Keysight 34465A reference replacement. - Page 1
Forum section: electronics / testgear

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2025, 06:33:04 pm
content: I've acquired recently a reasonably priced 9-year old Keysight 34465A multimeterwhich is still holding well inside 1year accuracy on those measurements I could check in my lab (DCV at 1V and 10V, ACV at 1V and 10V, Resistances from 1 Ohm to 1 Gohm.I would be completely happy with my purchase (even the original fan is reasonably quiet, much quieter than on another 34465A I used at work a few years ago) if not for one rather unpleasant nuance - the original reference was very stable over all these years but it exhibits quite typical ~1ppm "jumps" which are rather annoying in an otherwise nice unit.Fortunately, to replace the reference chip you only need to undo four screws (the reference is plugged into a small 4-way socket co can be removed and replaced without soldering) . Unfortunately, after such a replacement you would need to re-adjust the meter, and that means sending it to a good calibration lab, preferably to Keysight themselves. I also quickly discovered that out of three spare LM399 chips I bought from Mouser a few years ago only one is reasonably stable and quiet, however it also jumps occasionally though at much larger intervals than the original chip. I also had to my disposal a couple of ADR1399 chips which could be a reasonable (and potentially less noisy but that might be not very noticeable in this meter as I've discovered) replacement for the original LM399A.There is a problem however with such a replacement, the ADR1399 requires a much larger Zener current to work properly (at least 3mA) and also requires an RC compensation network in parallel to Zener pins. If the second requirement is pretty easy to meet, soldering the ~5 Ohm + 1uF to the Zener pins on the socket (without the reference in it, obviously), the first one is more difficult as it requires at least some understanding of the reference circuit and how to increase the current from about 1mA for the original LM399A to 3mA for the ADR1399. I had to work out the connections and it appears that 1.5K resistor between pins 5 and 7 of U707 IC (AD706J) does the trick. The ADR1399 is quiet and surprisingly stable for a brand new chip, now I plan to wait for a few hundred hours before sending the meter to Keysight UK.CheersAlexP.S. - to avoid a possible confusion - both voltage measurements graphs are taken at 10V input from Fluke 731B and the vertical scale is the same however on the second graph with ADR1399 the meter is nulled (so we see only deviations from the measured value - in this case 10.18692V , which means the ADR1399 zener voltage is about 2% lower than that of the original LM399A.

author: HighVoltage
author_rank: Super Contributor
timestamp: February 20, 2025, 06:46:07 pm
content: What an interesting topic.I have owned a few 34465A instruments over the years and they have all been extremely stable, even much better than the 34470A and none of them had this 1 ppm jumping.Looking forward to you long term measurements on this mod.Thanks for sharing

author: Kleinstein
author_rank: Super Contributor
timestamp: February 20, 2025, 07:09:13 pm
content: If there is a significant difference in the voltage of the LM399 and ADR1399 the ADC of the meter may want some extra adjustment. The ADC uses 2 auxiliary ADCs and it is not clear of these ADCs use a reference derived from the main reference or maybe a different (ADC internal) ref. voltage.So after a larger change in the main ref. one may want a kind of extra factory adjustment step. I don't know when the relative size of the parts is measured. A wrong value there could result in higher than normal DNL error (and related noise). It may be possible to check with rather fast (non AZ mode) conversions (e.g. 10 kSPS range), a small amplitude (e.g. some 10 mV ? in the 10 V range) triangle waveform and than looking at the histogram.The jumps look like the "normal" 0.5 ppm (maybe a little more, but not much) jumps. The more frequent jumps don't look nice, but would average out better than rather infrequent jumps.So far all the 4 LM399 that I have used had such jumps, though a bit more symmetric. 3 units at a comparable rate and one much slower.There is chance that the lower noise of the ADR1399 could be visible in some cases (with a low noise DUT in the 10 V range). The effect may be more visible with more than 10 PLC.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 20, 2025, 07:40:48 pm
content: Quote from: Kleinstein on February 20, 2025, 07:09:13 pmI’ve added a P.S. with some information on the graphs, both were taken using a low noise 10V source Fluke 731B. The ADR has about -2% voltage from the original LM399A.Another interesting fact - while voltage and current measurements results changed due to the reference voltage difference, the resistance measurements did not change as these rely on the relative values of resistors but not the actual voltage reference value.CheersAlex

author: Zucca
author_rank: Unknown
timestamp: February 20, 2025, 08:07:33 pm
content: You are tempting me to open up my 34411a and see what I can find...Nice job....

