamp: May 17, 2025, 04:16:17 pm
content: Check outhttps://www.eevblog.com/forum/metrology/3458a-logging-via-windows-app-revisited/

author: coppercone2
author_rank: Super Contributor
timestamp: May 17, 2025, 04:23:52 pm
content: these things are around x-ray machines usually in the building. someone should see whats going on with that u180I wonder if die reflow is a thing. Just reheat in a vaccuum and rebond the wires. If its gold, it should not have oxidized much and might reflow. Maybe a ultrasonic transducer on top to agitate the molten solder a bit too? or tapping it with a tiny solenoid?or just de solder it, lap it to flat gold and resolder it on a new carrier using gold tin solder. Its not a 300W CPU, I think it should be OK if the bond line thickness is a little different

author: Zondar
author_rank: Regular Contributor
timestamp: July 06, 2025, 04:53:37 pm
content: Hello All,I finally opened the instrument up, and it's as pristine inside as it is outside. All capacitors look in perfect condition (no Rifa's, by the way), as does everything else. The prevailing wisdom is to replace the electrolytics regardless, but as I'd rather not disassemble the unit further unless an actual problem is identified, I'll let those sleeping dogs lie for now.This version has the updated digital section that contains two SRAM chips with "snap-hat" backup batteries. I replaced the batteries while the machine was powered on. Unfortunately, both batteries were completely dead, showing only a few mV (should be ~3.4V). So I conclude that the calibration data was indeed lost, as was more or less expected.The good news is that the most serious-sounding errors were gone after the battery swaps. Upon turning it on:Acal all required.Scal all required.Secure required.After-startup stored error messages:110, "calibration required -- scal"And that's it! No more flatness DAC convergence error or any others, including after running ACAL! Yay! So I surmise that the instrument is in working order except for the loss of calibration.I have little to no need for an 8.5 digit DVM, so an expensive professional calibration is not an attractive option. Perhaps I'll try an amateur-grade one, which ought to cost maybe $200 in parts. Aside from having a buddy at NIST (which I don't), I'm not sure there are a lot of other options.Thank you for reading and for any feedback or suggestions.Edit: After finding some dates on it, it appears to be from 2004.

author: Grandchuck
author_rank: Super Contributor
timestamp: July 06, 2025, 07:37:24 pm
content: Nice Work!

author: CatalinaWOW
author_rank: Super Contributor
timestamp: July 06, 2025, 10:26:59 pm
content: You may want to join the USA Cal Club. It slowly circulates a very high standard reference provided by XDEV and has a couple of decent resistors included. Cost is basically shipping.https://www.eevblog.com/forum/metrology/usa-cal-club-round-3-1/msg5871944/#msg5871944

author: Zondar
author_rank: Regular Contributor
timestamp: July 31, 2025, 03:19:06 pm
content: An update:After replacing the backup batteries, the only errors remaining were related to the lack of calibration.I disabled the "secure" feature that blocks unauthorized calibration attempts, which also eliminated the "secure required" message.To familiarize myself with the procedures, I performed faux DC V and resistance calibrations, just using a well-regulated power supply and a common $0.02 resistor as "references." A KS 34465A was used to measure them and to compare with the post-"calibration" 3458A results.The post-"calibration" DC V performance essentially matched the 34465A's. The resistance calibration was much less successful, but that was due to the resistor's value wandering so much that it couldn't be pinned down. After this, only the "SCAL required" (AC calibration required) message remained.Next, having requested participation in the "Cal Club", Brian (bdunham7) reached out to me. It turned out that we lived close enough nearby, and he very generously offered to have me bring the unit to him.Brian's experience and equipment carried the day. Compared to his references and instruments, the calibration appeared well within (short-term) specifications.I left the unit with him so he could play with it. It is a virtual time capsule, having been used only briefly and then put in climate-controlled storage. So Brian is pretty much burning it in for the first time, and is collecting some data on it in the process.I thank Brian for his time and effort! Even if I had received references in the mail via the Cal Club, I could not have done as good a job as he did, especially on the AC calibration part. I look forward to hearing the results of his investigations.Thank you again Brian!

author: bdunham7
author_rank: Super Contributor
timestamp: July 31, 2025, 04:50:27 pm
content: Well, the news is grim. Following the -18A service note, I logged these parameters. I keep hoping the drift will settle since this meter has been dormant, but it looks to be accelerating if anything.DATE TIME 10V preACAL 10VpostACAL CAL72postACAL TEMP AMBIENT NOTE7/29 2PM -------------- -------------- .985426363 ? ? (~27C) Drifted +6ppm during the time it took to do the AC SCAL procedure! ACAL done again.7/30 8AM 10.0000201 10.0000003 .985424165 41.0C ? Had drifted up to nearly +5ppm after ACAL the night before, but drifted back down overnight.7/30 2PM 10.0000558 10.0000023 .985418895 41.5C 26.2C7/30 8PM 10.0000988 10.0000030 .985407328 41.8C 27.8C Was noticeably drifting up after ACAL. It seem to gain 2ppm or so right after ACAL, within 10-20 minutes.7/31 8AM 10.0001000 9.9999985 .985394132 38.9 25.0CAmbient temp measurement taken right in front of meter which is on a bench with it's feet up. Actual room temperature is a bit more reasonable, 24-25C. The unit has drifted more than 32ppm in less than two days.The initial calibration procedure went OK except that the final AC SCAL procedure took some time to figure out as we were using ad-hoc methods. The earlier parts were done using a Fluke 732A and the USA Cal Club 10k resistor (aka Mr. Leaky). The initial results looked pretty good, but this morning I've redone the CAL 0 as there was more offset than I'd like to see, like 50-100 counts at times. This is probably at least partly due to the short warmup time before the initial cal was done as well as my non-ideal summer environment.So, if any 3458A gurus would like to comment or at least offer sympathy it would be appreciated. It is a bit frustrating because this meter is mint--if you told me it was removed from a sealed factory box yesterday I'd have no reason to doubt it. I know the U180 on the A3 board is the known cause of this, but are there any others? And might it settle after running a while or is that just wishful thinking? And as of now, are there any options other than mailing it back to Keysight?I've attached an A3 schematic in case anyone would like to educate me about what exactly goes wrong with these. And I'd be even more curious if HPAK has rectified the issues with newer models or if the U180 hybrid design just has this 'feature' baked in and we have to live with it forever. The meter has drifted up over 2ppm in the time it took me to post this!Edit: Now, 2 hours later, it is up 14ppm on the 10V range.

author: iMo
author_rank: Super Contributor
timestamp: July 31, 2025, 06:05:04 pm
content: Hmm, and I was thinking the using of the simple zener droppers (for creating 15V from 18V) is the domain of the cheapo 34401A only..

author: Kleinstein
author_rank: Super Contributor
timestamp: July 31, 2025, 07:01:50 pm
content: There are also newer meters with a failing U180. So the issue was not fully resolved, more like a reduced rate of failures. I am afraid the failures will be a thing even for the black models. AFAIK there is no public knowledge what exactly happens to the chips.The main option seems to be sending the meter to Keysight for an A3 board replacement.

author: Grandchuck
author_rank: Super Contributor
timestamp: July 31, 2025, 07:24:39 pm
content: That is sad newsI never heard of one settling down.

author: factory
author_rank: Super Contributor
timestamp: July 31, 2025, 09:08:18 pm
content: Quote from: iMo on July 31, 2025, 06:05:04 pmAt least the diagram shows non-polarized capacitors for these Zener dropper supplies, they probably don't suffer from going short circuit, as the 20V rated tants (on 15V rails) used in the 34401 do.David

author: coppercone2
author_rank: Super Contributor
timestamp: July 31, 2025, 11:07:01 pm
content: What can it be on the ASIC that is doing this?https://xdevs.com/pow/u180_stk/What semiconductor parameter is varying so much that it causes a temperature sensitivity?Could it be a cracked bond wire ? or have people determined its something on the silicon?I think you can do a die reflow if you reattach the bond wires after wards. Die solder can have 'voiding' but you would think that is well solved by HP manufacturing process. Like cracks that propagate through voiding. I bet you can even just very slowly sand the back of the chip off and rebond it yourself with gold solder to a new die.There is a trap here like everywhere, they have cheap and expensive die bond solders. The gold content varies. I wonder which type of die attach solder they used. If someone is trying to cheap out it sounds like there is alot of criticism of the low gold alloys.Typical die attach solder should be like less then 400C to melt it.I thought if the die attach solder is low gold and it cracked up, then maybe the silicon has thermal strain and it like warps very slightly because of a thermal differential that is formed under the die when its connection to the thermal ground is cut by a crack effectively undermining the 'castle' on top .BTW the difference is 20% vs 80% gold composition for a die attach solder typically. I feel like there is a high probability that a bean counter got into the gold.I mean what if all it takes is a vacuum oven and wire bonding machine to fix this??Does anyone know how big the die is for the U180, is it different in size compared to other chips? If the die is irregular in size compared to other things it might be a reason to think its cracked die solder ?Excluding the die (somewhat easy fix) what other mechanisms of degradation of integrated circuits can cause the type of behavior as seen by malfunctioning u180s? The thing that comes to mind is low gain from a transistor that has drift of the added atoms.I find wanting to know what causes this kind of behavior to be more interesting then the entire meterSometimes I wonder about this high drift that follows over current in semiconductors. Could some of it be attributed to the die attach solder melting and shifting (from the strain the chip is under in the board) and then resolidifying and then leading to drifty behavior?? Maybe in some cases the bond wires survive and it just lets like a bubble or void form under the die from the chip-under mechanical-stress ? So it remelts itself like crooked. Then it works but it has very high strain because its no longer properly connected to the thermal 'ground' so it like wants to warp? The silicone does not have good lateral heat transmission. The bond wires certainly seem like they might have at least a tiny amount of play in them (since its like arches). Some sort of 'mooring' can also maybe make the chip die solder re-solidify wrong, because its wired after its bonded normally. It might not work if its bonded then re die soldered. I mean like op-amps. I definitely heard of damage that results in 'high drift' or unstable offset.

author: srb1954
author_rank: Super Contributor
timestamp: July 31, 2025, 11:29:54 pm
content: After running a drift check for a month, with regular ACALs, my 3458A showed no lessening of the drift rate. The CAL72 parameter seemed to continue drifting at almost constant rate, averaging 1.52ppm/day.After getting the A3 board replaced the CAL72 drift rate dropped to approx 0.003ppm/day; a vast improvement!

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 05:13:08 am
content: Quote from: srb1954 on July 31, 2025, 11:29:54 pmI could work with 1.52ppm/day. This one drifted 91ppm since lunchtime! You can't even complete a full ACAL without it drifting off 3-5ppm by the time the AC and stuff is done.

author: iMo
author_rank: Super Contributor
timestamp: August 01, 2025, 06:26:39 am
content: Quote from: Kleinstein on July 31, 2025, 07:01:50 pmPerhaps they do not manufacture the new U180 anymore (that would be quite expensive, imho).. They perhaps use the old stock, perhaps the chips which were binned as "lower grade" 30y back..

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 05:36:15 pm
content: I need to understand a few things:First, 91 ppm on a 10V signal is a ~0.91 mV error. This does not sound remotely reasonable, whether "drift" or not.Next, do the "10V pre-ACAL" measurements represent accumulated error over multiple ACAL's, or are they "reset" to a degree after each ACAL? If it's "reset", as I have been presuming, then the measurements above appear to show that the so-called "drift" is accelerating, which doesn't sound like other people's experiences with "drift."Then, I can't find a reference for what "cal72" specifically represents. Can someone clear that up for me?Also, yeah, using diode drops to go from 18 to 15V is surprisingly cheesy.Responding to thoughts by coppercone2: I have nearly 40 years of experience with IC design and packaging. I would not expect "drift" to be due to bond wires failing. The "strain" idea is more interesting and plausible, but U180 is in a robust ceramic package, and so again I would not expect that to be an issue. Electromigration (I think that's what you meant) - distortions or open-circuits in metal traces caused by high current vs. inadequate design - is at least a remote possibility, but in this case it's implausible since it's barely been used.Anything is possible, but since it's a virtual-virgin of an instrument, I'd have a hard time believing that any IC would be failing.Edit: I found a die photo of U180. As many here would already know, it is a hybrid containing an IC and a separate resistor array. I struggle to think how this part would result in such an error solely due to age (in this case).Twenty year-old electrolytics could easily be a problem, but assuming that they are the polarized capacitors in that diagram (not sure), none seem positioned to be a target of suspicion unless they were grossly failing. I also wouldn't expect resistors or ceramic capacitors to be a problem.Any counter-arguments? What is left?

author: Grandchuck
author_rank: Super Contributor
timestamp: August 01, 2025, 05:53:11 pm
content: https://xdevs.com/pow/u180_stk/

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 06:27:03 pm
content: Quote from: Zondar on August 01, 2025, 05:36:15 pmIt is indeed a ludicrous amount of drift or whatever you call the error. I'm still hoping for some ideas from the more experienced people that have been down this road before arriving at any conclusions.QuoteYes, if I've shown a pre-ACAL measurement it would represent only the drift since the last ACAL. I do have some more data that I'll post soon, but I've turned the meter off so as to try a cold start and see how it behaves. Yes, the drift appears to be rapidly accelerating.QuoteCAL72 is the active 10VDC calibration constant. CAL 70-74 are the in-use DC calibration constants for the 100mV, 1V, 10V, 100V and 1kV ranges respectively. A cal constant of 0.985xxxx means that if you lost all your cal data the meter would be off by about 1.5%, which I believe is about what you observed. These CAL70-74 constants are the ones actually used when displaying the readings and are updated by the ACAL process, the ones set during the artifact calibration are elsewhere and do not update during ACAL.QuoteThis was a very common design technique from that era for devices that needed a lot of different supply voltages at fairly low power. Using a 3V zener is a bit unusual though, but if the output isn't critical I suppose it works.QuoteIllya Tsemenko (userTiN) of xDevs.com is no longer on this forum but you might want to contact him. He's put a lot of effort into this issue and additional thoughts from someone with your experience might be helpful. I've no idea about the specific issue here, but custom hybrid ICs have a long and storied history of known failure patterns on high-end test gear.I did one more test before I shut the meter off last night and that was one suggested originally byTiN, using a shorter aperture. I set NPLC = 0.005 and observed that the accumulated error decreased. I'm not sure exactly why--something to do with the slope drivers--but apparently that is confirmation that the problem is actually U180.

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 06:36:39 pm
content: Thanks for the link. That page only shows the IC die and not the resistor array. Although I can't be sure of the technology being used, itdoes not looklooks like CMOS.It does use gold bond-wires. Aside from the color, you can tell because there's a melted "ball" attachment to the bond pads rather than a wedge. That all looks fine.You can also see marks on the bond pads from a test-phase prior to packaging and bonding.Interestingly, it uses aluminum bond wires to connect between the IC and the resistor array rather than gold. I wonder if that was to suppress bi-metal contact issues vs. aluminum wires on aluminum pads, or if it was simply a matter of convenience.

