Fluke 8505A - persistent offset condition on shorted inputs - Page 1
Forum section: electronics / general

author: Rax
author_rank: Super Contributor
timestamp: February 06, 2023, 06:33:26 am
content: I am working on an 8505A that's exhibiting some persistent offset voltage displayed on the condition of shorted inputs.DC Signal conditioner adjustment of R53(ZERO) restores a zero reading on the display (it always does). Later (hours later, maybe a day, etc.) a persistent reading of a few uV on a shorted inputs condition would come back. This fluctuates pretty wildly, from 6-7uV to less than 4-500nV and sometimes "spontaneously" going back to a 0.0000(-3)V display reading. The average faulty condition seems to be around 2-2.6uV.Not sure this is at all relevant, but the unit had completely riot behaviors at the beginning (random and truncated display readings, rendering it completely useless and making it constantly reboot) which was traced back to a faulty motherboard. Replacing that restored operation of all functions. Units behaves very well to calibration adjustments - everything is within range and easily adjustable to compliance.I can't figure the cause of the offset issue though. C15 on the A/D module was removed to eliminate the possibility of its leakage causing this.

author: Rax
author_rank: Super Contributor
timestamp: February 06, 2023, 10:39:34 pm
content: My current working hypotheses:Leaky FET switches in the DC Signal Conditioner. I have some 2N4391s, 2N4393s, and maybe some J111s that may (?) work if it gets there.Some contact or board/switch leak in the input module. I'll likely pull this out and see if anything needs some TLC. This is because when I switch between F/R (to clarify: the F has the short, the R does not), I am not getting quite the consistency of switching in the display I'd like to see (so the switch may not quite be working 100%)Looking forward to other thoughts and input.

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2023, 01:32:57 am
content: So you are seeing up to 60-70 counts and typically ~25 counts of variation up and down at random over an extended period?Typically you would think that leakage/bias/tempco/etc would be found in the sensitive input circuitry found in the vicinity of Q18/Q19 and R52/R53, but remember that the 100mV range has 100X gain as well. So if you leave it shorted and switch up to the 1V range, do you see the same absolute variations, which at 10X gain would be 6-7 counts max and 2-3 counts typical? Also, have you checked the input bias current when you have adjusted R53? Can you put your 1M + capacitor across it for some time and see if the bias current is relatively stable or not?I've attached the schematic in case anyone wants to comment. You can't easily replace a lot of the parts in the center because they are all factory matched for tempco. As for leaky FETS, there's really only Q14/Q15/Q16 to worry about and at zero volts and zero gate voltage on all except the active one, I think you would only see the issue in the range controlled by the leaker. I could be wrong. I suppose you could remove them and jumper the pads for Q14 source and drain to eliminate that as a cause.

author: Rax
author_rank: Super Contributor
timestamp: February 07, 2023, 02:12:48 am
content: Quote from: bdunham7 on February 07, 2023, 01:32:57 amIf I switch to the 1V range (one up), I get a measurement fluctuating between 0.000006-7V. A little weird, because in the mV range I see something like .0033-.0035(-3), so not sure how that matches.Quote from: bdunham7 on February 07, 2023, 01:32:57 amGood point, I'll double check. I have not readjusted the bias every time I've adjusted the zero (not quite every time, but most times). Namely, I have not adjusted it when it was still within the specified range, so there was no adjustment needed.

author: bdunham7
author_rank: Super Contributor
timestamp: February 07, 2023, 02:34:12 am
content: Quote from: Rax on February 07, 2023, 02:12:48 amThe offset could vary a tiny bit between ranges or perhaps there is actually an issue there, but what I was thinking was leaving it in the 1V range for an extended period of time and seeing if it wanders around a proportional amount. Also, the actual gains of the physical amplifier are X8 (1V range) and X64 (100mV range), that's why the max voltages for those ranges are not 200mV and 2V, but rather ~312mV (20/64) and 2.5V (20/8). That gets sorted out in software obviously, so it shouldn't affect transfers between ranges, but there probably is some room for a little weirdness.

author: Rax
author_rank: Super Contributor
timestamp: February 07, 2023, 04:33:30 am
content: Quote from: bdunham7 on February 07, 2023, 02:34:12 amI will do that next, but for now, I removed the short and placed the RC across the input and so far I am not seeing it compliant (about twice the top of the acceptable range at about .0060-.0080(-3)), but it doesn't seem to fluctuate very much either. Of course, to compare the graph with the short readings, a much longer plot is needed.

author: Rax
author_rank: Super Contributor
timestamp: February 07, 2023, 03:52:43 pm
content: Quote from: bdunham7 on February 07, 2023, 02:34:12 amNot an extended period of time yet, but while the 100mV range shows an offset of about 2.2-2.6uV, the 1V seems to dwell at around .000005. It seems pretty proportional, given the datapoints from yesterday.Going up the range, on the 10V I see .000036-.000038V of offset.I think the above may rule out the FET switches, as the effect persists on all ranges.I wonder if I should just go ahead and re-null the ZERO and adjust the BIAS, and then watch it for a while and see if it drifts off.