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 21, 2025, 08:46:19 am
content: I also own a 34465A.Its LM399 reference was quite ok concerning the usual noise of this kind of reference chip. Long term stability was excellent, only a few ppm in DCV over the past 10 years.I made a comparison between the 34465A and the '470A with the LTZ1000A (from HighVoltage), and demonstrated the better StD of the LTZ1000 based instrument.So I recently replaced the 399 by a clone LTZ1000 reference board - successfully - and already re-adjusted most of the modes and ranges.My LTZ1000 has more stable BMF resistors, and runs on about 60°C oven temperature only. Long term stability should be better than the original '470A.I still have to improve the warm up drift, due to my thermal concept, but then I will send it out for complete re-adjustment. I guess, I will report details here afterwards.Frank

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 21, 2025, 03:54:18 pm
content: Here is a 6h measurement of 10V output from Fluke 731B by HIOKI DM7275 and Keysight 34465A (ADR1399) in parallel, 100NPLC, 5s intervals. During these 6 hours the temperature in the room increased by about 1.5C, the Fluke has tempco about -1.5ppm/C, so there is about -2ppm change in the value. The Keysight is nulled as it is still not adjusted to display correct values. Vertical scale is 5uV = 0.5ppm per division.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 12:07:59 am
content: I did change something in the K34465A and the 10V noise halved - see the graph below. Now the Keysight is fairly close to the HIOKI .1 hour run, 10V from the Fluke 731B, measured again simultaneously by the 34465A (nulled) and DM7275. Vertical scale 2uV =0.2ppm per division. For comparison I've added a reasonably flat 1h part of the previous measurement on K34465A (roughly the second hour). Hmm... now I need to think what to do with this discovery.CheersAlex

author: voltsandjolts
author_rank: Unknown
timestamp: February 22, 2025, 02:43:09 am
content: Nice find!Is the mod particular to the ADR1399 or would it also be applicable to a meter using the original LM399?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 03:13:51 am
content: Quote from: voltsandjolts on February 22, 2025, 02:43:09 amMy first impression is that the original LM399A is less susceptible to that effect and noisier in the first place. The ADR1399 seems more sensitive to the airflow changes, so I’ve just disconnected the fan. I need to double check if it is the reference itself which is the culprit and if it is possible to eitherRun the meter with the fan disconnected continuouslyor shield the reference better.I would prefer the first option.CheersAlex

author: voltsandjolts
author_rank: Unknown
timestamp: February 22, 2025, 03:34:48 am
content: Yeh, I'm no fan of fans either.https://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/msg2655327/#msg2655327

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2025, 11:17:33 am
content: Quote from: Alex Nikitin on February 22, 2025, 12:07:59 amHi Alex,compared to what the noise has halved?In your first post, you've used NPLC 10, now NPLC 100..How do you acquire the data from the 34465A?Do you read them via GPIB, or do you use its internal data acquisition mode?What's the resolution in either case?There's an obvious difference between the Hioki and the 34465A, I guess.FrankPS: I think I will publish my first results as well, comparing LM399 vs. LTZ1000A reference, and a T.C. measurement.. Stay tuned.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 11:27:47 am
content: Quote from: Dr. Frank on February 22, 2025, 11:17:33 amI’ve used the logging option on the K34465A saving data on a USB stick. On the last graph the purple trace is with the fan on and the orange is with fan off, both at 100NPLC, 1h longCheersAlex

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 22, 2025, 01:21:12 pm
content: Quote from: Alex Nikitin on February 22, 2025, 11:27:47 amOk, i've done both, saving on 465As memory and over GPIB .. evidently same resolution (8 digits), but I still wonder whether there is a slightly different math / averaging method.Anyhow, I found out years ago, that the internal air flow of the 465A/470A clearly creates additional noise, as the air flows directly over the solder joints around the reference(s).So I have put a small plastic box over the reference area, and a poly foam beneath, which mitigated this effect.Frank

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2025, 02:22:25 pm
content: Thermal fluctuations can effect both the reference part and also just the input in the form of an offset. The fan bings temperature variations from the outside faster in side. In some cases not having the fan can also cause thermal instability. In addition without the fan the thermal stabilization after turn on will take much longer (e.g. some 5 hours instead of some 1 hour). The overall higher temperaure can also make a difference.Turning off the fan has both positive and negative effects.

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2025, 06:28:32 pm
content: Quote from: Kleinstein on February 22, 2025, 02:22:25 pmYup, this would be the common sense view on any instrument with fan cooling.If it wasn't needed for stable operation a fan wouldn't be installed by the OEM in the 1st place.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 06:42:20 pm
content: Quote from: tautech on February 22, 2025, 06:28:32 pmIt is not that simple. A multimeter design is a collection of compromises and having fan is one of these. It is, for example, perfectly possible that without the fan the high current ranges would not meet the specifications. If you never use the meter to measure 3A continuously perhaps the fan can be disconnected?CheersAlex

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2025, 06:49:20 pm
content: Quote from: Alex Nikitin on February 22, 2025, 06:42:20 pmYou overlook the warmup time to meet spec.Instruments designed requiring fan cooling will stabilize faster to meet spec in normal operating environments.