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 06:46:46 pm
content: Updated data log:DATE TIME 10V preACAL 10VpostACAL CAL72postACAL TEMP AMBIENT NOTE7/29 2PM -------------- -------------- .985426363 ? ? (~27C) Drifted +6ppm during the time it took to do the AC SCAL procedure! ACAL done again.7/30 8AM 10.0000201 10.0000003 .985424165 41.0C ? Had drifted up to nearly +5ppm after ACAL the night before, but drifted back down overnight.7/30 2PM 10.0000558 10.0000023 .985418895 41.5C 26.2C7/30 8PM 10.0000988 10.0000030 .985407328 41.8C 27.8C Was noticeably drifting up after ACAL. It seem to gain 2ppm or so right after ACAL, within 10-20 minutes.7/31 8AM 10.0001000 9.9999985 .985394132 38.9C 25.0C7/31 2PM ---in use---- 10.0000051 .985310891 40.3C 26.5C DMM was used in ratio mode for some other work w/ multiple ACALs. 84ppm drift in 6 hours!7/31 9PM 10.0009787 10.0000088 .985220166 42.1C 27.0C Over 91ppm accumulated drift since last ACAL (7 hours). Note that there is now some drift even before ACAL is done.There seems to be about 226ppm total drift since the start of the log. The change in the displayed output is noticeable even in less than 1 minute and since the ACAL process does other ranges after 10V, there is now almost 1ppm of drift just waiting for the rest of the ACAL process to finish.

author: voltsandjolts
author_rank: Unknown
timestamp: August 01, 2025, 06:52:39 pm
content: Some more details on U180 here;https://www.eevblog.com/forum/metrology/hp-3458a-ad-board-spare-part-availibilty-and-cost/msg3682681/#msg3682681

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 06:55:14 pm
content: Quote from: Zondar on August 01, 2025, 06:36:39 pmHere are links to the xDevs page. You can scroll down to the repair A/D drift sections and one of those has some additional pictures. I'll post links to any other info I find, I think there is a better photo somewhere of the entire assembly but I can't find it right now.https://xdevs.com/fix/hp3458a/https://xdevs.com/fix/hp3458a/#a3drift

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:02:04 pm
content: Quote from: bdunham7 on August 01, 2025, 06:27:03 pmI'm not ready to accept anything as "confirmed."

author: iMo
author_rank: Super Contributor
timestamp: August 01, 2025, 07:26:27 pm
content: Doublecheck all +/-15V voltages. When a dropper zener fails it gets usually shorted and you will get 18.5V there and that may cause further damages..

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 07:44:54 pm
content: Quote from: iMo on August 01, 2025, 07:26:27 pmI'll open it up and look at everything I can think of when Brian is finished with his experiments. I may need a stable reference, though.Ironically, if the "drift" continues to increase, it may make things easier to diagnose or confirm.

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 08:06:24 pm
content: Quote from: Zondar on August 01, 2025, 07:44:54 pmThe schematic I posted is the original design and there have been multiple updates so those zeners might not even be there. The xDevs page has extensive information on measuring the 3 reference voltages and observing them drift. Your 34465A will be sufficient for this, especially at the rates of drift we're seeing. At much lower rates of drift--say 0.5ppm/day--you might need a month. Given that the rate of drift is so extreme, I still think there's an outside chance that something else has gone awry. If anyone has seen these rates let us know.I'm pretty much done. The cold start simply revealed that it drifts down during the warmup and then starts going back up again once stabilized. So for a period during warmup the effects counteract each other. So it's time to pop the lid off....

author: iMo
author_rank: Super Contributor
timestamp: August 01, 2025, 08:40:52 pm
content: QuoteThe ceramic packages with that metal lid may get an air leakage. When not tested after manufacturing on that leakage it may easily happen the chip like U180 will slowly "die" (corrosion or surface contamination)..I saw somewhere 2 tests - they put the ceramic packages into pressure chamber (gas) and then looked at bubbles when in liquid, or, they put the chips into liquid under pressure and weighted them afterwards, heavy chips were trashed..

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 09:13:32 pm
content: Quote from: Zondar on August 01, 2025, 05:36:15 pmThat is what I think too. I just read about IC's for a long time but I never worked with design or manufacturing.I keep thinking of Au20/Sn80 solder vs Au80/Sn20 solder. The price difference is an order of magnitude. Both are for die attach. But if you read about it you hear of voiding. To me if there is voiding and crack propagation under it, then the crack depending on how it forms can laterally thermally isolate certain sections of the circuits that allow for islands of heat to form, heat that would normally be sinked strait down into the substrate. I think this could make strain on resistors and such and cause offset and gain drifts that are erratic.What about the size of the die compared to other dies? I know bonding problems occur when dies get bigger. Same for gluing parts on. Large ceramic parts (i.e. heater) are known to get cranky when they get glued, so normally people like to use paste which decouples them mechanically.

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 09:15:31 pm
content: Quote from: iMo on August 01, 2025, 08:40:52 pmChips are packaged in air anyway, and I seriously doubt this one is any different, so "leakage" shouldn't be a cause.

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 09:21:10 pm
content: I thought they are at least packaged in clean dry air?Could it be moisture ? But at most that would corrode the bond pads I think.But 1ppm to me... seems in line with what you would expect from strain gauge effect? Its small but not that small. I know its dependent on the geometry of the tracesOne weird idea is that maybe there is a wet tantalum or electrolytic nearby that happens to have a seal fail around the same time as a u180 seal and then the u180 absorbs some of the chemical emissions from the part. But I don't know what that would do other then destroy a bond wire, which should be very easy to find by anyone actually doing a failure analysis. I imagine standard procedure would be to re-bond it?

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 09:27:38 pm
content: If someone actually understands the IC, is there like a region of circuits that dissipates significantly more power then the rest? I wonder if you can use a tiny ultrasonic probe to probe on top of a known 'hot spot' of the IC to see if the die attach failed under that point or has a discontinuity at least. My whole idea hinges on the idea that the die itself might not be a great 'heat spreader' and requires direct contact on 100% of its surface area to function normaly.Or maybe its possible to de-lid the part and look at the top with a thermal microscope? I managed to get about 6x magnification on my TI by building a lens adapter and a stand for it that lets it work as a thermal microscope with fine adjust. I don't know how much zoom you can get from it. Maybe enough with a few lens to see if something is wrong or suspicious at least?And another test might be to use a dial indicator to see if its attached crooked, by sweeping the chip interior and sweeping the die.I hate IC problems, to me its like having a malfunctioning toilet (fluidic ASIC). It feels so god damn hopeless. But there is ALWAYS the chance its just the 'wax ring' and that its possible to fix without any damaged ceramic structures!!! often you can just lift it up and put a new ring.I still have dreams about possibly making some kind of basic wire bonder that uses micrometers and those bendy hinges that sits on a old microscope. I mean a small cheap slow one, not a giant apparatus...I wonder how low tech it can be, if you are willing to make it so you need to do 30 minutes of fine adjustments per bond wire. I always suspected there is a big cost cutting that can be done on these machines that would make their usability go down enough that it would be way too ridiculous for commercial customers to use it, but it would be possible for a hobbyist to use it since they are willing to spend the entire sunday to rebond 3 pads. We can tolerate more stress then commercial users (once the six packs come out). I.e. is it possible to make some kind of single use 'cartridge' that is manually assembled with a bit of wire rather then developing a whole feeder mechanism (black hole). Like if you can do 1 pad, then take the head apart to insert another short strand of gold, instead of trying to make it so its 'belt fed'. Like a bolt action instead of a autocannon. Along with other... design changes.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 01, 2025, 10:05:34 pm
content: U180 has 2 parts. A resistor array with the resistors heavily interleaved. So even if 1 resistor is getting hot, the power is still spread out over quite some area. The power should not be that much: like some 10 K and 12 V as one of the highest power ones. This would be some 15 mW and maybe some 50 mW overall.The other part is JFET switching with a few flip flops and gate drivers. Here it is not clear if this is more CMOS or more TTL like gates. Chances are the JFETs itself have very little power and the main power would be the synchronization flip-flops. These would likely be fast and may thus be a bit power hungry.Silicon is a good thermal conductor and thus no so much localized high temperature or dependence on good thermal contact everywhere. The resistor substraight may not be such a good thermal conductor, but should still not get locally that hot. A loose die attachment could still cause a change in the mechnical stress.While the drift of the ADC is in most cases a problem with U180, there is a small chance that other issues (like the supply) can also cause drift. So it would be worth checking the supplies.If one has a good meter, one could also check the +-12 V ref. voltages - at least in some cases with a bad U180 the drift already effected these voltages. So an issue in the resistor part (could still be more trouble too).

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 10:23:08 pm
content: Quote from: coppercone2 on August 01, 2025, 09:13:32 pmOne of the problems in explaining a change from in-spec to "drifty" is the fact that it's apparently a permanent change that creeps up over time (I haven't heard anyone complaining about a sudden change). This mostly rules out things like transient temperature-related changes.Another big question is whether drifting is inevitable - does it eventually happen to all of them, and the non-drifty ones are just late to the party? I don't think the answer is clear, is it?HP/Agilent would not be scrimping on materials for such an exotic and no-doubt very expensive chip. It's hard to say if solder or conductive epoxy was used in this case from what little I've seen.I discount the crack idea because that would probably cause a discontinuous change or a failure of the entire chip. Voids can and do occur when using high-volume, high-speed packaging, but probably not with the care that HP would have put into these parts.Strain on the resistor network is plausible. It would change the resistor values slightly, but the question is by how much, and how much a change would affect performance. For example, matched resistors on the same die and under the same strain would likely stay well matched.Also, such strain would have to benew and permanentfor the performance to change from normal spec to "drifty." Temperature changes would be the obvious source of a change in strain, e.g. by mismatched TC's, and that would be seen on its own if it's large enough - but I don't see how it would cause permanent changes. What else is could?Another possibility is poisoning of the resistor network by outgassing from the attachment material (e.g. solder) or the lid-attach material. However, I presume that the resistor network includes a good protective passivation layer. That might not be good enough over lengthly time, of course, but I mostly doubt this one too.The size of the die could be estimated by measuring the width of the package cavity vs. the die. I haven't done that, but it's pretty easy. Larger dies are more susceptible to issues than small ones, but I don't see explanatory power here since they are all the same size anyway.The fact that this machine has barely been used might offer a clue.

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 10:45:36 pm
content: Won't it be gradual if there is a crack propagating between tiny bubbles ? And the longer it gets the worse it gets.I am imagining like tearing foam slowlyand I thought all the solder used for this application is either 80 20 or 20 80 and has no flux (vacuum bonded)I also wonder if there could be wiskers in high tin die bond solder. Could a tin wisker grow in a void and 'jack up' the die ?Wiskers are known to grow basically by themselves without much external stimulusI have not yet seen a flux that is for ceramics or carbon. All the solder used is either vacuum melt or ultrasonic. Flux actually prevents it from working. They do a gold coating on things before gold soldering, but its also done in a vacuum (as far as I can find out)You could have residual.... acid? from the polishing step I guess (if they do chemical mechanical die polishing), but you would think all the bake outs they do would take care of that.One thing I did notice is that some people use a strong chemical treatment for ceramics before soldering or plating, I think a boil in phosphoric/sulfuric acid to 'etch' alumina slightly. Like 36+ hours. But its hard to imagine any chemical being the problem, the processes should be similar for all the HP ASIC chips they havehttps://www.sigmaaldrich.com/US/en/product/aldrich/667447There would have to be some nuance to the chip that makes it get a different treatment. I think the bulk 'processing' steps must all be pretty much the same for all of their silicon ASIC devices.**I have seen manufacturing problems that relate to objects of unusual size going through the same process, for instance if you have something that is too large to fit in the vessel you like to use, and there is some alternate thing made up for it that is of low quality. Kind of like how for most of your DIY de-rusting your use a bucket, but then you come across some object of irregular size and it ends up being laid out in a garbage bag pool inside of a lamp cover! Or something deep getting incorrect torque because it requires an adapter no one bothered to calibrate. That is why I am so interested if there is anything special about that chip that might make it go through a alternative path to other ASICS that HP uses, which seem to be generally OK (maybe aside from the analog MUXs that seem to break alot). Another one is changing ovens, there is always chaos when an alternative oven is used for something. We don't actually know how ovens work. There is theory about temperature and airflow and whatnot but I see its about as useful as tea-leaves when the curing oven is changed for a well tuned manufacturing process.

author: Zondar
author_rank: Regular Contributor
timestamp: August 01, 2025, 11:16:37 pm
content: Responding to a few other comments:I had presumed that the "digital" portion contains more than just digital logic. Otherwise, I'm curious why JFET technology was used when CMOS would have been far cheaper and easier. The supply voltage looks like it's 5V, too.There are what looks like capacitors on the die, but I don't think they are active components. And then there are what looks like some mix of strong amplifiers or buffers and possibly switches. Anyway, I'm sure they had their performance-related reasons for JFETs.The full-resolution die photo is excellent. It's good enough that some simpler areas can easily be reverse-engineered. However, the densest sections look to be very difficult and probably too much so. Still, has anyone attempted this?If it's purely digital logic, then the logic style wouldn't be terribly important. The final amps or buffers could be an exception.I'd expect the resistor substrate to be silicon too.Interleaving the resistor array (this is common when precision matching is needed) reduces the probability of things like temperature or strain causing problems.Yes, of course U180 is the most likely source of these problems, but I'm curious: has anyone desoldered one and replaced just the chip? Or has a chip ever been studied in isolation? Related, I note that a few of the resistors could possibly be measured in in-situ if certain jumpers or connectors are opened. I doubt it would reveal much, though.I'm slightly warming up to some sort of poisoning idea. Hmm.

author: coppercone2
author_rank: Super Contributor
timestamp: August 01, 2025, 11:23:28 pm
content: This photo right? 100mbhttps://xdevs.com/doc/HP_Agilent_Keysight/3458X/u180/result_1.jpg

author: bdunham7
author_rank: Super Contributor
timestamp: August 01, 2025, 11:46:32 pm
content: Quote from: Zondar on August 01, 2025, 11:16:37 pmYes, people have started down this path but AFAIK nobody has gotten terribly far.Illya (xDevs) has desoldered and transplanted U180 chips for testing, but only from scrap and used sources. A new, genuine U180 is pure unobtanium for any of us mere mortals. At one point Illya did make a cobbled-up network of external resistors and said that 0.2ppm/hour was the best he could get as far as drift goes. He gets an A for effort, but at the end of the day he could only replace the chip or board as was available.Maybe they had a production issue that they just don't want to discuss and they also ended up with a huge stockpile of them to forestall obsolescence.

author: Zondar
author_rank: Regular Contributor
timestamp: August 02, 2025, 12:10:37 am
content: Quote from: bdunham7 on August 01, 2025, 11:46:32 pmI can't comment about today, but I think integrity was important during the golden old HP days. So I wouldn't believe they would continue to sell something that they know to be actually defective.That raises the question of whether they, then or now, ever considered it a significant defect. Hard to say. Maybe they do think a certain level of planned obsolescence is acceptable? More likely they just expect to be servicing the units on a yearly basis, and it, along with any other problems, will be caught and fixed if and when they happen. They knew the NVRAM batteries absolutely would fail, and they didn't consider that a defect, right? Plus, no-one buying these new and expecting to have it serviced regularly is terribly worried about money.

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2025, 12:15:29 am
content: What if you made a distributed project where a bunch of volunteers take a grid section of the die and identify the parts in it. It could be a fun thing where you get together volunteers and have say a 3 month period for everyone to get their analysis together and stitch it into a picture.I think the problem is burn out . So everyone that can read it takes 2 square inches and translates itOr a program that randomly gives a web user part of the die on request that is unsolved.Kind of how you do archeological digs, you make a grid map and assign people to dig at different 'squares'.I feel like it could work in a Wikipedia type thing, just assign carthesian coordinates to each 'page' and the picture could be 'interactive' much like the picture of the table of elements they had up there somewhere where you can click a box.Once things are identified like 'resistor' 'amplifier' then people less skilled in semiconductor level engineering can analyze it with a normal EE methodIt's too big and boring for one person. That is like eating a crate of matza

author: Zondar
author_rank: Regular Contributor
timestamp: August 02, 2025, 02:27:24 am
content: Quote from: coppercone2 on August 02, 2025, 12:15:29 amThe problem is that the dense areas are so crowded that it's difficult to make out what's connected to what with any certainty. This is compounded by the fact that the wavelength of light imposes a limit on what can be seen, in this case making the jumble a little blurry, too.