author: m k
author_rank: Super Contributor
timestamp: February 07, 2023, 05:51:53 pm
content: Do you have other modes available?Maybe checking what's happening in RT6 can give something out.RT5 is another point but there you may need an external trigger, different part of the filter may be in the middle every now and then.

author: Rax
author_rank: Super Contributor
timestamp: February 07, 2023, 09:40:05 pm
content: Quote from: m k on February 07, 2023, 05:51:53 pmNot sure what you mean by that. Other than DCV? I have the ohms and RMS options installed, but not the current shunts.Quote from: m k on February 07, 2023, 05:51:53 pmAssuming both are in the DC Conditioner, I was unable to find RT6 (just RT5). Even so, I'm not sure I can easily probe this without an extender.On further observation, I am becoming increasingly weary of the inputs board and its switches. On a 2uV offset condition with shorted inputs, if I switch to rear from the front knob, the offset would go high (which I assume to be fine, as I have no short on the rear input), but if I go back to the front by switch, it would not necessarily go back to the 2uV indication. Not immediately, and not every time, and sometimes I feel it'd even improve the reading bringing it closer to 0.0000(-3)). So there seems to be some inconsistency of switching (mechanical contact issues?), or there's some grounding issue (hopefully not some MB PCB leak) related to the GUARD circuitry which triggers this behavior. Not confirming this is the case just yet, it may just rock the boat enough when looking at low level signals that it seems to be pertinent.I just did (another) comprehensive clear to make sure no constants (correct or less so) are messing with the hardware zero point. These have been cleared recently, in case anyone's wondering.

author: Rax
author_rank: Super Contributor
timestamp: February 08, 2023, 01:31:54 am
content: I am currently doing another readjustment of DC Cond ZERO (R53) and BIAS (R52). After all, I haven't readjusted these after eliminating (temporarily, for troubleshooting purposes) C15 from the ADC module. In doing this, I am using a cardboard sheet to put on top of the unit between tweaks to emulate conditions which would occur when the case is on (warmed up regime).In a prior run through this, it has not disturbed my ADC ladder/linearity adjustments - but rather helped. Which tells me the DC Conditioner is just playing with the zero level, shifting it around.

author: 1audio
author_rank: Frequent Contributor
timestamp: February 08, 2023, 06:17:57 am
content: I would try jumpering the hi to low on the board to remove issues from connectors and switches. Also check the dcr from connector to the board to look for funky connections. You could be fighting some thermocouple that's not obvious.Those are not easy to work on.

author: m k
author_rank: Super Contributor
timestamp: February 08, 2023, 10:58:38 am
content: All RTs are available through backplane but it's still not easy.Maybe underside is finally the easiest way.FR4 proto boards China is selling have edge connectors, but you must modify them with heavy hand, luckily only few contacts are needed.Old PC ISA boards have better connectors, if you can find any, and you must modify them also.I've made few KiCad protos, but their possible probe connection quality is totally unknown.By selecting Ohms you can change some DC SC relay inputs to RT1, Ohms output, guarded bus 23.I guess at least uV situation should be visible somehow.By selecting RMS you can bypass the whole DC SC.Then input is going to RMS board before DC SC relays.DC SC output RT6, guarded bus 5, is also blocked and RMS output RT6 is activated.Next in line is Active Filter, from RT6 to RT5, and then A/D.You can also try shorting input and backplane.Taking out Ohms and RMS modules is also a difference.If nothing is not changing anything it must be pretty close to input connectors.

author: Rax
author_rank: Super Contributor
timestamp: February 08, 2023, 02:02:51 pm
content: Quote from: m k on February 08, 2023, 10:58:38 amThank you, mk, very thoughtful input. Will explore what you're pointing out.For now, as I said, I've went through the R53/52 adjustment one more time, very methodically, lid on top of the unit, long pauses between each adjustment. I left it running overnight and it seems to be holding its adjustment on the 100mV range, at least. BIAS was at the top of its allowed range, on average, factoring its variations.Not sure how solid this .0000V +-.0002(-3) is supposed to be. In my case, it's very rare it'd stick within that range with its fluctuations (it's more like .0003-.0004V plus and minus fluctuations). And it'd drift up and down the range, within variations of just the last count (say, centered around .0005, some other times around .0002, and maybe then slide down to the nominal .0000V) - is this normal?...Also, the bias has bigger fluctuations, may go well beyond .0030 on either side (to even three digits occasionally), though I've been watching for a center of the variations to be at zero.These fluctuations seem to indicate to me there's something wrong with the DC Conditioner input circuitry. Maybe I should just be looking for a spare DC Conditioner and see if that makes it behave differently.