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 22, 2025, 07:10:33 pm
content: Quote from: tautech on February 22, 2025, 06:49:20 pmIt would be cool if manufacturers included info for use without a fan (when possible), or what internal temps require cooling. Some labs never shut their meters off, so warmup time doesn't matter to them.It would be nicer if they all used smart fans rather than cheap ass always on stuff. I'm pretty sure my Keithley DAQ has a smart fan in it. It's also the quietest of all my meters with fans.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 08:08:35 pm
content: A “smart” fan is the last thing you want in a ppm capable meter, you would never get the required stability. It’s either always on and stable speed or no fan at all. I do prefer fanless meters, that was a reason why I’ve bought the HIOKI DM7275 and my other precision meters in use all have no fans - HP3456A, and two Keithley 2015. The warm up time is not a problem when meters are on 24x7. Fans also add vibration which occasionally might be a problem.I had been asked once at my then job to investigate a possibility of running a piece of equipment we manufactured without a fan. It was a small unit, a similar size to the 34465A and with a small fan producing a continuous air flow across a number of boards and blocks. I run the unit in an environmental chamber with and without the fan and found that with a perforated top cover the unit keeps in specs and has no hot spots as long as the ambient air temperature is below 30C. I strongly suspect that the K34465 will work happily without the fan in a normal lab environment.I will nevertheless try to improve the performance of my K34465A with the fan by shielding the reference as I need to send the unit for a full adjustment and I suspect Keysight would not be happy with a unit that has a disconnected fan…CheersAlex

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2025, 08:19:09 pm
content: Quote from: Alex Nikitin on February 22, 2025, 08:08:35 pmYet I lost a fanless Tek DSO that was top vented in a dusty and humid environment (uninsulated garage).Now very wary ofanytop vented instrument !

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 08:24:40 pm
content: In this particular case the unit was supposed to work in a semiconductor factory, so the environment was as controlled as technically possible. My home lab is certainly not up to the same standard but not as bad as to clog the ventilation slots with dust!CheersAlex

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2025, 08:33:53 pm
content: Quote from: Alex Nikitin on February 22, 2025, 08:24:40 pmIn the Tek I lost clogging of vents was not the issue but instead top vents allowing for the entrance of dirt/dust onto the PCB then humidity and corrosion did its evil work.

author: zepto
author_rank: Regular Contributor
timestamp: February 22, 2025, 09:27:08 pm
content: When I saw your post in the buy/sell thread, I figured you bought it to upgrade to 34470A. Were you planning to upgrade the reference even if it wasn't jumpy? I expected the LTZ1000, not the ADR1399.The 65A is interesting because you can be certain that a higher class of reference can provide improvement. Are there other multimeters, new or old, that would benefit from an upgraded reference, or are other components the limiting factor in terms of noise/tc/drift?Would it be worthwhile to thermally isolate larger parts of the analog circuitry than just the ADR1399?Thank you for sharing your experience.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 09:58:47 pm
content: Quote from: zepto on February 22, 2025, 09:27:08 pmI have already a meter with the LTZ1000 reference - HIOKI DM7275 and I also think that the hardware on the K34465A is not good enough to justify the LTZ. The ADR1399 on the other hand, looks like a simple upgrade and can be just as stable from my experience. If I can resolve the air flow problem I will be happy with it. It appears that with the fan off it shows a lower noise at 10V than the original LM399A (if not looking at "jumps, that is). As I've said in the first post, I would be happy with the meter if not for the "jumps", so this upgrade was not planned and certainly not to the LTZ level.Quote from: zepto on February 22, 2025, 09:27:08 pmMy guess is that if the meter/source noise/stability is limited by the used reference noise/stability (or the reference is "jumpy" as in my case here), the ADR1399 might be a good upgrade path.Quote from: zepto on February 22, 2025, 09:27:08 pmMost likely no. Even the reference should not be more thermally isolated than it already is (only screened from air movements), otherwise the temperature stabilizer could be unhappy. The problem is not a thermal leakage (you always have to remove some heat from a circuit) but temperature gradients in time and space, so you need a certain thermal mass to stabilize and a thermally conductive board or coupled heatsink to equalize. That however is only useful if applied with precision, knowing particularities of the circuit in question otherwise it is easy to make things worse. I personally only would look to change or modify something reasonably well made if there is an obvious and understood problem - as it was in this case.CheersAlex