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2025, 02:29:49 am
content: Is that photo at the limit of what you can do optically? I don't have a feel for such things.Anyway if it was all explained on a big circuit diagram it would at least limit the amount of unknownsmaybe if it was narrowed down to a few unknown patches it could get studied with some kind of better equipment by someone capable.Unless the whole thing is like that.Its like one of the most powerful DMM and its being held up by some little flaw no one understands. kind of ridiculous considering how important people consider it to be for electronics

author: Zondar
author_rank: Regular Contributor
timestamp: August 02, 2025, 04:19:04 am
content: Quote from: coppercone2 on August 02, 2025, 02:29:49 amYes, it's pretty much at the limit.It's possible to etch away the insulating layers, leaving the metal traces and contacts suspended in air, and then preferably use an electron microscope to photograph it. Electrons have a far smaller wavelength than visible light and hence can image much finer features. But even then, you won't be able to see beneath any metal with an EM, and it looks like there are broad swathes of second-level metal covering portions of the circuitry.

author: Zondar
author_rank: Regular Contributor
timestamp: August 02, 2025, 05:20:30 am
content: I looked at the full resolution photo again, and it's not as bad as I originally thought.For example, I can already see that each of the 18 structures in the upper section contains two multi-stage single-ended to differential drivers plus two very large single-ended drivers, plus some other random logic. The larger structures near the bottomare mostly just two-stage driversare the current-steering switches.Producing a full schematic would be very tedious and time-consuming, but a basic schematic of a few of the major sub-sections like the upper blocks looks semi-doable with a few hours of effort. I'll give it a try soon and see how it goes.

author: iMo
author_rank: Super Contributor
timestamp: August 02, 2025, 07:58:34 am
content: To reverse engineer the chip would not be a big problem today. The issue could be the final price of U180 in say 10000 quantity (I would consider the absolute minimal amount which makes sense) and the costs of a replacement and adjustment. Moreover the new chip might require the changes in sw as well. So such an U180 rescue operation by Keysight might cost you $5k+ at the end, provided KS is even willing to mess with XX thousands of none-corporate users owning the meter world-wide.. I am pretty confident KS made this calculation many times in past, the only result so far being the Black edition reusing the old chips..

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2025, 09:09:28 am
content: The U180 chip is really central to the 3458 ADC performance. KS did a major update one the PCBs, but they still kept the old U180 chip. They may still have quite some chips (could be just dies).A new replacement may look reasonable easy at frist glance, but the problem is that it is hard to predict the actual performance (especially the INL) to the 0.1 ppm level. So there it no guarantee it would work well in the frist try.For a different company it would not make that much sense to do a direct replacement. One would more likely start with a new design for a high performance DMM. The ADC in the 3458 was really good for it's time, but it still has it's weak points (besides the bit high failure rate). For the higher higher speeds the performance is no longer that good compared to ready made chip. Keep in mint that the good INL is the 10 PLC. Already 1 PLC mode is likely worse and at 10 kSPS it is quite different. A more modern design would likely use 2 separate ADCs for the slow (e.g. 1 PLC and more) and fast case ( e.g. < 1 PLC to maybe 1 MSPS). The competing Fluke meters aready go that way.The high end DMMs are still a pretty small market and developement and testing can be quite expensive.

author: chilternview
author_rank: Frequent Contributor
timestamp: August 02, 2025, 09:39:29 am
content: Quote from: Kleinstein on August 02, 2025, 09:09:28 amThis has always been a puzzle to me. I would assume that the drift issue would have been known to HPAK and they would have spent time analysing the cause, given that it is key to the 3458 performance. So why no redesign, after 35+ years? They have addressed other issues (like the Elantec comparators) so why not U180?Perhaps they do have a stock of the originals, enough to satisfy demand, and so a redesign was not cost effective. And it may be the case that the fab that made them is long gone and there is no modern foundry that can make them to the original spec. Of course admitting to a problem that may cause recalls would be very costly too. But without inside info it remains a mystery.

author: aronake
author_rank: Frequent Contributor
timestamp: August 02, 2025, 11:51:33 am
content: Quote from: Zondar on August 01, 2025, 11:16:37 pmI have made some swapping of U180 between boards. Its pretty easy to desolder with a good desolder gun. Conclusion I came to was that the fault I saw on one A3 board moved to the borad where i placed, the U180. And the "broken" board started to work when it got another U180.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 02, 2025, 01:06:33 pm
content: Why KS did the redesign of the 3458 is a bit odd. A big point for the meter is the legicy. Otherwise a complete new version would make more sense.There are a few reports on other defects (especially the comparators) with the A3 baord, but these usually result in a non working meter and not just high drift. The drift shown is really extreme and in many cases with so much drift one would end up with a convergence error.

author: bdunham7
author_rank: Super Contributor
timestamp: August 02, 2025, 01:58:48 pm
content: I'd like to ask a favor of any users that have a 3458A on their bench. Could you post at least the first three numbers of your CAL72 parameter?From the front panel press "shift", C, down arrow (meter reads "CAL?) then "72, enter".Zondar's meter is now showing CAL72 = 985.203185e-3, the few other ones I've seen in random places have been much closer to 1. I'm just wondering if it is normal for this to be shifted by 1.5%.

author: Zondar
author_rank: Regular Contributor
timestamp: August 02, 2025, 02:15:28 pm
content: A schematic of U180 would only be to add to the understanding of the chip. There is virtually zero chance that replacement chips could be fabricated by and for the community, regardless of the investment. Perhaps a daughter-card could be made, cobbled together from this and that, but it's very unlikely that the performance would match sufficiently to be a realistic replacement.Out of frustration, people here endlessly speculate about what is going on with the chip and how HP/Agilent/KS is positioned regarding them. My own speculation, with zero facts behind it, is that they made a significant excess of wafers on an old fab line while it was running (extra wafers are relatively cheap) and still have plenty of die left, and then have made new batches of completed chips as needed over the years.By the way, is it absolutely confirmed by HPAK itself that the logic chip is JFET tech? Planar JFET and MOS tech can superficially look very similar. It's so frustrating, since the resolution isjustbelow what is needed for this to be clear. (If CMOS, then the odds that the community could make new ones shoots way up, though it would still be difficult and expensive. The resistor network would then be the bottleneck.)We need to get together and picket KS headquarters, holding signs saying "FREE THE U180!" They will quickly cave, right?

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2025, 03:10:28 pm
content: Speaking of matching resistors, have you ever heard of diamond alloy?They have a substrate that has conductivity 2.5x of pure silverhttps://en.wikipedia.org/wiki/Dymalloy(this is the old version that has weaker thermal conductivity)There might be a missing link in thermal coupling that can allow better performancehttps://rhp.at/en/applications-products/thermal-management/diacool-heat-sinksnot sure who had the ~1000 coefficient versionfor people experimenting, some expensive solders conduct alot more heat then regular electronics solder. Pure tin is alittle better then normal solder. The high gold ones used for die bonding are way better, and theoretically you might be able to solder a component with them for some interesting 'hybrid' designI wondered before about doing interesting things with heat pipes filled with odd solvents too (they have some rather unique thermal properties)

author: Grandchuck
author_rank: Super Contributor
timestamp: August 02, 2025, 03:40:11 pm
content: Quote from: bdunham7 on August 02, 2025, 01:58:48 pm1.00720355I did a little searching and found that value is not unusual:https://www.eevblog.com/forum/metrology/new-to-3458a/msg5870683/#msg5870683

author: Bill158
author_rank: Regular Contributor
timestamp: August 02, 2025, 04:01:02 pm
content: Quote from: bdunham7 on August 02, 2025, 01:58:48 pmBrian:Readings from two (2) 3458A DVMs. S/N 2823A122xx and 2823A124xx. Both purchased used in 2006 or so. 124xx had dreaded U180 problem from the start. 122xx never had a problem since I have had it. Stable as the Rock of Gibraltar.0.999084326 This is an A3 board that has never been replaced as far as I know.0.989985819 This is an A3 board that was purchased from Agilent ($1,200 with a $200 refund for defective board) as a replacement board when they would sell us a replacement A3. Stable as the above Rock since 2006.I don't remember how bad the U180 drift was on the defective A3, but something like 2 ppm/day (I think).Bill

author: floobydust
author_rank: Super Contributor
timestamp: August 02, 2025, 07:25:19 pm
content: The HP3458A design is discussed inHP Journal April 1989.Wayne Goeke snippets that explains U180 secret sauce a bit:--------------------------------Resistor NetworkThe resistor network has several requirements. The most stringent is to obtain the lowest ratio-tracking temperature coefficient possible. It is important to keep this coefficient low because the gain of the ADC is dependent on the ratio of the input resistor to the runup reference slope resistors. An overall temperature coefficient of 0.4 ppm/°C was achieved for the ADC. Even at this level, a temperature change of 0.1°C results in a five-count change in a full-scale 8-1/2-digit measurement. (Autocalibration increases the gain stability to greater than 0.15 ppm/°C.)Another requirement for the resistor network is to have a low enough absolute temperature coefficient that nonlinearities are not introduced by the self-heating of the resistors. For example, the 50kΩ input resistor has a input voltage that ranges from +12V to -12V. There is a 2.88 milliwatt power difference between a 0V input and a 12V input. If this power difference causes the resistor to change its value, the result is a nonlinearity in the ADC. A 0.01°C temperature change in a resistor that has an absolute temperature coefficient of 1 ppm/°C causes a one-count error in an 8-1/2-digit measurement. The network used in the HP 3458A's ADC shows no measurable self-heating nonlinearities.The final requirement of the resistor network is that it maintain the ratios of the six slopes throughout the life of the HP3458A. The tightest ratio tolerance is approximately 0.1% and is required to maintain linearity of the high-speed measurements. This is a relatively easy requirement. To maintain the ADC's 8-1/2 digit differential linearity at less than 0.02 ppm requires ratio tolerances of only 3%.SwitchesA last major concern for the ADC design was the switches required to control the inputs and the slopes. Because the switches are in series with the resistors, they can add to the temperature coefficient of the ADC. A custom chip design was chosen so that each switch could be scaled to the size of the resistor to which it is connected. This allows the ADC to be sensitive to the ratio-tracking temperature coefficient of the switches and not to the absolute temperature coefficient. Another advantage of the custom design is that it allows the control signals to be latched just before the drives to the switches. This resynchronizes the signal with the clock and reduces the timing jitter in the switch transitions. The result is a reduction in the noise of the ADC.PerformanceThe performance of an ADC is limited by several nonideal behaviors. Often the stated resolution of an ADC is limited by differential linearity or noise even though the number of counts generated would indicate much finer resolution. For example, the HP 3458A's ADC generates more than 9-1/2 digits of counts but is only rated at 8-1/2 digits because the ninth digit is very noisy and the differential linearity is about one eight-digit count. Therefore, when stating an ADC's speed and resolution, it is important to specify under what conditions the parameters are valid. Fig. 9 shows the speed-versus-resolution relationship of the HP 3458A ADC assuming less than one count of rms noise.----------------------------------It looks like the JFET switches' on-resistance is scaled to the resistance it switches. That might be what is drifting as a wild guess, because JFETs don't seem to age well.In the other repair thread, the 12V rail drifting causes troubles, but that would be just the resistor-divider changing value with temperature which seems a little odd for a failure mechanism.I would guess U180 made by Hewlett Packard’s Colorado Integrated Circuits Division (ICBD), likely designed on HP9000 with their Hybrid Circuit Design Module software. Converted into a profit/loss center in 1989 as part of HP’s new corporate strategy. Not sure how long before it got closed down.The predecessor HP3478A used two custom hybrid IC's (1983). U403 might have similar design but I don't know if it had some reliability issue.sourceI didn't see patents for it. Wayne Goeke left HP for Keithley.With component obsolescence, first knee jerk is to do a last-time buy and I assume there is strictly controlled stock of the part. Did they get enough for a few decades of support?

author: coppercone2
author_rank: Super Contributor
timestamp: August 02, 2025, 07:33:44 pm
content: I wonder if some kind of mems switches can be used for low frequency operation. The technology came a long way. I wonder if it might get rid of the resistance trouble you get with semiconductor switches on networks. Its always disappointing when you digitize a mechanical switch for precision things, reading the data sheet.Imagine the IC had a micro relay socket on it to put a replacable mems switch bank on, like little posts you can insert with something akin to a mechanical pencil

author: srb1954
author_rank: Super Contributor
timestamp: August 02, 2025, 09:41:04 pm
content: With a 2005 built Agilent labelled unit, last calibrated in 2013 and with a drifting A3 board:CAL72 = 0.983864592After replacement of the drifting A3 board:CAL72 = 0.984172346

author: Zondar
author_rank: Regular Contributor
timestamp: August 03, 2025, 12:14:14 am
content: I have the machine back from Brian, who also generously loaned me a "VREF-10" 10V reference.I monitored the internal test points 160 and 165. These are the internal +12 and -12 references. Both drift down (more negative) over time. Normally, this is a very bad sign.However, there is hope!Reading service note 3458A-18A, it says:"The internal reference assembly used in the 3458A are aged (stabilized under a powered-up state) and monitored for appropriate DCV time drift for use in the 3458A. Initially, virtually all references drift at rates too high for use in the 3458A. However, the rate of drift decreases with time...""If the reference assemblies are powered down for an extended period of time (such as being stored in a stock bin) the references revert back toward their “pre-aged state”. As a result the initial drift rate for the DCV reference may be high enough that the DCV function may be “out of specification” prior to the first calibration/adjustment interval (90 days or 1 year)."Except for ~3-4 days recently, this unit has not been powered on formany, manyyears. It goes on:"Operation of the 3458A for six weeks is sufficient for these references to stabilize under a powered-up state. Adjustment/calibration of the instrument after this initial six week operating period will eliminate this issue and the reference will be sufficiently stable to remain within specification until the next regular scheduled adjustment/calibration event."With this note in mind, I've set up the instrument to burn it in for ~6 weeks. I'll check it now and then and report back with any interesting results. Maybe it's a small hope, but I'll take it!

author: aronake
author_rank: Frequent Contributor
timestamp: August 03, 2025, 03:20:53 am
content: One trick some people have had success with is to disable the fan and let the machine run for some months to bring it within specification. But this is more for cases of +/- 2 ppm per day or less and somewhat linear drift. This 3458a seems to have some rather extreme jumps. But could be worth a try.

author: bdunham7
author_rank: Super Contributor
timestamp: August 03, 2025, 04:53:23 am
content: Quote from: Zondar on August 03, 2025, 12:14:14 amI fully support the idea of a long bake to see if anything changes, it never pays to hurry into these things. When you're done with that you might try measuring from pin 3 of U160 to pin 3 of U165 (ZR_HI2 to ZR_LO2) as this will be the actual reference voltage. At least I think it is, the schematics seem to be a bit imprecise.

author: TERRA Operative
author_rank: Super Contributor
timestamp: August 03, 2025, 05:06:42 am
content: Probably would be worth running an ACAL once a day for the 6 weeks to plot the change and see the trend, then you can see how the instrument settles (or doesn't).Shouldn't be too hard to automate with a python script or similar.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 03, 2025, 08:40:12 am
content: If both the +12 V and -12 V drift towards more negative, this is a fault for the amplifier / gain part (resistors in U180 or the OP-amps). A a change at the LTZ1000 ref. module would cause opositve drift for the 2 references, like one goind up and the other down.There is some hope that part of the drift is from settling after not using the meter for a long time. It is no uncommon that meter that are sold were not used for quite some time before. The shown drift is still extreme.It is no only the A9 ref. that may need settling after longer time of no using the meter. Other parts can show a similar effect. Part of this would be be humidty the causes the PCB material to swell and with the higher temperature when powered on will slowly leave the PCB over the time of a few weeks.

author: chilternview
author_rank: Frequent Contributor
timestamp: August 03, 2025, 02:46:27 pm
content: Quote from: bdunham7 on August 02, 2025, 01:58:48 pmCAL72 = 996.459150E-03CAL72 = 1.00187455E+00 (this one has Opt 002)

author: IanJ
author_rank: Unknown
timestamp: August 03, 2025, 04:59:28 pm
content: Quote from: bdunham7 on August 02, 2025, 01:58:48 pmDay 1: CAL72 = 0.984806743Day 2337 (6+ years) = 0.984803016(Includes 3 visits to Keysight for cal)Ian.

author: aronake
author_rank: Frequent Contributor
timestamp: August 03, 2025, 05:22:02 pm
content: PPM/dayMeter Cal last 6 month1 0.9838178 -0.011 Opt 22 0.9858763 0.0103 0.9798675 -0.1144 0.9844274 -0.0135 0.9889628 0.007 Opt 26 Dead A3

author: Zondar
author_rank: Regular Contributor
timestamp: August 04, 2025, 01:14:48 am
content: Questions for the collective mind:First, where can I find the "programming instructions" manual?Am I correct in assuming that the result of "CAL? 2" is a constant produced during calibration and not the actual current value of the 7V reference?Am I correct in assuming, in the A/D circuit diagram, that pin 3 of U160 (looks like it's labeled "ZR_HI2", from ZR_HI, but it's hard to read) is the actual 7 volt reference voltage? It does not closely match the above value, though it's in the ball-park. Furthermore, it is drifting, and in the space of a few hours it has dropped by 40-50 uV (10 uV resolution). This will then drop the +12V reference as well, as I am seeing.Also, does anyone know what the input "ZR_LO2", from ZR_LO, on pin 2 of U165 is? I'm guessing that it's the reference ground. It's currently about -0.00057V. This value has also dropped by ~40 uV in a few hours, dropping the -12V reference as well, also as I am seeing.If I have the above straight, then the 7V reference appears to be drifting badly. Does that sound right?(An aside: in the hi-res die photo, the lower section contains large current steering switches, not buffers as I though earlier.)Thank you.