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 22, 2025, 10:07:42 pm
content: Quote from: Alex Nikitin on February 22, 2025, 08:08:35 pmWhy would a smart fan with variable speed be an issue? Is smart fan the wrong term? If the goal is consistent temperature, that's far more effective at maintaining a constant temp. Always on or always off is more likely to be affected by ambient temperature changes. I'm pretty sure my DAQ6510 has a 3 wire variable fan, and it's the most stable of my meters.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 10:43:45 pm
content: Quote from: KungFuJosh on February 22, 2025, 10:07:42 pmThe screenshot you've posted is not very useful, with three different vertical scales for three graphs, an unknown timescale (is it in ms?) and no way to learn anything about stability in time (and we are talking minutes/hours/days). If you would like to see the effects of the fan speed changing you have to make the fan change the speed and than observe the voltage over an hour or so. That might give you some ideas but it would not cover all possible troubles for a variable speed fan. I've been to that rabbit hole and would stay clear. No fan is generally the best for accuracy unless you really must use it to remove the heat. Even than a stable and minimal required speed fan is the best option. Changing the fan speed will change the airflow path and that in a precise instrument could be a real killer.CheersAlex

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2025, 10:58:00 pm
content: A regulated fan can stabilize the temperature at the sensor, but it causes temperature gradients that depend on the fan speed and thus the external temperature. With a fixed speed fan, the overall temperature can change, but temperature gradients are largely fixed.Some of the more nasty thermal offsets depend on the temperature gradients. Another point could be a possible resonace at a certain fan speed.It still depends on the details if a regulated fan is good or bad.Without a fan one can still get thermal fluctuations - I see this with my DIY meter in the form of thermal oscillations.A downside of a fan the the noise and more dust collecting.There are meters with a not so stable ADC, that may not improve much from a better reference. I would consider the HP3457 and 34401 (and the related 34420) as examples. For the 3457 there is a report of an upgraded reference with not much change.The LM399 run quite hot and it is rather hard to improve significant on the isolation.I have not seen any negative effect from adding some isolation.The LTZ1000 can be different with a usually lower temperature and square law heater, where less power comes with less loop gain for the temperature regulation. The LM399 heater is kind of linear, using the heat from transistors and thus control loop gain not much effected by the power.

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 22, 2025, 11:10:56 pm
content: Quote from: Alex Nikitin on February 22, 2025, 10:43:45 pmTime scale is minutes. Look at the grid panel with statistics.Thanks,Josh

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 22, 2025, 11:17:41 pm
content: Quote from: KungFuJosh on February 22, 2025, 11:10:56 pmWhat was used as the source?CheersAlex

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 22, 2025, 11:30:22 pm
content: Quote from: Alex Nikitin on February 22, 2025, 11:17:41 pmIt's a VREF10-001 r9.Thanks,Josh

author: zepto
author_rank: Regular Contributor
timestamp: February 23, 2025, 12:13:50 am
content: The graph is somewhat misleading. The scales are not consistent for some reason.

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 23, 2025, 12:23:08 am
content: Quote from: zepto on February 23, 2025, 12:13:50 amClassic user error. I never figured out how to sync the chart until today...after he mentioned it. I mostly looked at the grid panel for the PP and averages.I'm going to do a new test with everything properly synced on the chart.Thanks,Josh

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 23, 2025, 12:28:16 am
content: You also better to monitor the ambient temperature, as something is seriously wrong with your graph- over a short period of time two meters show a difference between the initial and the final voltage values, the third is not. Something is amiss. It could be that the Keithley compensates the reference tempco somewhat…CheersAlex

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 23, 2025, 12:33:56 am
content: Quote from: Alex Nikitin on February 23, 2025, 12:28:16 amI will. I have a 4th meter I can monitor temp with. That test was almost 5 hours, so notthatshort. My office is tiny, and it's winter here in New England. It's 27F outdoors here right now, and that's warmer than the day I did those tests. 😉

author: The Soulman
author_rank: Super Contributor
timestamp: February 23, 2025, 12:51:15 am
content: Josh, even with that existing data you can adjust the scales on the charts.On the tab "scales for charts" select on the appropriate lines "si" instead of "auto" than manually enter the min/max values, use the same difference (span) between min and max for all three meters.A span of 20uV should work well in this case (2ppm full-scale, 1/10th ppm per division).

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 23, 2025, 01:09:00 am
content: Quote from: The Soulman on February 23, 2025, 12:51:15 amThat would be true if I logged the data. Thanks for the tip though! Now I know two ways to sync the meters. I like your way better, because using the sync control makes it harder to tell which is which.Thanks,Josh

author: Simmed
author_rank: Frequent Contributor
timestamp: February 23, 2025, 01:11:23 am
content: for those who remember the wine chiller experimenthttps://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/450/maybe it is easier to compare using Adev chart ?(for some reason the server no longer display pics in that thread, but i guess i did save the page as pdf)i try to look for the alavar website, but it is also no longer thereso i put the 2 below(x86 stuff)1 is for Adevthe other makes "fake noise" for testing(rename txt to exe)these files are old, so i assume they are clean inside my drivei think python has some Adev stuff also (allantools?), but i have not tried them.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2025, 09:28:33 am
content: The Allan deviation is a different way to look at the data, especially for really long data sets. It is still worth to look at it also in the time domain. Some parts (e.g. correclation with temperature or popcorn jumps) are more visible in the time domain.A check of the temperature effect is defintely a good idea.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 23, 2025, 01:26:46 pm
content: Here is a better comparative graph for fan ON/OFF, vertical scale 2uV = 0.2ppm /div, 2ppm full scale, horizontal 500ms/sample, 30 min in total. The Fluke 731B as a 10V source, results are nulled for 10.18691V offset. So far the reference behaves well with no noticeable drift.CheersAlex

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 23, 2025, 03:38:42 pm
content: I found an older thread where this air draft noise has already been demonstrated:https://www.eevblog.com/forum/metrology/turning-fan-off-to-reduce-dmm-voltage-noise/msg2596671/#msg2596671Here's the thread from 2020, where I already have measured the different references w/ and w/o a draft protection.https://www.eevblog.com/forum/metrology/buyers-beware-misleading-34470a-reference-modules-(clones)-on-ebay/There's a nice picture, where you can see, that the fan is directly blowing over the whole reference circuit, what a design flaw!The plastic box clearly has reduced the noise of the 399.I have now permanently assembled the LTZ1000A inside my 34465A, by replacing the precision DIL socket by the correct 8x male pins.The stability / noise measurement look exactly the same as 5 years before, and the instruments needs about 8h to stabilize to < 0.2ppm after turn on and ACAL.Frank

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 23, 2025, 04:33:14 pm
content: I posted a 10 hour DCV test in the Keysight thread including the csv data:https://www.eevblog.com/forum/testgear/keysight-6-5-digit-bench-dmms-34460a-34461a-and-34465a/msg5828335/#msg5828335

author: Dr. Frank
author_rank: Super Contributor
timestamp: February 23, 2025, 04:47:54 pm
content: I have stabilized / made a run-in of the LTZ1000A for about 2 months, after which period it stabilized to the usual drift of < -1ppm/yr.After turning on of the instrument, it takes half a day to stabilize completely; the first 2h are the normal warm-up time which you see as well for 'normal' 34465A, the following drift I assign to the temperature stabilization inside this plastic box. Maybe I cut a hole on top the LTZ1000A, so that its heat can escape.The vertical scale division is 2µV or 0.2ppm. I use a FLUKE 7000 running on mains.After stabilization, I made a 24h run and zoomed into 1h of data. Scale division is 1µV or 0.1ppm. The 1h-noise is about 480nV or 0.048ppm, maybe double the noise of a 3458A.Then I estimated the T.C. by attaching an NTC inside the 34465A through the holes in the case. I cooled down the room temperature by 1°C, which caused the same temperature decerease inside the 34465A. This gives about 0.35ppm/K, in comparison to the specified 2ppm/K.Frank

author: KungFuJosh
author_rank: Super Contributor
timestamp: February 23, 2025, 05:54:32 pm
content: Quote from: Kleinstein on February 22, 2025, 10:58:00 pmI asked @E-Design about the DAQ/DMM series fan, this was his response:Quote from: E-DesignI think that's pretty darn smart.

author: branadic
author_rank: Super Contributor
timestamp: February 23, 2025, 08:27:06 pm
content: QuoteFrank, can you perform an FFT on that data please? It seems to me that there is some frequency visible in that measurement, that is not related to noise.-branadic-

author: Alex Nikitin
author_rank: Super Contributor
timestamp: February 23, 2025, 09:04:53 pm
content: Quote from: Dr. Frank on February 23, 2025, 04:47:54 pmHmm, my modified K34465A with ADR1399 and disconnected fan gives roughly the same noise level at 10V input and 100NPLC, just under 0.3ppm p-p. Below is another graph to illustrate it, with 1uV/div scaling. The downward slope is due to the room temperature going up by about 0.3C and the Fluke 731B tempco (which is the reason why I run the HIOKI meter in parallel to monitor the reference) . What is the point of using the LTZ in this meter? I also feel that the unit stabilizes much quicker, at some point I will measure the warm-up performance. I think the ADR1399 is probably the right reference for the 34465A as there is no obvious improvement in the noise and possibly in the long-term stability as well, compared to the LTZ1000. And much simpler and cheaper.CheersAlexP.S. Added Allan Deviation for 10NPLC, 30 min run, for a 10V FS .