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2025, 05:08:00 am
content: Quote from: Zondar on August 04, 2025, 01:14:48 amI think it is all in Chapter 7 of the regular manual. If you find more somewhere let me know and I'll add it to my collection.QuoteYes, AFAIK that constant is determined during the 10V artifact calibration. What number are you seeing for this?QuoteI was not able to trace through the entire schematic to confirm this, but looking at the A3 diagram it seems to me that you should measure the reference volt with the positive lead of your DMM on the ZR_HI2 and the negative lead on ZR_LO2 and that should be a steady reference voltage. If it is steady but both are drifting down together relative to AGND, then I think the reference itself is good and the combined drift is the result of something else. I haven't figured out the rest of the circuit so it might be the result of the shift in the 12V references, not the cause. But, as I said, either the schematics available are incomplete or else I have not yet managed to find all the relevant details.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2025, 08:11:51 am
content: U160 is the OP-amp to do the 7 V to 12 V amplification step. Pin 3 should be the 7 V reference and this should be resonable stable. The problem could be in the ground connection that is used. ZR_lo is the low side of the reference and this to a large part is the analog ground for the ADC. It could be slightly different from the ground at the input, but 570 µV of difference looks rather high, as there is current compensation in most places. The connection should include some connectors (A9 to A1 and A1 to A3 boards).The difference from the input ground the ZR_lo may not be so critical as the AZ mode will correct for this.

author: Zondar
author_rank: Regular Contributor
timestamp: August 04, 2025, 04:13:35 pm
content: As of this morning, the 7V ref and its ground have dropped another 50 uV each, with a total drop of 0.65 mV. The difference between the two, being the actual 7V ref, has not measurably changed. The +/- 12V references also sagged by a similar amount, as expected.It's too soon to be sure, but I think the above has mostly stabilized. The values did not change after a DC ACAL, which is at least consistent with the notion that it's just a baseline difference between the boards. I would have expected such a shift to have settled much faster than I observed, though (e.g., a few hours at most, considering temperature changes).What I think I can say with reasonable certainty is that the downward drift of the +/- 12V references seem mostly or completely due to baseline shifts in the 7V reference and its ground. That is, I don't think those shifts, at least, are due to U180.Meanwhile, the digitized output has drifted from 10V to 10.0007V and is still increasing at an approximately-constant rate. Now that's bad!I didn't record times very accurately, but I'd estimate a 50ppm per day drift of the CAL72 value. Also very bad.I admit that I don't yet fully understand the ADC section. But assuming that the +/-12V and 5V have settled to constant values, what else is left to shift slowly but steadily? I'm not sure what to look at next.(I had been looking for the list of CAL? queries. They are actually in the calibration manual.)

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2025, 05:14:35 pm
content: The shift between the input section ground and the reference low side (= ADC ground) is not due to U180. It is more the resistance between the two, e.g. from the connectors or just the copper. Another point could be the resistors used for the ground current compensation (e.g. R188 on the A3 board). These are more normal resistors, not special higher grade ones. The DMM should not be effected very much from a difference in the grounds.The 5 V at the ADC is anyway not super critical it is only used for the fine part of the result and thus only very small part at 10 PLC or 1 PLC, though it gets more with faster readings.The ADC gain factor that is measured for CAL72 is hardly effected by reference drift at all. It is more resistors inside U180, especially the 40 K for the reference and the 50 K for the input path to the integrator. The 7 to +-12 V gain step also contributes. So that are the resistors connected to pins 2-14 at U180.With so much drift in the CAL72 value I am surprized to not get an AD convergence error. So there is a slight chance that the drift here could be from a different reason than U180. This could be something like routing the input signals to the DC amplifier. Excessive input current from a bad JFET or gate drive could also effect the CAL72 part, but would be repairable. As a crude test one could check if there is excessive input current, if it is possible to convince the meter to do some DC measurement.A defect in the switching could also effect some more CAL constants to give unusual or drifting values. So maybe have a look at these too.

author: Zondar
author_rank: Regular Contributor
timestamp: August 04, 2025, 06:21:38 pm
content: An earlier post suggested looking at the Zener diode drops used by the various op-amps. These vary a lot. For example, U170 (input section) shows the negative side at only 2V drop and the positive at 2.5V. If these are supposed to be 3V Zeners like the schematic says, they don't seem very good. On the other hand, I don't think the op-amps would particularly care. Any comments?I haven't been able to locate R188 for sure yet. In the schematics, is it the one just below the ZR_HI and ZR_LO jumpers? I sure wish they were clearer; what a pain, and it calls for an Optivisor just toguessat the designations (but begger's can't be choosers). I can't find it on the board either, but my A3 board is updated from the available schematic, and there are definitely differences.I don't suppose there's a reference describing all those other test points?It's easy to "convince the meter to do some measurements" - no errors, everything works fine, it just drifts. Right after ACAL, it's very accurate, too. I could arguably use it to measure itself with greater accuracy than my 34465A, as long as I acted fast.

author: bdunham7
author_rank: Super Contributor
timestamp: August 04, 2025, 06:21:53 pm
content: Quote from: Zondar on August 04, 2025, 04:13:35 pmI'm assuming the drift is when you measure from ZR_HI2, 12VREF+ or 12VREF- to AGND? Can you pinpoint exactly where you are connecting to AGND (or if not AGND, where and what)? If you measure from 12VREF+/- to ZR_LO2 do you see any drift?That brings up two questions regarding ADC operation (which I also don't have figured out). First, since the integrator, input, etc seem to use ZR_LO2 as the reference and not AGND, why does the drift relative to AGND matter? This may turn out to be a silly, obvious question but I don't have the full schematics available right now since I'm travelling. Second, how are ZR_LO2 and AGND actually connected? My hypothesis that cause and effect may be the reverse of what you are thinking is based on the idea that AGND may be derived from and referenced to a sort of "rail-splitting circuit" that would provide a signal that was exactly halfway between them. IDK how it would work, but I'm looking at those resistors and signals inside U180 that are labelled AGRND1, AGRND2, SUM, etc.

author: Zondar
author_rank: Regular Contributor
timestamp: August 04, 2025, 07:03:55 pm
content: Quote from: bdunham7 on August 04, 2025, 06:21:53 pmI'm not sure which ground you mean by "AGND." There appear to be at least three: A, A2 and A3. Presumably, they should all connect together somewhere, e.g. in a star configuration, but I haven't seen how yet.Inside the U180 package, the names for grounds change (e.g. to AGND2, etc.), but that's confusing, so I'm just using the names as shown for the external components.ZR_LO2 and A/A2/A3 are not connected together. There is less than 1mV difference between A3 and A2.I'm measuring the reference values using the "A2" ground associated with the +/-12V references. Specifically, I'm clipped to the ground side of C157, a decoupling cap below the -12V reference op-amp. That seems good to me (no?). I do see that A3 is used for the +5 reference, but I haven't been using that thus far.ZR_HI, ZR_HI2 (just post-jumper ZR_HI), ZR_LO, +12, -12, +5 all appear stable at the moment - no drift, at least none within the resolution of my 34465A, and it didn't change since yesterday. So 12V to ZR_LO, etc., also appears stable.I don't think the circuit should be (very) sensitive to the 7V reference baseline - that is, where ZR_HI and ZR_LO sit vs. "ground."

author: Kleinstein
author_rank: Super Contributor
timestamp: August 04, 2025, 10:26:04 pm
content: The position at the decoupling capacitor may well be a poor choice for the ground point.ZR_lo is connceted to ground on the A9 ref. PCB, near the LTZ 1000 ref..There is likely not just 1 star ground. Chances are the ZR_lo, analog ground on the A1 PCB and the power ground for things like decoupling are treated separate. The power power does not care about a start ground.The OP-amps would not care that much about the exact supply / drop at the zeners. It is known that low voltage zeners like 2.7 or 3 V are often not accurate and temperature dependent.With the DMM somewhat working it would rellay help to check the input current, e.g. as the voltage read with a high resistor (like 1M or 10 M) or via the drift with a 10 nF PS or PP capacitor at the input. A similar test would be looking at the 100 V range with an open input. This has the 100 K from the divider at the input and use the 1 V range at the amplifier - so excessive drift of the input bias would get visible. The absolute value is hidden by an offset that is subtracted.

author: Zondar
author_rank: Regular Contributor
timestamp: August 05, 2025, 12:38:35 am
content: Using my 34465A directly, the input current is virtually unmeasurable. Maybe it's 1uA. This was while the 3458A was on the 10V scale with a 10V input. To do better, I'd have to employ a trick such as you are suggesting.You would propose using ZR_LO(2) as the ground for measurements? I guess that makes sense, and I'll see how that looks.Where would you measure the "input bias"?Thank you.

author: Zondar
author_rank: Regular Contributor
timestamp: August 05, 2025, 02:16:42 am
content: Why do I suddenly have a desperate desire for a scanner card in my DVM? It's you - you metro people - it's your fault, I say!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2025, 09:02:38 am
content: ZR_Lo (e.g. at the jumper for ZR_lo and ZR_lo2) would be a good ground for measurements at the ADC and reference voltage levels.For measurements more at the input there would be a different suitable grounding point - maybe at the 1 A range shunt.The question with input bias is if it is < 20 pA as it should be, or maybe in the 1 nA range, which could cause some smaller issues (e.g. with the 100 / 1000 V ranges). 1 µA could already be enough to upset ACAL and cause problems with the CAL72 value.Looking at the 10 V or 1 V or 100 mV range should not make a difference for the input bias.

author: iMo
author_rank: Super Contributor
timestamp: August 05, 2025, 09:32:49 am
content: Quote from: Zondar on August 05, 2025, 12:38:35 amIt is my understanding Kleinstein suggests to measure the input bias at the DMM input.You may charge a 10nF capacitor (wired to the input posts) to say 10V and then observe the speed of the discharge (like XX V/sec). The input current will be aprox, imhoIb = XX[V/sec]* 10e-9like 5mV/sec with 10nF is 50pA.

author: Zondar
author_rank: Regular Contributor
timestamp: August 05, 2025, 04:55:45 pm
content: Oh, that! It's 1.69 pA.Right now I am looking at ZR_HI-ZR_LO. With an early look it kind of looks like it may be drifting. I will log it for the next 24 hours to learn more.A question: If this makes sense, and if people know, what is the expected "gain" in drift from the 7V reference to the +/-12V ones. In other words, if the 7V reference drifts by 1ppm, how much of that might show up in the +/-12V references? The same?Thanks.Edit: I'm not sure if the 7V reference is exactly "drifting," but it's already apparent that it's not super stable.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 05, 2025, 09:54:21 pm
content: 1.7 pA of leakage is very good and healthy. It gets more tricky if the leakage is only in the switching part (Q16 to Q21) with the 7 V ref. Large leakage there could be visible in the 100 V range comparing the reading with an open input and a short. Because of the divider one is however not very sensitive. It would be only an effective resistance of 100 K. So 1 nA would result in 100 µV that would be just well detectable in the 100 V range.However this test would not check for leakage at Q16 (the turned on JFET are unlikely to leak).The 7 V ref. signal is relatively low impedance and 1 nA of leakage should not cause a very large error.The 12 V are scaled from the 7 V. So 1 ppm change at the 7 V would result in 1 ppm change at the 12 V.The CAL72 value is anyway only the change of the ADC gain from nominal. So a change in the 7 V reference would not effect CAL72. It is more the gain factor from 7 V to +-12 V (actually the difference from the +12 to -12) and the resistor ratio at the integrator.

author: Zondar
author_rank: Regular Contributor
timestamp: August 06, 2025, 02:59:46 pm
content: I logged the 7V reference voltage overnight (ZR_HI-ZR_LO). I can't figure out how to post an image of the data, though.The data showed an obvious bow of 6-7 uV P-P, apparently due to temperature changes, with the voltage rising as the temperature dropped. I don't have a thermometer, but the P-P temperature change dropped maybe 5C or a little more at night. I believe this is within the temperature coefficient spec ((0.5ppm*10V+0.01ppm*10V)*5C=27.5ppm). The linear trend-line was almost completely flat.Here's something interesting: The rate of drift in the digitized output has clearly fallen since I got the machine back. Brian's last measurement was 200 ppm per day. Now it has fallen to about 30 ppm per day. Only another two orders of magnitude to go, and everything will be fine!

author: Grandchuck
author_rank: Super Contributor
timestamp: August 06, 2025, 04:21:53 pm
content: Here's hoping.

author: bdunham7
author_rank: Super Contributor
timestamp: August 06, 2025, 05:31:20 pm
content: Quote from: Zondar on August 06, 2025, 02:59:46 pm7µV would be 1ppm, how can you determine whether that is attributable to the source or to the instruments measuring it?As far as the problem being "baked out", that's a reminder not to rush into things! If it got down to say 1ppm/hour, the meter would become quite useful even though it would still be considered quite broken.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 06, 2025, 06:22:08 pm
content: If the LTZ1000 reference is measured with the KS34465 meter chances are that the drift and noise are mainly from the 34465 meter and not the LTZ1000.Anyway the ADC gain drift as measured from CAL72 is a different thing than drift of the actual reference.A point to check could be the supplies - a problem with the supplies could also result in odd readings and things can drift there too, e.g. with mains voltage.Another easy point to check can be the noise with a shorted input - things like supply ripply would likely also cause additinal noise.

author: Zondar
author_rank: Regular Contributor
timestamp: August 06, 2025, 07:27:57 pm
content: Yes, I'm aware that my 34465A will contribute noise and drift, and likely more than the 3458A's, at least in terms of noise. In fact, merely probing something like the 12V reference changes the 3458A's digitized output by 20 or so counts. But that's the best I have and at least it sets an upper-limit.The supplies in the ADC section, at least, look normal. This morning I calculated the drift of CAL72. It was 36 ppm per day, so it's similar to the 30 ppm per day I calculated from the digitized value. I am not sure why the latter is lower than the former, but one day's data is really insufficient for accurate measurements.And yes, I know I need to be patient with burning it in. I'm just anxious to see things improve. If ppm/day drops again by tomorrow, I'll park it in a corner and wait a month. But I'll still be tempted to look every day.I'll ask a previous question again: On what basis is the U180 steering section thought to be jfet? Anything from HPAC? It wasn't mentioned in the HP journal articles. Why do I ask? Because it sure looks a lot like CMOS to me.Thanks.