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 04, 2025, 10:13:08 pm
content: Here is a small update, as I've looked for a simple way to reduce the fan influence so I could send the meter to Keysight for an adjustment. After some experiments I've added a Kapton "skirt" to the ADR1399 (see the photo attached) and the noise with the fan ON is now almost the same as with the fan OFF. I can live with that, so the meter is ready to go (the reference appears very stable so far).CheersAlex

author: mawyatt
author_rank: Super Contributor
timestamp: March 04, 2025, 11:58:19 pm
content: Nice 34465A modificationThe variation is likely due to random thermals across the Kovar leads, and your shield is blocking these thermals across the leads.We did something similar awhile back with our LM399 and LTZ1000 References with a 3D printed thermal shield.Best

author: KE5FX
author_rank: Super Contributor
timestamp: March 05, 2025, 02:06:19 am
content: Quote from: Alex Nikitin on February 20, 2025, 06:33:04 pmI just picked up one of the LG surplus 34465As. It cleaned up nicely, but if the LM399As are jumpy I don't want any part of that. Possibly a naive question, but couldn't the 2% difference between the ADR1399 and LM399 be handled by adding a non-inverting buffer? OPA388-class opamp, plutonium foil resistors, wave dead chicken in graveyard at midnight, etc.There will be some value for R2, which could be found with a trimpot initially, that will result in no shift in the displayed voltage when switching between the LM399A and ADR1399. At that point it seems arguable that the DMM's calibration would remain intact, assuming you don't care about the actual paperwork.

author: KungFuJosh
author_rank: Super Contributor
timestamp: March 05, 2025, 03:12:48 am
content: Quote from: mawyatt on March 04, 2025, 11:58:19 pmIs that one of your reference projects? Did you...share it? 😉

author: mawyatt
author_rank: Super Contributor
timestamp: March 05, 2025, 04:05:41 am
content: Quote from: KungFuJosh on March 05, 2025, 03:12:48 amYes, we did post about it on here somewhere. It has dual references (actually 3) so we could evaluate the LTZ1000 vs LM399 under the same conditions. Also did a LM399 based 10/1/0.1V Reference.Best

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2025, 08:24:11 am
content: Quote from: KE5FX on March 05, 2025, 02:06:19 amAdding an addition gain / divider stage would be quite some effort and could add drift. The voltage can be higher or lower, as the LM399 have quite some tolerances. Even though only a relatively small trim it would be quite some effect to adjust down to a few PPM back to the old value. With the higher ground current one may need a new calibration anyway. There could be an extra effect on the ohms ranges. Not sure how much of this is corrected with the ACAL part - the documentation I have seen so far was a bit vague.

author: Dr. Frank
author_rank: Super Contributor
timestamp: March 05, 2025, 10:49:13 am
content: Quote from: KE5FX on March 05, 2025, 02:06:19 amHello KE5FX,Your approach makes no sense (in technical aspects) to me.Even if you would compensate for this 2% difference, you would need anyway a completely new calibration, as we are talking about ppm uncertainty, not %.Fine tuning with a trim-pot is as well not feasible, as you again would need ppm resolution for the trim-pot. Such ppm trimming methods like in those old FLUKE calibrators (332, 335 etc.) always require very stable resistor networks, like those FLUKE wire-wound, T.C. selected ones, so I guess an electronic adjustment would be the most effective and straight forward way to go.A similar question was raised at the Metrology Meeting 2024 in Stuttgart, when the American volt-nuts presented the replacement of the LTZ by ADR reference inside the 3458A.. it was better to simply use the lower reference voltage by changing the firmware calibration margins, and re-calibrate the instrument, than to add a (more noisy) amplifier stage.Frank