author: Zondar
author_rank: Regular Contributor
timestamp: August 07, 2025, 10:01:52 pm
content: EDIT: I realized that I may have misunderstood something in the die photo. I'd like to understand it a little better. To avoid confusing people with material that I may have to correct, I deleted this post and the next.I'll comment later once I'm more certain about things.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 08, 2025, 08:42:56 am
content: If the switches are actually MOS or JFET would not make that much of a difference. If they would uses MOS it would likely be NMOS only switches.The logic part may well be CMOS. The CLIP schematics is a bit unclear around pin 53. If this pin is grounded, there would be no negative supply and n-JFETs could not turn off. However with a link to ground the diode CR155 makes no sense. R155 to the -18 V could still be some ground current compensation. A negative supply only via a 10 K resistor would still be odd. Considering that I tend to believe the switches are not n JFETs. P-channel fets are normally not that good, but could be easier to produce. After all the TL07x OP-amps also use p-channel JFETs.The resistor configuration is a bit different from the HP journal. There seem to be a few resistors / switches that are wired in parallel, maybe with options for separate testing. The main difference is that the fine slopes are not just higher resistors. To avoid impractical high resistors and more switches there is a kind of simple R2R type DAC around the 74HCT14 and the 5 V reference for a reduced reference voltage steps.Another part not covered in the HP journal is the zero glitch jump circuitry (U181,R184,R185). This part adds some +-200 mV to the input signal. Which setting is used is decided before the actual conversion and kept fixed for the AZ loop. This effectively avoids near zero conversions where the largest INL errors from dielectric absorbtion are expected.The first U180 revision may well have been more a first try before actual series production. Once the chip worked one would avoid touching it and doing any changes to the ADC if possible.

author: Zondar
author_rank: Regular Contributor
timestamp: August 08, 2025, 11:45:13 pm
content: I made an error, earlier, and deleted a reply to avoid confusion.Here is a correction: The output transistors consist of a single FET per switch, and the schematic and the die photo do substantially line up. This is good news.I'm still working on the die photo. Some areas are tough to read, but it's slowly coming together. I might comment again when I understand more of it.Sorry for any confusion.

author: martinr33
author_rank: Frequent Contributor
timestamp: August 09, 2025, 02:30:35 am
content: A few points on the ADC board. I have access to a stack of about 15 of them, mostly dead because of U180.1) Not all U180s drift. Agilent published a note suggesting that drift was limited to a batch of units in the early 2000s, but all units can drift. TiN spent a lot of effort trying to debug some, or create a replacement. No success.2) "Good" units have no significant drift. That's nice for long-term precision logging.3) Drift appears to be an accelerating problem, starting with minimal drift, then blowing out of the 24 hour spec, the going crazy, then resulting in no readings at all.4) One of the contributors succesfully remediated drift in a lower drift unit with a baking cycle. I have no details, and not mine to give. I do know that he melted some parts.5) Keysight now charges so much for a replacement ADC that it makes more sense to pay their fixup fee for the whole unit. Plus, they have pulled all parts pricing from the web site and you have to call in for anything.6) The unit does have a second ADC for very fast low-precision measurements. In a unit with a dead "main" ADC, you can do some limited testing by setting to NPLC 1, or maybe fewer digits (need to check the manual).7) Many eBay units are drifters. The sellers may know nothing of this problem, or may be well aware of it. Drift testing is important when you first get a unit.

author: aronake
author_rank: Frequent Contributor
timestamp: August 09, 2025, 04:09:47 am
content: Quote from: martinr33 on August 09, 2025, 02:30:35 amMaybe you could donate a really bad U180 to Marco Reps for him to do a super detailed picture of the die?

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 05:13:07 am
content: Quote from: iMo on August 01, 2025, 08:40:52 pmQuote from: martinr33 on August 09, 2025, 02:30:35 amBaking may help, indeed, at early stages when the drift starts (in order to get the moisture off the chip).But not with entire board in case of this meter. Remove the chip off the pcb and bake it for 2-4 days at 120-150C in dry air, for example.I would send a chip to Noopy..PS: is the lid on the U180 soldered or glued? I cannot see it properly on TiN's picture. The leaks might be with both types.

author: MiDi
author_rank: Frequent Contributor
timestamp: August 09, 2025, 07:14:10 am
content: U180 is fully RE, it consists of a decoder (digital inputs) and negative edge triggered D-FF followed by a break before make driver for each switch pair (AGND & SUM).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2025, 08:12:30 am
content: Quote from: martinr33 on August 09, 2025, 02:30:35 amThere is no 2nd ADC. The same basic part is used also for the faster conversions. It is only from some 100 µs integration down, that a different input path and lower resistor parts are additinally used. The faster conversions may be less sensitive to some defects and this way not show the same error message as slow integration.Even if there is no error from failed convergence, a drifty U180 may also cause an higher than normal DNL error. So it is not only the gain drift as a problem.

author: chilternview
author_rank: Frequent Contributor
timestamp: August 09, 2025, 09:42:16 am
content: Quote from: martinr33 on August 09, 2025, 02:30:35 amI don't know if Keysight replaced individual components on the 3458A in the past, but in the last few years they only replace whole boards as part of any 'Return-to-Keysight Repair'. At least, that's been my experience in the UK.

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 10:51:08 am
content: Quote from: martinr33 on August 09, 2025, 02:30:35 amHi @martinr33I agree on most points.Regarding 4, that was TheSteve, he sent the full A3 board thru the reflow oven multiply times, and it cured it to “acceptable” levels, not that I would recommend doing exactly that, at least isolators needed to be replaced if I remember correctly.Timing of your post is interesting as I have been having some late-night ramblings in the xDevs IRC chat about this matter. As far as I can tell and understand, it must be the resistor array, causing the troubles of drift. If one look at the picture of the dies, the passivation is brown and I assume it is Polyimide, that is somewhat hygroscopic, combined with the fact that U180 is sealed with epoxy only, moisture is a plausible explanation in my opinion.I have a planned experiment to see if U180 can be revived, current plan after some discussion in chat is:Remove lidStrip Polyamide with 90% DEA / 10% MEA or just with 100% DEA, it should be safe for the NiCr layer.Dry and bake and apply FC-742 passivation.This is not perfect as there is still possibility to get moisture in over time, but better than Polyimide. Another option would be to transfer the dies over to a new carrier that is hermetic. While fun, it is a lot of work.What matters for 3458a U180 is the ratio stability, what I don’t yet know is how to revive devices that has gotten into the fault of run down convergence, I don’t know if this is something we can fix in FW or if they are just gone. But at least I am going to try on drifty U180 to see if that can be stopped with the steps above.Baking can and will probably work, if patient, but problem is that you have two slow gas barriers, the passivation of the resistor network and then the epoxy seal, they are not gas proof, but it creates a low pass filter so it takes a lot of time to reverse the process and you are still left with the risk that it can reoccur since the passivation of the die is hydroscopic.Best way to dry out without stripping/taking the lid off would be to apply vacuum and then heat the IC, but it still takes a long, long time.TiN did a lot of experiments, but for external resistors, there are two points not bonded out to the carrier, that is also something that can be tested, by wire bonding out all the legs and add external network, but since ratio stability is so important, I fear it will be hard to make any conclusion from it.What is really interesting for me is that you mention you might have access to 15 boards with bad U180, what I am doing requires a bit of trial and error, I currently have 4 bad boards, but if you have any to spare, do let me know. I want to try to debunk the U180 drift once and for all, but not guarantee, this is experimental.There also is already die photos of U180 out there, but if anyone wants to send one, send it to Noopy, not Reps(He is to fresh in the game).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 09, 2025, 11:38:53 am
content: Chances are that the run down convergence error is due to a problem with the smaller slopes. These a realized with the odd R2R like DAC via the HCT14 chip and the 5 V reference. So quite a bit of this is externally acessible / replacible. If the error is not too large an adjustment of the external R154 could be an option.The convergence error is likely from a smaller reference that is too weak and thus needing more than the preset maximum of some 5 to 7 cycles.Too strong a minor reference may not cause an convergence error, but it could than be the next smaller one that is off.For just a test during bake out (with the chip removed from the PCB) the resistors at the pins 18,19,21,23 would be accessible for a measurement.

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 12:36:20 pm
content: Quote from: Kofen on August 09, 2025, 10:51:08 amchatgpt-5 told me thatQuote

author: tggzzz
author_rank: Super Contributor
timestamp: August 09, 2025, 12:42:22 pm
content: Quote from: iMo on August 09, 2025, 12:36:20 pmIf you ask the same question repeatedly, will the LLM repeatedly change itsanswerresponse?https://www.eevblog.com/forum/chatgptai/the-incredible-shrinking-resistor/msg5989271/#msg5989271

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 12:53:02 pm
content: ..re U180 it quoted mostly eevblog and xdevs.com, so I trust the first answer.. Everything else is just a math..

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 01:02:50 pm
content: Quote from: Kleinstein on August 09, 2025, 11:38:53 amMight be, it needs testing. I will run u180 resistor thru the thermal chamber, before and after. All of this needs a methodical testing to be certain. A bake out is not enough for most cases and is short sighted, it has been shown in the past. Goal here from my part is not short fix, but rather long-term fix. But as you rightfully say, it might need trimming on external resistors (or even internal).One last step IF resistor network is the problem, is to replace it, but that is a very, very, very very costly experiment.EDIT: Should be added that I can wirebond out the last legs of the resistor array on spare pads for testing.

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 01:56:53 pm
content: It seems that no one understands the drifting. I don't mean what iscausingit, I mean what IS it? A change in current? Voltage? Resistance? Something else?One speculative theory I have contemplated recently is increased off-current (leakage) through the steering switches.In a former life, I dealt with radiation damage to MOSFETs (in physics experiments). With it, charge gets deposited and trapped in the gate oxide layer. This causes the transistor thresholds to shift. This then causes transistors to become either more difficult to turn off or to turn on (depending on if it's N or P). It's anearly-permanent change, and eventually the transistors leak so much or get so hard to turn on, that the chip's performance is degraded, and beyond that it will stop functioning altogether.Now, I don't think radiation damage is the cause, but the same result can happen if transistors are over-stressed in other ways - and this chip must handle higher than normal voltages. If Vds (source to drain voltage) is too high, it accelerates electrons to too-high velocities (they are called "hot electrons"), which can enter the gate oxide, where they get trapped. Similarly, if Vgs (gate to source voltage) is too high, electrons will be pulled from the source into the gate oxide.The trapped charge can be partially removed by annealing. This involves - guess what - baking the chip! A gradual reduction in performance? Eventually failing all together? Able to be partly restored by annealing? A lot of this sounds familiar!So let's look at a pair of steering switches: Typically, the switching is from a reference to ground or to a sum node. If it's steered to the sum node, but the transistor to ground (which is nominally off) is leaking, it will pull or push on the sum node slightly, causing a small error. Interesting!Taking an N-channel MOSFET as an example, there is a "simple" way to increase the off resistance of a leaky transistor: You reduce Vgs, even if Vg has to go negative with respect to the source. I wonder if this is testable at all? I wonder if one of the supplies or grounds can be manipulated to increase the off resistance of associated steering transistors? Well, probably not without ruining the performance in some other way.How about looking at different setups, i.e., does one speed or gain setting, etc., result in more or less drift? One could look at which or how many transistors are off. For example, a leaky "off" transistor through a low-value resistor would show more net leakage than one through a high-value resistor. The "fast" mode uses lower value resistors, right? And I've heard a claim that faster digitization shows less drift. If true, then maybe this is another clue.What do people think? Is there an obvious flaw here?

author: Echo88
author_rank: Frequent Contributor
timestamp: August 09, 2025, 02:02:41 pm
content: Looking forward to the U180 experiments Kofen

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2025, 02:26:42 pm
content: Quote from: Zondar on August 09, 2025, 01:56:53 pmWell I certainly don't know, but given the experiment by TiN (xDevs) where he basically bypassed part of the resistor network with external resistors and had some success, I've assumed it was the values of the resistors changing. But lacking any real understanding of the specifics of the internal U180 operation I'm still at "I don't know".QuoteGiven the number of people that have suggested baking by various means may help, perhaps you should do a non-destructive in situ baking operation. Just put a heating pad under/around the meter, add a blanket, block the fan outlet a bit, etc to get the internal temperature up to 70-75C (measured via the menu) and let it run for a while.However, as to the mechanism you suggest, would trapped charge result in the continuous, large unidirectional drift that we see? I've seen similar instruments have temperature variance, various sorts of noise, reference shifts, null shifts, etc but I don't know that I've seen any instruments that just set off on a a 200ppm/day journey in one direction and just keep going. That would be bad even on a mid-grade handheld meter!QuoteYes, it was mentioned on xDevs that selecting a short aperture will reduce the displayed drift. IOW, if you let it drift 100ppm in the 8.5-digit mode, shortening the aperture to NPLC 0.001 will result in less displayed error. I did this with yours but I think I used NPLC 0.005 so that I could still read the display. I did see a large reduction in the displayed error. That's why I think I said at one time that perhaps the problem was in the minor slope driver area, but I think that was just a reflection of my lack of understanding.QuoteI can't speak for everyone, but for my part I'm happy to have you lead the way and hope to learn something in the process. How hot do you think you need to get the chip to release trapped charges in the oxide layer? And how hot can you get a hybrid chip like that without damaging it? My experience with HP and Tek hybrids of other kinds is that they don't do well in high temperatures and need to be carefully soldered (if they aren't socketed) as well as cooled properly in operation. Also, when they do fail than supercooling them often restores them temporarily, although that behavior is well known with discrete parts as well.

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 02:35:24 pm
content: I've been reverse engineering U180. I'm not quite done, and in particular I haven't finished this section, but here's a quick follow-up to my previous post:If you reduce digital Gnd2's (pin 53) voltage a little, maybe by 0.5V to start with, then the low logic state of the circuit controlling the steering switches should also be lowered by 0.5V. This should reduce Vgs and increase the off resistance of the steering switches.Everyone's drifty U180 gets fixed with this one little hack!Well, maybe not, but if you have a board that's drifting but still functional, it's worth further investigation.Edit: It looks like -18V is served from the "inguard" power supply. Jumper JM2 looks like a good access point. Or else R05 (3320 Ohms, it looks like) can be adjusted. If these parts are exposed without removing a board, I might try this.Another edit: The inguard supply is not exposed directly. However, its bottom-copper is. I need to locate R05 from the opposite side, maybe from a board photo.

author: coppercone2
author_rank: Super Contributor
timestamp: August 09, 2025, 02:38:26 pm
content: you can make a custom heater for the chip by raising it off the PCB. I wonder if you can. Lead extensions from wire, a 1 inch thick block of PCB material (insulator) then a heater and heat spreader + grease all under the chip.

author: aronake
author_rank: Frequent Contributor
timestamp: August 09, 2025, 03:19:57 pm
content: There seems to be two types of drift, one somewhat random, quite some general drift that can be slower ans quicker and even change direction and occasion jumps. I can understand this drift. But the other kind, somewhat stable drift maybe 0.5 to 2 ppm per day over quite some time. Thisone feels pretty odd. And most A3 boards have some small but stable drift over time.

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 03:43:04 pm
content: Zondar, if we flip it, how are you so sure it has to be the silicon die and not the resistor array? I dont disagree with your theory, but seems far more unlikely than resistor drift.A simple test I my self have done is to heat the U180 and by repeating that, it was stable for around 3 days, this aligns with others experiences as well. And in my book, rule out trapped charges.