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 05, 2025, 09:26:23 pm
content: Actually, trimming the reference amplifier should be perfectly possible using a reasonably stable separate voltage reference and (if available) another accurate meter for control. You measure the external reference before and after the LM399 replacement, then trim the internal reference to get the same measurement of the external reference as before. I might try to do just that to get the meter reasonably operational before I'll send it to Keysight UK. I need to trim the reference about 1.87% up, it should be possible with a well padded trimmer (a permanent resistor close to the required value and a trimmer to cover, say, last 1% required, for about +/-100ppm trimming range, or even less. If R408-b resistor in the circuit from the first post is (say) 50K in value, it might work with about 2.67M permanent resistor + 50K multiturn trimmer in series, and with tempcos, say, less than 5ppm/C for the permanent part and standard 100ppm/C for the trimmer, the resulting added temperature influence should be about 0.1ppm/C. The potential advantage of this approach is keeping the V/I calibration reasonably close to the original state (R measurements are not affected anyway). There are some possible pitfalls in this approach however it might work OK in this particular meter if my assumption that the reference output is at a single point and trimming only one resistor will get the required result. If that is not the case this will not work. I might however try it as it is reasonably simple and easily reversible.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 07, 2025, 12:05:53 am
content: It looks like I need ~82K from pin6 of U707 to ground to get the reference voltage back to the original value. I had to order a low tempco 79.6K resistor from Mouser and I have a nice 5K metal foil trimmer to go with it. Let's see what happens. Meanwhile here is the graph of the warmup from cold of the modified 34465AM, measuring 10V from the Fluke 731B, 10NPLC, 1hour (horizontal grading at 5min/div, vertical 1ppm/div). Fan ON, Shield ON.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 11, 2025, 12:17:26 am
content: Well, I've tried it and it doesn't work as I hoped. On one hand it allowed me to adjust the reference with a sub-ppm resolution and get stable 10.00000V display for 10V input, on the other hand the meter produced three errors on the start-up test. So there is more to it as I suspected however I had no time to investigate further. The meter is restored to its previous condition and will go to Keysight UK soon (I've received a quotation from them at last) .CheersAlex

author: mrthanhapple
author_rank: Contributor
timestamp: March 11, 2025, 12:34:22 am
content: hi everyoneI have a 2020 generation 34461a model and it seems to measure capacitor capacitance inaccurately. I don't know if it's the same or if it's faulty, although it still has a capacitor scale. I measured smd capacitors, it almost couldn't read them, the numbers displayed jumped around, and when measuring electrolytic capacitors, it could be read but the results seemed inaccurate. I recently upgraded to FW from version 3.0.2 to the latest version 3.0.3 using USB, through a file shared by a friend.

author: KungFuJosh
author_rank: Super Contributor
timestamp: March 11, 2025, 12:58:30 am
content: Quote from: Alex Nikitin on March 11, 2025, 12:17:26 amHow painful was the quote?

author: Alex Nikitin
author_rank: Super Contributor
timestamp: March 11, 2025, 03:00:25 am
content: Quote from: KungFuJosh on March 11, 2025, 12:58:30 amRoughly what I’ve expected, my total is still well below the new meter price.CheersAlex

author: Alex Nikitin
author_rank: Super Contributor
timestamp: April 10, 2025, 08:24:31 pm
content: Almost a month passed and I've received the 34465AM back from Keysight UK, fully calibrated with near-perfect results. I've asked how much they would charge for a new handle and the price with the "repair" included (i.e. a new handle) was 2.5 times that of a full calibration only, so I've politely declined the "repair" part. Perhaps I could find a handle only on eBay or something... .On the first glance the meter is now not more than 5ppm off from my 10V/10K in-house references.CheersAlex

author: KE5FX
author_rank: Super Contributor
timestamp: April 10, 2025, 08:50:16 pm
content: Quote from: Alex Nikitin on April 10, 2025, 08:24:31 pmI just received a handle and pair of rubber bumpers fromthis seller. Took a few weeks to get them but they look great. They are either OEM items or OEM-quality copies. It says 15 available/5 sold, but the page also says 'Ended', so who knows.

author: Someone
author_rank: Super Contributor
timestamp: April 10, 2025, 10:34:42 pm
content: Quote from: Alex Nikitin on April 10, 2025, 08:24:31 pmService is always a target for increasing profits (screwing over the customer). Sad when parts used to be available at reasonable/believable cost.Shipping will be unpleasant on those parts so try and find something in country. I have several in excess if you get stuck.

author: HighVoltage
author_rank: Super Contributor
timestamp: April 11, 2025, 07:44:56 am
content: If you still need a handle, let me know.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 12, 2025, 10:24:11 pm
content: Another month passed and I use the upgraded and calibrated 34465A extensively, today I've repeated the 10V measurement from Fluke 731B (10NPLC, 30min) and placed it together with the original one from the first post. The fluke has about -1ppm/C tempco, and to my knowledge gives 10V +/-1ppm around 23C ambient. Today it was a hot day and the lab temperature was about +28C, so around -5ppm deviation was expected. The fan is ON, the skirt is ON. Altogether it was a very useful exercise and I am happy with the results.CheersAlexP.S. - I've decided not to replace the handle after all, as the meter sits nicely on my bench without it, thank you to all who offered to help!