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 04:41:23 pm
content: A very nice must read is also TiNshttps://xdevs.com/article/life_with_3458/

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 04:43:44 pm
content: Quote from: Kofen on August 09, 2025, 03:43:04 pmYes, it could just as easily be the resistor array. I'm just speculating on what in the switching part could be to blame (if it's to blame), and gate leakage is the best theory I have at the moment. It fits in most ways.That said, "likelihood" of this or that is mostly speculation. The idea of moisture getting in and contaminating the resistor array, for example, requires several improbable things to go wrong together, whereas gate leakage requires only one (improbable) thing to go wrong. Occam's Razor works more often than not.But this is the first time that I've heard that "baking" improves the situation for only a few days, and then things revert back. That's indeed confounding if so.Another mark against the gate leakage theory is my own 3458A. It has virtually never been used, so trapped charge from mechanisms like I discussed seems much less likely in mine vs. one that has been on for 20 years.A point about the steering transistors: They have very large W/L ratios. This is intended to reduce on-resistance, but it also increases the amount of gate leakage. There willalwaysbe some, and these transistors will have a lot (relatively) regardless.Also, depending on the resistor values, some switches have even larger W/L's, namely 4 times larger in many cases. These should have 4x the base leakage amount. I'm not sure how to integrate this into my pet theory yet.In the end, if some sort of test is possible, and one may be for the gate leakage theory, then it should be tried. The resistor hypothesis is probably harder to test, but it should be investigated too if you have an idea of how and have a U180 to test (in a likely destructive manner?).The reason I hope that it's gate leakage is that there may be something to do about it. Hope is good!

author: coppercone2
author_rank: Super Contributor
timestamp: August 09, 2025, 04:49:44 pm
content: I noticed something in general, mechanical seals are never unlikely to break. People always say that it won't leak but then it some how leaks. I just mean when containers are engineered. You are always guaranteed that they will never leak by most engineers (and they will scream that it wont' leak until they look like a cherry). Then it turns into a disaster. I noticed nothing brings as much confidence to engineers as the seals they develop. The big public ones are the titanic, space shuttle thrusters and that carbon fiber submarine, but of course I mean small engineered devices, pipe seals, valves, etc. When someone tells me that it won't leak I just think "ok right". Its like the same as saying it wont scratch, that it won't rust, that the paint won't come off or that the glue will never fail to bond

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 04:57:03 pm
content: Oh, wow, I just realized something:My unit's serial number is within the range called out by HP as being subject to excessive drift!So they will take it back and rectify it for free, right?

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2025, 05:03:24 pm
content: Quote from: Zondar on August 09, 2025, 04:57:03 pmI would ask just to see what they say. You never know!

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 05:03:43 pm
content: I dont quite agree here Zondar, while I see your point, I find it far more likely to be the NiCr array that we know are very moisture senitive. The silicon die I assume has been passivated with SiO2 so it should be quite stable, the same cannot be said about the resistor array that has (in my opinon) only Polyimide passivization. I can't see how a trapped charge can be kept and keep making drift over long time? It should be impossble. I have U180 to test and resistor array is easy to test in thermal chamber, we will see how it goes.coppercone2 Keep in mind this is not an hermetic sealed device, it is not gas proff so break or no break, humidity will diffuse into the ASIC.

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 05:08:12 pm
content: Quote from: Kofen on August 09, 2025, 05:03:43 pmI agree it's a definite possibility. I'll be very interested in your results if you manage to test the theory.

author: Kofen
author_rank: Regular Contributor
timestamp: August 09, 2025, 05:10:09 pm
content: I will report my findings, but it is a few months out, I have other projects I need to finish first.

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 05:12:48 pm
content: Quote from: bdunham7 on August 09, 2025, 05:03:24 pmI will, actually. But I don't think so. Even HP's note implied that it was on the owner, not them. And of course it's something like 20 years later, too.I'm on their site right now. A per-incident repair of this unit costs $6,079. I presume that would include recalibration, but if not, that's an extra $1,154 per incident.I submitted a support request. I'll provide their answer here when I get one.

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2025, 05:22:40 pm
content: Quote from: Zondar on August 09, 2025, 05:12:48 pmYikes! It wasn't that long ago that they would sell refurbished A3 boards for about $1300.

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 05:46:30 pm
content: As per Imsai guy's latest video on his new 3458 repair (w/ error 202, part 7) he mentioned he perhaps will donate his meter, incl. 5 genuine EL2018s..PS:Quote from: bdunham7 on August 09, 2025, 05:22:40 pmFrom my experience working for large corps. those might easily be "list prices" only, where the registered active customers typically pay much less. There was a discussion in past on how KS wants to deal with "registered companies" only..

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 07:21:44 pm
content: Quote from: iMo on August 09, 2025, 05:46:30 pmI can hope. I did beg for some sort of accommodation, such as just getting an A3 board to install myself. I expect the answer will be no.I was registered on their website 20 or more years ago and still have access. In fact, the site immediately found the serial number of my unit, and they even still have a record of selling it to me. (If anyone needs a document or something that they are denied access to, I can probably get it.)

author: iMo
author_rank: Super Contributor
timestamp: August 09, 2025, 07:26:26 pm
content: So you are registered.. Now, are you "active"??With $XXXXk purchases per year you get all that services free of charge, perhaps..

author: bdunham7
author_rank: Super Contributor
timestamp: August 09, 2025, 08:39:11 pm
content: Quote from: chilternview on August 09, 2025, 09:42:16 amTheir 1989 Component Level Information Package which they provided (apparently not completely willingly) to certain customers clearly states that their repair strategy is to replace or exchange boards and that they did not actively support component level repairs. So I think whole-board swaps have been their method all along, although obviously there has to be a department tasked with refurbishing the boards themselves and that would necessarily mean component level replacements. They probably have some fancy one-off test jigs and one or two guys working in a room in a basement somewhere.

author: Zondar
author_rank: Regular Contributor
timestamp: August 09, 2025, 10:44:51 pm
content: The test I was proposing, lowering digital Gnd2, isn't straight-forward. Lowering that node runs into the CR155 diode, which resists lowering the voltage.Despite that, I did try lowering it by about 100 mV, with no readily-noticeable change. I wouldn't have expected that to make a big difference, but I haven't felt like lowering it any more, and I certainly haven't felt like modifying the board, e.g. by disconnecting CR155.Honestly, I'm a little confused about the many supplies and grounds, and where they all are with respect to one another. I'll have to do some more studying and probing to figure that out.Meanwhile, I think the idea was a good one worth trying, but alas, probably too good to be true.

author: MiDi
author_rank: Frequent Contributor
timestamp: August 10, 2025, 02:45:47 pm
content: Quote from: Zondar on August 09, 2025, 02:35:24 pmhttps://x.com/kenshirriff/status/1448398125828239364https://x.com/VPitushivdd2 & gnd2 is the supply for the FET drivers, with two exceptions: rusn1 & rusn2 are supplied by mod_in1 & 2 (input FET modulation).gnd2 is at -0.6V, does it make a significant difference driving the gate even lower?Decoder logic:EDIT 2025-09-12: corrected ladsn (was inverted)

author: Zondar
author_rank: Regular Contributor
timestamp: August 10, 2025, 04:12:28 pm
content: The idea was to reduce the low logic level voltage of the steering-switch drivers, set by Gnd2, decreasing Vgs of the steering switches when they are off, to hopefully increase off-resistance. While I didn't exactly disprove the theory, I couldn't see evidence that a very small reduction (100 mV) was helping, and I couldn't push that much further without disconnecting diode CR155, which clamps Gnd2 to about -0.6V. (Also, I can't see posts on X unless they are fully public.)I've given up hope that my machine will ever get drift sufficiently under control, but it's a pity to just put it in storage or sell it to some othernewbie suckerfully-informed buyer.One idea to keep it on my bench would be to make a pin-compatible functional analog to U180 to drop into its place, and I've begun some work towards that. (I'm aware that there are a few others out there with this interest too.) Would it restore my machine back to 8.5 drift-free digits? In a word, no! But it sounds like a fun and interesting project.

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2025, 06:13:09 pm
content: Quote from: Zondar on August 10, 2025, 04:12:28 pmHas the drift resumed or accelerated or is it still tapering?Let us know if you decide to leave it by the curb for recycling.

author: Grandchuck
author_rank: Super Contributor
timestamp: August 10, 2025, 06:46:28 pm
content: You have had it for a long time. Set it aside and wait. Something might turn up?

author: bdunham7
author_rank: Super Contributor
timestamp: August 10, 2025, 06:51:57 pm
content: Quote from: Grandchuck on August 10, 2025, 06:46:28 pmI have a storage locker dedicated to that proposition.

author: Zondar
author_rank: Regular Contributor
timestamp: August 10, 2025, 08:18:54 pm
content: Too late. The trash guy spotted me hauling it to the curb, and was like "Hey man, you tossin' that? I'll take it!"CAL72 has been stuck at around 30 ppm per day for the last few days, so there's alongway to go before it's in spec. I'll still set it aside and let it bake for a month. Maybe I'll look for a small heating pad for U180, as someone suggested.Meanwhile, except for being sure I understand all voltages and signal levels, I'm pretty much done with RE'ing U180. I'm starting to look for suitable parts for a work-alike. First thing I found? It's surprisingly difficult to find pin grid array sockets these days, at least from U.S. suppliers.

author: Zondar
author_rank: Regular Contributor
timestamp: August 14, 2025, 03:58:30 am
content: I wanted to see what a U180 clone might look like, so for the sake of discussion, I threw together some parts! Please note that the schematics are incomplete and incorrect.Most resistors can be specified as 0.01% and 2 ppm, costing between $9-$19 each.The decoding logic is expected to be on a CPLD or FPGA. Here it's constrained by the need for 5V logic levels, with an ATF1502AS being one of the few options. Unfortunately, these old 5V ones are rather large, forcing an extended or double-sided board.The main circuit is a 4 channel SPDT switch, a TI TMUX6234 (there are a few other options). Some features:* Independent control on each of 4 channels.* Very wide analog signal range: rail to rail +/-18V.* Compatible with 1.8+V control signals.* Break-before-make switching.Some important values at 25C:* 3.6 Ohms on-resistance.* 0.2 Ohms miss-match between on-resistance values.* 1.6 Ohms on-resistance flatness.* 0.15 Ohms / degree C temperature drift.* 0.02-0.04 nA off leakage.* 1 ns break-before-make time.* 3 pC charge injection.* 90 ns switching time.Many of these specs will not be good enough to actually replace a U180, but it's fun to imagine that it could.I also wanted to see if it would fit within the constraints of the U180 pin-grid-array package. I haven't routed it, but it looks like it might fit, at least if the board is double-sided.It's all fun and games until you desolder your precious U180![ Specified attachment is not available ]

author: Zondar
author_rank: Regular Contributor
timestamp: August 14, 2025, 04:02:27 am
content: Trying to post the schematic again:U180.pdf(296.65 kB - downloaded 70 times.)

author: bdunham7
author_rank: Super Contributor
timestamp: August 14, 2025, 04:37:01 am
content: Quote from: Zondar on August 14, 2025, 03:58:30 amI wouldn't rule it out because of two things. First, IDK how HP would have produced something that reliably had extraordinary specs without laser trimming (unless perhaps they did use laser trimming...) and I suspect that a large part of the stability and precision is due to everything being at the same temperature. Second, even if they had remarkable process control, if you replicate it with modern small SMD parts you have an additional weapon at your disposal--selected parts. If you make a version that works at all, then you can figure out which values and ratios are the most critical and how much they matter. Many things will be calibrated out, I've no idea which will be critical for linearity.There was a guy that replicated the commonly faulty U800 horizontal output hybrid on Tek 2400-series oscilloscopes. It apparently wasn't completely perfect but it worked as a replacement for an unobtanium hybrid. One issue for him was that the Tek U800 is actually not complete unobtanium (I have a spare myself) and the Tek scopes it goes in are not all that valuable. You are a lot less likely to luck into a good HP U180 at a flea market. I'll keep an eye out anyway.

author: iMo
author_rank: Super Contributor
timestamp: August 14, 2025, 05:45:15 am
content: Quote from: Zondar on August 14, 2025, 03:58:30 amFirst step is to have the complete schematics. Then set the expectation, like "my replacement U180 will be only 5x worse than the original" (talking here the TC and parasitics in the switches). Now, how it will demonstrate while operating the meter? Will the software accept that? Will the selftest somehow pass? Etc., Etc.I’ve often talked with friends about things like, “How did this guy come up with that idea or invention that made him rich?” And the answer we usually arrive at is, “Simply because he was the first to try it.”

author: chilternview
author_rank: Frequent Contributor
timestamp: August 14, 2025, 07:27:32 am
content: Quote from: bdunham7 on August 14, 2025, 04:37:01 amI thought it was the matching between resistors that was important, rather than their absolute values. And matching of a single chip implementation is going to be good, way better than a discrete one. Still as you say with small SMD parts, selected for value matching, it may be just possible.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 14, 2025, 11:26:49 am
content: Value matching in many places should not be that critical. The critical point for the resistor is the relative TC and for the input to the integrator also linearity (low voltage / power effect). Here resistor arrays can be a rather good option. The specs are often quite conservative as they are for the whole resistor range.For the switches the jitter can be relevant. The 3458 switches them quite often and thus can see quite some noise from jitter. E.g. the jitter from 74HC4053 would be well visible. 74LV4053 would likely be jitter wise likely just OK. Jitter specs are hard to find. Often high speed comes with low jitter - so one may want relatively fast switching switches.The switches direct a current to ground or a virtual ground. So no higher voltage rating needed. So even just 3 or 5 V switches would be acceptable.Lower voltage switches tend to be faster and lower capacitance. A high capacitance (e.g. > 20 pF) would also add to the noise.A tricky part can be the switch for the input path: here one would need either relatively low resistance (like <= 5-10 ohms) or have the same signal dependent gate drive only for this switch. This is to counter the current dependent on_resistance.With lower resistance for the switches matiching to the resistor ratios would be less relevant. The problem is than however the capacitance.For the flip-flops for the main ref. paths jitter would also be critical. An FPGA / CPLD may have extra delay modulation from cross talk / ground bounce. So one may have to consider separate Flip flops for this part.To simplify matching of switches to the resistors one may even consider changing the resistor values and ref. voltage. So maybe +-14 V reference and 50 K instead of 40 K for the main ref. paths.

author: Zondar
author_rank: Regular Contributor
timestamp: August 14, 2025, 04:13:40 pm
content: Quote from: iMo on August 14, 2025, 05:45:15 amThis is a real concern. The 3458A's operation is very particular and finicky, and it will throw error codes, even to the point of stopping operation, if it thinks anything significant is off. This is good if you are very concerned that your measurements are accurate, but not so good if you are experimenting like this.Concerning some of Kleinstein's comments:The TC of the switch is 0.15 Ohms per degree C. So with say a 5 degree temperature drift, you would get about 0.75 Ohms drift. The resistor's are specified at 2ppm, which if my math is right comes to 0.2 Ohms for a 5 degree shift on a 20k resistor. So the switch is about 4x worse in that respect for that resistor.The on-resistance of this switch is about 3.6 Ohms (so the same or less than the numbers you mentioned). My guess is that U180's switches are in that ballpark too. Depending on the voltage range, it can dip lower than that in the middle of its range by about 1.6 Ohms. So if it's connected to say a 20k resistor, there would be roughly 0.008% variability in the net resistance depending on the voltage. U180 is likely around this too, but probably worse since it's a single FET vs. a complimentary switch - it would go up very substantially as you approached the switch's Vdd. If necessary, a similar resizing of the switches as in U180 (changing on-resistance depending on resistor values) is possible, simply by doubling up switches, etc., but that comes with other tradeoffs that I would not recommend.I chose a wide voltage range switch because the switch's operating voltage range in U180 is shifted downward by ~0.6V (I assume so signal ground is above the switch's lower rail). One that does not have separate supplies for the control and switch range, or isn't tolerant of voltage deviations, may not do. But there are other options out there that might be a bit better (most are worse, including the ones you mentioned).I can't say too much about jitter because there aren't any specs on that either for U180 or for the switch. However, for typical circuits including FPGAs, we're probably talking about less than 1ns of jitter. The clock that A3's FPGA uses operates with a 20 MHz clock, so a 50 ns clock period would be about the shortest time of interest for the sake of sheer functionality. I'm not sure how to evaluate this without more info. Likely it would just have to be tried.In the end, something like this would probably function OK (if the machine didn't choke), but it would be somewhat down on performance in various ways. What's very unclear, and possibly unknowable, is how the net performance would suffer under all the various conditions and ranges, etc. Also, would it just continuously drift too, or would it help finally reveal what the famous drift problem is due to?I'd be very willing to actually make and try this, but I don't think I'd want to sacrifice my own A3 board. I wonder if there is a known-good board with a dead U180 out there that could be sourced?If people think this is actually practical and worthy of trying (and if a spare U180 or a knowledgeable and experienced volunteer with one is available), then I'd propose to start a new project thread on it.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 14, 2025, 05:21:05 pm
content: The way the switches are used they see a voltage essentially at ground level. The break befor make time is usually so short that in this time the capacitance is enough to prevent to voltage from going up more than a few 10 mV or so.For this reason the relevant R_on is only at the GND level and little need for the 2nd FET. With a CMOS switch power from 0 and 5 V linke the logic, only the N-mos part would turn on anyway and the p-mos part would only act as charge compensation (with many types too much as the p-mos part tends to be larger to get a somewhat constant resistance).For the jitter 1 ns of jitter would be horrible. In my ADC I can see the differene between 74HC4053 and 74LV4053 with jitter that is likely in the 2 ps vs 1 ps range. This assuming the LV series about as fast as the AC series. The 3458 ADC would be even more sensitive to jitter as the modulation frequency is higher by about a factor of 3.Even worse than jitter can be delay modulation (e.g. the FPGA delay modulation by some output states or ground bounce). This would be result in an INL error. It still depends on the details - I got away with the timing in the AVR µC and only with specific setting just saw some effect (e.g. Uart and ADC clock effecting the result). It's only the larger reference switches (like 10 K and 40 K to +-12 V) that are so critical with jitter. Probably worth an 74AC175 to be on the safe side.The 3458 uses FETs of different size, likely to get a constant ratio of resistor to switch ratio. This way the relative TC stays constant even with relatively high resistance switches. My version uses 20-25 Ohms switches with 50 K resistors (ORN array) and I get a low TC for the overall ADC gain in the 0.5 ppm/K range, which is not bad.I would expect the switch resitstance in the 3458 more in the 50 ohms range. It still depends what type of switches this are. The main candidates are n-MOS or p-channel JFETs as there is no negative supply to turn off a n-channel JFETs. For very low resistance switches there would be little need for the extra substrate correction with the input signal.I still see 2 options to go.a) use rather low resistance switches, like in the 2-5 ohms range and not or little care for matching. (could use higher R with higher resistors for the fine part). One might skimp on the fast mode path (10 K input resitor) as this mode is anyway not super accurate.b) uses switches with suitable scaled resistance (e.g. different for the 40 K ref. and 50 K input). The resitance could be higher, like 10-30 ohms. the other point to adjust the gate voltage for the input switch (e.g. add ~1/2 of the voltage drop at the switch - that is what should be the purpose at line 28-30 of U180). This would mean a separte switch chip for the input path(s). One may be able to use a reduced supply (like 4.5 V) to get the 25% higher R_on to make up for the 40 K / 50 K ratio.One might skimp a bit on the fast mode path (10 K input resitor) as this mode is anyway not super accurate also with the 3458.

author: Zondar
author_rank: Regular Contributor
timestamp: August 14, 2025, 09:20:39 pm
content: Kleinstein, have you attempted something like this yourself? I did see photos of some external resistors being hacked into a U180. Was that you? Or have you designed a whole ADC?One ps is the delay through a coax that's a fraction of a mm long - far less than the size of a U180, so it's not much! But sure, even tiny flaws can be made visible if you look hard enough.Actually controlling signals down to 1 ps is an engineering feat in itself. Nothing in in the DVM actively does this that I'm aware of. Instead, the circuit relies on the jitter being low enough just due to the parts that are chosen. (I doubt that the parts I've chosen are either much better or much worse.)The switch I specified has ~3.6 Ohms on-resistance. I mentioned that one could parallelize multiple switches for scaling if desired.Obviously, there are many things that can and even would go wrong.

author: coppercone2
author_rank: Super Contributor
timestamp: August 14, 2025, 10:13:50 pm
content: Is it possible to put the u180 in a test circuit that could tell something about the faults via thermal stimulus. So you have a known state with a known spec to check whatever conceivable specs there might be (even if they are not relevant to the function of the DMM). I mean anything you can measure. Say stuff like power consumption. There might be stuff you can measure that is not directly related to the chips function in the DMM using test equipment.Finding a irregularity here might determine what the fault is.I thought just putting a small strip heater on top of it with a thermocouple.And another idea might be a 'grid heater' to see if you can determine if heating a particular area on the chip causes a disproportionate response.Even without a heater, a test circuit might still let you do something that you can't normally do. Like a v sweep..,. and current measurements on every pin, and load impedances.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 14, 2025, 10:16:13 pm
content: A fixed delay or skew ( difference between different switches) is not a problem. The point is jitter as random noise in the switching time that contributes to the noise of the ADC. The point is getting the PWM ratio for the ref. switching repeatable. This are some 3 µs per step and thus some 0.3 ppm from 1 ps chance in the switching time. It is not about the absolute accuracy, but the stability. Low jitter helps, but the 74AC logic and a normal crystal clock (not a mems one or especially bad oscillator) is largely good enough. Jitter is one of the easier points.The other issue is a modulation of the delay (e.g. via the supply voltage to the switches or with ground bounce), e.g. from previous run-up steps. This can lead to INL errors.I have not done experiments with an 3458 ADC - just read most available information. I have build my own ADC low noise variant, that reaches a slightly lower noise level. It uses 74LV4053 (started with HC4053 like the 34401) switches and 50 K resistors from a resistor network (ORN or NOMCA). So my experiance is with slight different hardware. I have done quite some calculations to the noise of MS- ADCs.

author: Zondar
author_rank: Regular Contributor
timestamp: August 14, 2025, 10:42:10 pm
content: Quote from: coppercone2 on August 14, 2025, 10:13:50 pmIt's soldered in, and I'm not inclined to remove it, at least yet.If I can gain access to the bottom of the board while the machine is running, I can at least probe all of U180's pins. Or else some of those pins should be available by tracing paths to nearby components and probing those. I was intending to track down as many of those as possible next.Kleinstein: I did some calculations and came to the same conclusion: Jitter is at least close to counting. But as jitter is not a common specification for these kinds of parts, I don't see that there's much I can do about it except hope it's good enough.As to my 3458A: It has regressed, and even started throwing codes due to how off it is. (Like many have found, faster conversions still work fine.) So frustrating!

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2025, 05:10:37 am
content: FYI - here is a picture from imsai guy's series on his repair (part 8 ) - an fpga chip replacement and an EL2018 replacement boards.. So the U180 will be the third to replace there..PS: the fpga used looks like a 3.3V one with 9 pieces of 3.3V<->5V logic drivers/voltage level converters..

author: aronake
author_rank: Frequent Contributor
timestamp: August 15, 2025, 06:01:32 am
content: Quote from: Zondar on August 14, 2025, 10:42:10 pmWith a decent desolder gun, its actually pretty easy to remove from a board and solder back. I have have done some U180 swapping between some boards for testing. And the 3458a has been working after solder back. The pins on the U180, despite quite many of them are solder and desolder friendly.

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2025, 06:07:59 am
content: The first thing for the experimentalists I would recommend - to find a suitable sockets - several males (for the replacement boards) and a female for the A3..

author: wanghar
author_rank: Contributor
timestamp: August 15, 2025, 09:38:05 am
content: I see someone using such a socket on the A3 board, likely for the purpose of testing U180. This can be referenced.

author: MiDi
author_rank: Frequent Contributor
timestamp: August 15, 2025, 10:27:38 am
content: Quote from: Zondar on August 14, 2025, 04:02:27 amHad a quick look at the schematics:PIN10/14 are PIN15/16, PIN10/14 are at -12ref1/2RU resistors are 80k instead of 60kThe 50k (DC_AD to rusn1) could be just connected from Pin 6 to 11 or 13, e.g. 2x100k in parallelAttached corrected schematics & U180 switch connections (die shot fromhttps://xdevs.com/pow/u180_stk/)

author: MiDi
author_rank: Frequent Contributor
timestamp: August 15, 2025, 11:29:27 am
content: U180 switch resistance:rusn1 ~14Ωrusn2 ~3.5Ωrusn3 ~9Ωladsn ~12Ωzero ~30Ωall measured to sum

author: iMo
author_rank: Super Contributor
timestamp: August 15, 2025, 11:34:46 am
content: Quote from: Zondar on August 14, 2025, 03:58:30 amI think the xc9572xl or xc9536xl would work fine (5V input tolerant and small package - 44pins flatpack, easy to solder)..PS: and ISE14.7 is free..

author: coppercone2
author_rank: Super Contributor
timestamp: August 15, 2025, 08:56:13 pm
content: not sure if its also possible to measure the switches switching time?Also what are those jumpers with the plastic bit on them?>Is it this?https://www.digikey.com/en/products/detail/harwin-inc/M7681-46/3922855i think yours look a bit more robust then thatI am thinking to buy this onehttps://www.digikey.com/en/products/detail/samtec-inc/SNT-100-BK-H-H/2664749

author: Kleinstein
author_rank: Super Contributor
timestamp: August 15, 2025, 09:25:40 pm
content: Quote from: MiDi on August 15, 2025, 11:29:27 amAre the values for rusn2 and rusn3 swapped. Rusn2 should be relatively low priority, while the ratio from the DC input (rusn1) to the AC input (rusn3) should ieally be around 5:1. Chances are it would not be super critical and a different ratio would only effect the fast mode to a small amount. There is anyway no extra drive of the gate supply with the input in the AC mode.

author: razvan784
author_rank: Regular Contributor
timestamp: August 15, 2025, 10:18:00 pm
content: Hi. I've been intrigued by the "input FET modulation" portion, on which there is little information online or in the CLIP. Kleinstein has just touched upon ithere. I also think it's an attempt to compensate the remaining square-term error of the FET switches by adjusting their gate drive voltage with the signal level. See the attached simulation which I did some time ago -- the on-resistance is apparently much lower in reality.Notations:ref: reference signal, ideal resistor instead of MOS r_dsonm: MOSFET drain, uncompensatedmc: MOSFET drain, compensatedThe error seems significant at 8 digits even with the real r_dson; to me it seems that a U180 replacement must include this supply adjustment mechanism for the input switches, while keeping the slope switches supply constant; the challenge is doing this while also maintaining the other matching characteristics that a monolithic implementation provides.

author: MiDi
author_rank: Frequent Contributor
timestamp: August 16, 2025, 07:25:34 am
content: Quote from: coppercone2 on August 15, 2025, 08:56:13 pmSure, it on the list for characterization of U180.The jumpers are from local supplier:https://www.reichelt.de/de/de/shop/produkt/kurzschlussbruecke_rot_mit_grifflasche-9018Quote from: Kleinstein on August 15, 2025, 09:25:40 pmThe measurements are from a dead U180, so possible that they are off, but not swapped.The results do not only include the bare Rdson, but the connections too.rusn3 e.g. has ~9Ω to sum, but ~56Ω to agnd.A look on the die shows it has quite a long and thin wire connection to agnd and a much shorter to sum.Comparing the switches, rusn3 should have lower Rdson than rusn2, as it has more fingers (MF-MOSFET).

author: Kleinstein
author_rank: Super Contributor
timestamp: August 16, 2025, 09:01:40 am
content: In the logic description it says that the path to the ladder resistors (SW3) us usually connected. This would make sense for the run-down, but not for the run-up phase. In the rundown the 50 K impedance from the ladder would compensate the input path that is no longer connected. So here it makes some sense to have the small ref. signals from the 50 K and lower voltage. This was especially an issue with a simper, 1 amplifier integrator. It is no longer than important with the 3 OP-amp version, but it does not hurt. When also active in the run_up part it would add quite some noise (e.g. increase the noise gain for the integrator from 2.25 to 3.25 and add the johnson noise of 50 K) - with this it would have a hard time to reach the observed noise levels.For a direct 1:1 copy of the U180 one has a difficulty in getting switches with the 4 to 5 (or actually 8 to 5) resistance ratio to get a constant switch to resistor ratio.With the adjustment of the supply voltage to get a more linear resistance at the input one would already need separate switch chips for the input and the references. So ideally one would want switches with on resistance of something like1 x 5 ohms for the input (50 K resistor), separate supply with linearity correction4 x 8 ohms for the ref signals (80 K each)1 x 5 ohms for the ladder part - may not be all that critical1 x 50 ohm for the reset , here no need for matching but good isolation, maybe even 2 stage swiching to be on the safe side1 x 1 ohm for the AC mode input - not that critical as the resistor is off chip anyway and thus not that well matched4 x 2 ohm for the 20 K ref. resistors for the AC mode1 x 1 ohm for the 10 K to ground - should be not that criticalThe fast / AC part is less accurate anyway with the input resistor external. So one may well get away with also higher resistance switches for the AC path.As U180 is current sterring, it does not really matter if the reference currents are from 12 V and 80 K or 15 V and 100 K. The gain to get from 7 V to 12 V is also part of U180. As far as I have seen the +-12 V refrence levels are not used much for other purposes. One is the ground current compensation and one use is in generating the ~4.5 to 5 V supply for the input switch. This last point would need a change in a resitor on the A3 boad if a +-15 V ref. would be used. The supplies to the OP-amps to generate the +-15 V should still be high enough with a 18 V raw supply and not too much series resistance (could reduce tham if really needed). The advantage of using a +-15 V ref. instead of +-12 V would be the option to used off the shelf resistor arrays (100 K or 50 K). Another point would than be easier matching in the switch resistance with 2 switches in parallel for the input.Resistor arrays would only need TC matching in less need for a super low absolute TC. 2 NOMCT 50 K arrays should be enough for the really critical part: 4 x for the input and 4 x 2 for the references.

author: Zondar
author_rank: Regular Contributor
timestamp: August 19, 2025, 10:46:53 pm
content: A few things:First, thank you MiDi for the corrections.For a first prototype where functionality is a bigger question than performance, I'd say using single low-R switches is good enough. However, if it's essential, then switch resistance should be addressed now. I'd defer to more knowledgeable and experienced people on these choices.I need to fully understand what voltages should be expected at the switches, and also which if any switches or resistors are less critical. I've presumed that the 5 resistors associated with the "minor slope drivers" are less critical simply from observing that they are driven by standard 74xxx parts - true? Any others?A smaller CPLD or FPGA package would be great! However, the steering die includes two digitaloutputs, which presumably should be 5V, and modern ones are almost all 3.3V max or lower. I'd have to check what those outputs go to (the on-board FPGA would be my guess) and what voltage levels it's compatible with.To stay within the outline of U180, it's necessary for the board to be double-sided. I'd prefer that the "top" be everything other than the FPGA so all the signals can be easily probed. If an open-frame socket like in the photo above is found (it's surprisingly difficult!), then hopefully there's enough room for a thick old clunkly CPLD to fit within the "open" portion of the socket.There's also the possibility of hanging portions of the board outside the boundary of U180, but there are a lot of obstacles in that area.So what shall I/we do? I can update the schematics and start working on the layout, but of course choices for switches, etc., should be finalized before that.Perhaps I should start a different thread solely on this project?

author: iMo
author_rank: Super Contributor
timestamp: August 20, 2025, 04:51:50 am
content: For first prototypes you would need to have a pretty good access to every component there (and a lot of test points where you can connect 4 scope probes and other meters easily and firmly). So a larger board will be necessary, imho (like 100x100mm for example, or even bigger). The connection to A3 female socket via long pin headers thus the pcb will hover over all the obstacles. Perhaps some mechanical mount should be 3d printed for the prototype board(s) as well (the pinheader connection will be mechanically weak).And feel free to start a new thread when you feel yourself dedicated to this project - I am pretty confident it will be watched by many..

author: aronake
author_rank: Frequent Contributor
timestamp: August 20, 2025, 04:58:29 am
content: Quote from: Zondar on August 19, 2025, 10:46:53 pmDifferent thread sounds like a good way forward. What has been written here so far seems to be one of the more serious attempt in really figuring out the U180, so this would likely be a very well read and commented thread.Excellent work, input and comments from you and other here so far!!!