author: nwind
author_rank: Regular Contributor
timestamp: May 17, 2025, 06:33:24 pm
content: So, what would be best recommendation for 34465A improvement?Better reference is for sure and looks like ADR1399 with airflow shielding is the most natural choice. Is there any detailed description of the process?Perhaps Alex Nikitin can help?Does it make sense to do it with LTZ1000A or ADR1001, or it's unnecessary complication and overkill?I would appreciate any good suggestions since I have couple of 34465A (great instruments). One of them a bit more "jumpy" and "drifty" than other one, and both of them can be improved with lower noise and overall drift VREFs.I assume with any VREF replacement it should go for the full calibration thereafter, and preferably to Keysight since there is no way to do it properly at home (correct me if I'm wrong).Top one is one which is drifty. Kind of worst than 34461AAll of them running continuously for ~3 days with reasonably stable ambient ~21C +/-1C, short banana plug wire shorting inputs. ACAL was used on 34465 before measurements. AutoZero is disabled on all of them and Null was pressed before data collection started. 10 PLC for all.

author: voltsandjolts
author_rank: Unknown
timestamp: May 17, 2025, 07:02:40 pm
content: FYI if anyone wants to join the party, there is this 34465A on bay right now, seller would accept offer for £775.https://www.ebay.co.uk/itm/256931900591I have no connection with the seller.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2025, 08:12:41 pm
content: The non AZ mode naturally has quite some low frequency noise and drift. For a long term low voltage DC measurement one would definitely use the AZ mode. The reference would be mainly relevant in the 10 V range, when measuring an actual voltage and not just a short.There may be a small effect with a short too, but that should be a small effect, as the 3446x meter have at least a little filtering for the reference.How much the non AZ mode dirfts is likely not effected by the reference - this is more a thing of the zero dirft of the input amplifier and this can naturally vary between units.

author: nwind
author_rank: Regular Contributor
timestamp: May 18, 2025, 09:24:26 pm
content: Similar with AZ enabled and with medium filtering (50 samples rolling average). 10 PLC

author: nwind
author_rank: Regular Contributor
timestamp: May 18, 2025, 10:37:35 pm
content: Funny, but 34410A shows much better distribution curve than both 34465A. Perhaps 34465A built-in histogram tool isn't the best, but I don't think so it would explain shape.10 PLC for 34410A, AZ is enabled. No filtering since it doesn't have any.Two results from two different AI tools to analyze and create diagram

author: Kleinstein
author_rank: Super Contributor
timestamp: May 19, 2025, 07:26:00 am
content: It is a bit unclear which ranges are used. Seems to be autoscale and thus the 100 mV.The noise (Stddev) with the 34410 is quite a bit higher. An additional white noise source can make the histogram look more even and "better". For a histogram test one would normally want digital filtering, especially a rolling average turned off. The averaging would reduce the number of effective samples and thus a more random histogram. A rolling average can also smoothen out holes (if they exist and thus mask potential issues).

author: nwind
author_rank: Regular Contributor
timestamp: May 19, 2025, 06:58:16 pm
content: You are correct, 34410A is a bit noisier in comparison, so I'm doing more controlled retesting for all of them with the same settings and statistically big enough dataset collected with PC

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 20, 2025, 09:03:02 pm
content: I've tried to repeat the same settings as described above, K34465AM with a short, at 10NPLC, medium filter, nulled at the start.CheersAlex

author: nwind
author_rank: Regular Contributor
timestamp: May 20, 2025, 10:16:37 pm
content: Thank you Alex,It looks better, but obviously depends for how long you run it.I did it for ~24 hours.Could you help with VREF upgrade questions (post above)?Nikolai

author: Alex Nikitin
author_rank: Super Contributor
timestamp: May 20, 2025, 10:29:03 pm
content: Quote from: nwind on May 20, 2025, 10:16:37 pmI did explain it with photos, should be enough, it is not a difficult upgrade. If you have a specific question, just ask.CheersAlex

author: nwind
author_rank: Regular Contributor
timestamp: May 24, 2025, 12:40:34 am
content: Thank you Alex!Nikolai

author: iMo
author_rank: Super Contributor
timestamp: June 07, 2025, 05:57:52 pm
content: For the reasons I still do not understand well I fully missed this interesting thread..I made some experiments with my lowest noise 399 (incl. the additional RC low pass filtering before the AD706) in my 34401 and I found out the soldering of anything directly to its socket pins (from the bottom side of the pcb, of course) increased the noise fluctuation (perhaps because of the larger thermal gradients).My 1399 reached 3khours burning-in already (and it shows half the noise compared to the HP399) so perhaps it is the high time to put it into the 34401..

author: aronake
author_rank: Frequent Contributor
timestamp: June 07, 2025, 06:28:42 pm
content: Great mod! I will hack one of my Keysight 34461a with a ADR1399

author: Kleinstein
author_rank: Super Contributor
timestamp: June 07, 2025, 07:46:28 pm
content: The 34401 has considerably higher ADC noise than the 3446x meters. One could still expect some improvement, but less obvious.