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2025, 09:38:44 am
content: The switches are all current steering and would only see a low voltage, like < 20 mV most of the time with maybe spikes during switching to some 100 mV. The same configuration is used in the HP34401 and Fluke 8845 with HC4053 switches powered from 0 and 5 V.For the switch resistance I don't think one would need that low values. If there is good matching even replatively large resistance should be acceptable. The numbers I gave in my last post are already at a level where matching is not longer critical (and maybe 40 K could work). With good matching even 10 x the values should be acceptable. My first ADC tests used HC4053 switches (~ 75 ohm Ron with 50 K resistors) and this still worked OK. The 34401 uses HC4053 with 100 K for the input and 30 K for +-10 V references.The reistors for the 5 V reference and the ladder part (37.5 K and 16.7 K) are less critical. The values are a bit odd. If I got the math right, the 16.7 K should be more like 16.6 K ( 16.6666 K minus the R_out of the HC14 which is some 60 ohms).Also the 20 K resistors and related switches for the fast mode should be less critical, as the 10 K resistor for the input is external anyway.A separate thread would make sense.

author: Zondar
author_rank: Regular Contributor
timestamp: August 20, 2025, 03:50:54 pm
content: If this turns into a real project, then it needs a name, right? How about "NU180"?The steering switch control supply is biased a little lower than normal, namely about -0.6V to 4.4V. This would be to insure that ground level is not right against the lower rail. Therefore, switches with separate Vdd and Vss supplies plus GND are preferred.I also consider SPDT to be mandatory since it's convenient, saves space, reduces some stray capacitance, plus they are widely available anyway. Direct, individual control over each switch pair is also mandatory. The more switches on a (tiny) chip the better, but 4 seems to be the limit for us.There's normally a trade-off between on-resistance and channel leakage current, charge injection and capacitance, with higher resistance doing better on the other parameters. However, I haven't found a great difference amongst the likely options. I'm still reviewing what's out there, but maybe Analog Devices ADG788 looks slightly better than the TI product I mentioned earlier.I'd want to keep stray capacitance to a minimum, so I'd be loath to raise it up on stilts or for it to spread over a large area. I'm hoping that all resistors and switches can closely fit U180's profile, maybe spilling over the edge slightly for the sake of a few resistors. The CPLD/FPGA could easily be farther off to one side, though.Having test points available would indeed be good, since there would be virtually nothing to attach to otherwise (though more capacitance again). Maybe a first prototype should lean towards ease of testing over performance.

author: Kofen
author_rank: Regular Contributor
timestamp: August 20, 2025, 04:16:19 pm
content: Looking foward to a seperate thread to discuss this further, but I think this need to be broken down into to sub tasks and even just make a prototype that is larger.Who knows maybe a new A3 board is in order, but beside, you can go a lot more outside the original footprint, there is quite the room in the 3458a.I think the main thing is to set the goal for what do we want to achive? For me, it would be to tackle it as a 8.5 digit ADC with, if not fully compliant, fast speed. Take that as a sperate project, then migrate.If the goal is just to "get it working" with 6.5 digit, I dont see the point.And starting a new project for a 8.5 MS ADC or other topology would be great, then rest of DMM follows quickly. I look forward to it and ready to contribute, but it should be another thread.So design spec, what are the goal? Design note, then design, in my opinon.Putting early constrains on size is asking for trouble,

author: Zondar
author_rank: Regular Contributor
timestamp: August 20, 2025, 05:49:39 pm
content: I agree that design goals would be good, but I think there’s more than one, and some would need to be staged.One goal is to contribute to the understanding of the infamous U180 drift problem. I still haven't seen more than speculation about it. Unfortunately, even if we knew, a replacement A3 or U180 would likely remain the only viable solution, as a repair of U180 is exceedingly unlikely. Therefore, it could at least be hoped that an open, fully-accessible replacement, including with parts that can be selected and changed at will, may help.So an initial goal for NU180 would be for a functional drop-in replacement; one which correctly works without throwing codes and which can provide a platform for learning and experimentation. It may not achieve the same performance due to less well-matched components or extra stray capacitance, etc., but if it works, that’s already a success, and for some better than a machine that is dead or otherwise unusable.Of course, the ultimate and most valuable goal would be a genuine 8.5 digit-worthy, drift-free replacement. This can't be promised and shouldn't be expected, but there's hope it can be achieved after a revision or two. The surface-mount resistors could be replaced with super-duper through-hole ones or a dedicated network, and so on.I also agree that the design can be broken up. For example, someone smarter than I am could perform an error analysis given the ppm's of the various parts, etc., to help guide the design.I’ll start a separate thread after doing a little more preparation.

author: Kofen
author_rank: Regular Contributor
timestamp: August 20, 2025, 05:56:17 pm
content: Yes, so if we break this simple down here:1, can U180 be fixed? That is something I am working on.2, can it be replaced with good enough performance, that is open and you have the lead.You start a thread on your ideas and I contribute when I have time and I'll start one on U180 "revival project" once that starts a few months from now or so.I am still looking for more bad U180/A3 boards, if anyone has some, DM me.

author: Zondar
author_rank: Regular Contributor
timestamp: August 20, 2025, 07:27:42 pm
content: Quote from: Kofen on August 20, 2025, 05:56:17 pmOh, please say more about your ideas for fixing U180!I'd also like to get a working A3 board with or without a bad U180. Without that, I could perform surgery on my own one once I'mstupiddrunkbrave enough.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2025, 07:29:22 pm
content: The minimum would be to have the 3458 accept the U180 replacement and not give a serious error message. This would mean having all essential function work to pass the self tests.The noise part would be relatively easy. The main points to possibly get higher noise would be from jitter and to a small degree the switch capacitance at the switches (from the switch to the ref. path resistors). So the noise may be a little higher, but likely not very much. If lucky one might even achieve slightly lower noise than with the original U180. This would especially be the case when changing from +-12 V ref. and 80 K resistors to +-15 V ref. and 100 K resistors. Than slighly lower (like 5-10%) noise is possible.The stability / TC would mainly depend on the resistors / resistor matiching. From my tests with ORN / NOMC arrays I would expect that acceptable performance (like gain drift of less than 1 ppm/K) would be likely. If needed one could think about some trim option to compensate. One is however in the range of observed performance - so the actual performance can vary. It would be very expensive to get resistors with perfromance that is guranteed specs that good.The tricky part is the INL with 3 aspects. One is from heating in the 50 K input path resistor(s), mainly leading to U³ type INL. This part is about the resistor quality. Matching can help if tightly coupled - so no need for super low absolute TC. The second point is possible hard (more localized) INL errors from an inferior layout and interference in switching times - this part is tricky and hard to predict. Finally there could be some DNL errors from not so perfect small contributions (between the 80 K and 20 K resisors and with the ladder part). It would mainly effect the shorter integration time, not so much with 10 PLC and slower. From the 3458 describtion the demand on the resistor ratio matching here should not be super high.From leakage currents the drift in the non AZ mode could suffer a little, but this is usually a less relevant performce part.Overall chances are one would get 8.5 digit performance, though maybe not all the way as good as the original.

author: Kofen
author_rank: Regular Contributor
timestamp: August 20, 2025, 08:08:52 pm
content: Quote from: Zondar on August 20, 2025, 07:27:42 pmWe already discussed it in this thread, started here:https://www.eevblog.com/forum/metrology/so-i-have-a-mint-condition-agilent-3458a/msg6005651/#msg6005651I have one A3 that can go, but it is missing gate array and processor so usesless for your needs. Rest I need for the project. I would let your U180 be alone for now, rather focus on a 8.5 digit ADC first, then adapt to U180.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 20, 2025, 09:29:07 pm
content: Building a 8 digit dmm would be quite diffenrent from a U180 replacement would likely be quite different. With more standard parts one would not directly copy the ADC of the 3458. Today I would more separate the slow precision ADC and the faster ADC (mainly for the AC ranges). Today there are plenty of ready made ADC chips to do the fast part (like 16 bits are 100 kSPS). Not needing speeds faster than some 1 ms integration the precision ADC can be build simpler. The Keithley DMM 6500, DMM7510 and Fluke 8858 already go the way with separate fast and slow ADCs.The alternative to a U180 replacement could be a replacement for the complete A3 PCB. After all the A3 PCB also has obsolete comparators and the FPGA with level shifters that may no longer be needed.

author: Zondar
author_rank: Regular Contributor
timestamp: August 20, 2025, 09:44:17 pm
content: Kofen: I re-read that, thank you. I wish you luck and hope it works! If it does, I'll be willing to try a few steps like that with my own U180.I'll agree with Kleinstein: developing a whole new DVM is probably easier than trying for a replacement A3 board. For one, how would you reverse-engineer everything going on inside A3's FPGA?

author: MiDi
author_rank: Frequent Contributor
timestamp: September 12, 2025, 11:01:05 am
content: Quote from: Kleinstein on August 16, 2025, 09:01:40 amChecked that on U180 and indeed the logic is inverted for ladsn (resistor ladder rundown):RURRUZRUSladsn sum3 Ωladsn agnd3 Ω000>3G13.9001>3G13.901111.9200Meg010>3G13.911011.9200Meg11111.9200Meg10111.9200Meg100>3G13.9

author: IanJ
author_rank: Unknown
timestamp: September 12, 2025, 03:06:31 pm
content: Quote from: Zondar on August 20, 2025, 03:50:54 pmSounds a great name!Hoping this project comes to life, seeing a working prototype would be awesome....I'm following closely!Can you imagine what it will do for the 3458A community!Ian.

author: Zondar
author_rank: Regular Contributor
timestamp: September 12, 2025, 07:42:39 pm
content: The project is being discussed in a separate "NU180" thread.To date I've submitted a prototype NU180 board along with a simple break-out board to test it with, plus a few small test circuits. They, and the parts I ordered for them, should arrive next week. The first version is to test functionality more than performance. It's quite uncertain that it will work well enough, or even at all, but it's a fun project that is worth a shot.

author: wanghar
author_rank: Contributor
timestamp: September 18, 2025, 10:18:50 am
content: I've never quite figured out the function of the resistor ladder circuit for the minor slope drivers on the A3 board of the 3458A. Did a quick simulation (skipped the 74HCT14 part), and my rough guess is that by changing the logic levels of the four control pins—S1, XS4, S16, and XS64—it can inject integration currents into the integrator that differ by roughly 4x. It's not exactly 4 times but pretty close. Still not sure if this part functions during the run-up phase or the rundown phase though.Thanks to MiDi for posting the KiCAD version of the A3 schematic in the other thread—I'm referencing a portion of it here.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 18, 2025, 10:52:17 am
content: To get exactly 1:4 ratios the resistors should be 50 K and more 16.666 K instead of 16.7 K . The 51 K instead of 50 K would be the lesser issue as it is on the LSB and thus should tolerate quites some tolerance.The 37.5 K and 16.7 K numbers in the schematics may include some rounding. It may only be relevant for the small part inside U180.The ladder part should be disconnected during run-up. The impedance of the ladder would nicely compensate the 50 K for the input resistor. So it would make some sense to already activate it (via the MOS switch) for the fast run-down that uses current from the main references. It would not be super critical with the 3 OP-amp integrator, but still nice.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 18, 2025, 04:30:06 pm
content: The differences to ideal currents of the rundown resistor ladder:U180 ladder resistor measurements:The measurements of the remaining resistors are attached (dead U180).

author: wanghar
author_rank: Contributor
timestamp: September 22, 2025, 05:22:06 am
content: Thanks MiDi for these data,.I'm guessing the res values in the last table represent the combined resistance of the resistor and switch in series, right? How were the specific resistance values in the resistor ladder measured? Given that at most only a single pin is brought out to U180, was an auxiliary constant current source used for measurement, or was some kind of mathematical approach like solving equations (e.g., x+y=a and x-y=b) applied as a workaround?

author: MiDi
author_rank: Frequent Contributor
timestamp: September 22, 2025, 11:36:26 am
content: For the +-12Vref paths the values include the switches, as they cannot be measured separately w/o probing the die.The switches for 80k are around 2x ladsn (~24Ω) and for 20k around 1/2x ladsn (~6Ω), so one can estimate the resistor values (seehttps://www.eevblog.com/forum/metrology/so-i-have-a-mint-condition-agilent-3458a/msg6012477/#msg6012477)The resistor ladder was calculated from the measurements of all pins to each other (see attachment).

author: wanghar
author_rank: Contributor
timestamp: September 22, 2025, 02:03:16 pm
content: Even though this is a faulty U180, the measured resistance readings all appear normal.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 22, 2025, 07:19:06 pm
content: The two 80k from +12ref are off by ~1% compared to its siblings from -12ref (and all other resistors), likely the resistors are off and not the 4 switches.It failed with classic "multislope rundown convergence error", first only infrequently until eventually every reading resulted in that error.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 22, 2025, 08:46:39 pm
content: A drift of the 80 K resistors would give some extra current also for the run-down phase. Likely one 80 K path to the +12 V and -12 V are used also during run-down to keep the impedance about constant. To much extra current from this can interfere with the later run-down steps.If this is the only problem one might compensate for this with changing R154 (604 K resistor to -12 V).1% off for the 80 K would be about as much as the smallest ref. step from the ladder part.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 22, 2025, 11:37:37 pm
content: Details of ADC operation:0.01NPLC, AZ off (thanks to xDevs for the capture):1NPLC, AZ off (thanks to macaba for the capture):EDIT 2015-09-25: fixed errors & fit lines to exact switching timesRundown 1NPLC, AZ off (thanks to macaba for the capture):EDIT 2025-09-24: Fixed errors & exact switch timesAdditional captures on xDevs:https://xdevs.com/fix/hp3458a/#diag3

author: MiDi
author_rank: Frequent Contributor
timestamp: September 23, 2025, 10:29:19 pm
content: Updated the rundown in last post.Wondering why there are these hold times and the different transitions between the slopes, S64 even engages before S256 disengages - weird.Did a quick check of the switch delay times for rusn1 to sum3 (midpoint of CLK & switch transition):On: ~25nsOff: ~30ns

author: wanghar
author_rank: Contributor
timestamp: September 24, 2025, 01:54:13 am
content: During the hold time between -S256 and +S64, the control pins S1/XS4/S16/XS64 of the res_ladder are set to 0/1/0/1, resulting in a combined current from the res_ladder and R154 (604K) to the integrator being close to zero. At the same time, ENR/SLA/SLB are set to 1/1/0, which turns on both 80K-12Ref2 and 80K+12Ref1 simultaneously, also supplying nearly zero current to the integrator. This configuration creates the 'hold time'. As for why it's designed this way, further investigation is needed.This capture has almost all the operation details. Thanks for sharing, MiDi! Is there a similarly detailed capture for the run-up phase?

author: Kleinstein
author_rank: Super Contributor
timestamp: September 24, 2025, 07:23:05 am
content: An additional holding phase in between the run-down steps can give the integrator and slope amplifier some extra time to settle. The larger step is turning of the previous reference and most settling starts from there. Starting the new slope is only 1/4 the size. The steps with the ladder should have no addition spike from charge injection (unless the layout has significant capacitive coupling).Some additional waiting in the run_down (after the fast part with the full references) can also help a little with the error from dielectric absorbtion. At least the fast part can settle before the final decision (last comparator reading) is made.It is odd to see the switches faster in turn on than turn off. The modern CMOS switches usually have it the other way around with a few ns of break before make. I don't see an issue with the short break before make time - if at all make before break could be an issue and may be reason to have the ferrites.