author: m k
author_rank: Super Contributor
timestamp: February 08, 2023, 05:42:42 pm
content: I have no idea what is typical with these machines, even less how back in the day typicals differ from typicals around these days.Guess is that more or less problematic relays must be a some sort of a latter typical, mechanical switches also.How to check if RMS is or isn't showing the DCV symptom is also a bit low of ideas.Maybe a longer time average of range/balance zero can show some notable differences, but is it a mark if there is none.My meters have also been out of regular use for a longer time so no idea after how long they can be treated as any kind of typicals.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 06:42:41 pm
content: Quote from: Rax on February 08, 2023, 02:02:51 pmI pulled out my 8505A, which I consider to be more or less working, and let it run overnight. This morning, with a short on the inputs, a towel over that area, AVERAGE turned on and the DC zero turned off, I get about 0.0005 (-3) with a few counts of fluctuation. Turning on the PEAK function and letting that run for some time (maybe an hour?) I recorded high and low peaks of 0.0009 and 0.0002, so 4 counts either way.The calibration routine is not a spec, those are allowed to be 'aspirational'. The spec for the DC100MV range states that the floor is 8 counts provided AVG is on and a DC zero has been done within one hour. Mine seems to meet that so I leave it be. Remember that these do not have any autozeroing or ACAL, so you are going to clearly see any drift in any point in the signal chain--it doesn't hide any of that.

author: bdunham7
author_rank: Super Contributor
timestamp: February 08, 2023, 11:25:02 pm
content: Quote from: Rax on February 07, 2023, 09:40:05 pmRT5 is the output of the Active Filter module. So this brings up something that is probably a shortcoming in the procedures in the manual, which is that there are three zeroes (DC Signal, Active Filter and ADC) and it is possible to get them pretty out of whack without anything in the calibration process catching it--at least until you run a pot to the limit. I have not had this issue, but it might be worth a check and is easy to do.Open the 8505A, flip up the DC Signal Conditioner and Active Filter tops. Turn the meter on, set to 100mVDC range and install an input short. Take another DMM set for mVDC and as much filtering as possible and connect the negative lead to TP1 in the Active Filter. Then with the postive lead check the voltages at TP3 of the Active Filter and then TP4 of the DC Signal Conditioner. Both should be reasonably close to zero. I have no idea what 'reasonably close' would be though! If they are out by more than you like, adjust R53 for as close to zero at TP4 (DC Sig) as you can get, then adjust R14 for as close to zero at TP3 (Active Filter) as you can get. These signals might be a bit noisy.Once you've done that, go back to and do 4-58 and 4-63 in the manual. Hopefully redoing 4-63 won't alter all your ladder adjustments, but I'm actually not sure. In any case, it shouldn't matter as you are just looking for a stable popcorn-free zero at this point.

author: Rax
author_rank: Super Contributor
timestamp: February 09, 2023, 12:56:35 am
content: Quote from: bdunham7 on February 08, 2023, 06:42:41 pmThank you for providing that data and overall information. A whole (work)day later I came back to seeing the offset at less than 8 counts (most of time within 2-3 of zero, or on zero). Not to jinx it, but maybe this confirms I may be in good shape and maybe my worry is not confirmed. I'll have to monitor if this gets back into the uV redzone during shorted input, but I feel this controlled stage may be my most reliable data.

author: Rax
author_rank: Super Contributor
timestamp: February 10, 2023, 12:14:05 am
content: Quote from: bdunham7 on February 08, 2023, 11:25:02 pmI redid the DC Cond ZERO and BIAS, then the ADC ZERO (which seems to be a prerequisite of the ACTIVE FILTER ZERO per p.4-22), then attempted the ACTIVE FILTER ZERO. But once I tried that, I think I understand why they require the AF input be shorted by itself (p.4-22) - I think the TP3(OUT) level tracks, and is higher than the TP4(OUT) - differently put, unless the AF is zeroed independently of the stage before it, one's chasing a fluctuating point driven by the slightly fluctuating ZERO of the DC Cond (which is how the "main zero" point from the shorted UUT inputs is "seen" down the line to the AF).In that spirit, I think I should be able to achieve the shorting of the AF input (the shorting of RT6 and RT2 on p.4-22, which I don't think I can easily achieve - where's this "Bus Interconnect Monitor?") by jumpering TP1/AF with TP4/DC SG?It's true that TP4/OUT on the DC SG is quite a bit a mislabel, as that's not the output of the module (RT6 is), but unless I grab the upstream leg of R1 on the AF module and ground it, not sure how else I can short its input (or what better options I have).

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2023, 12:51:26 am
content: TP3 (AF) is it's output to RT5 that goes to the ADC. TP4 (DC SC) is separated only by Q8, which should be conducting, from RT6 which is the DC SC output to the AF. The bus interconnect is a device that you install in place of the isolator for maintenance purposes. I think setting TP4 (DC SC) to as close to zero as possible relative to TP1 (AF) is probably good enough to then set the AF zero, but if you want to short it as in the manual I don't see why a jumper from TP1 (AF) to TP4 (DC SC). wouldn't be good enough. Or, you can peel the AF out of it's shell and solder a small wire to R1 and run it upwards so you have access to it. Soldering in some handy test wires is how I deal with not having an extender card on these and the 5100-series calibrators.

author: Rax
author_rank: Super Contributor
timestamp: February 10, 2023, 01:54:21 am
content: Quote from: bdunham7 on February 10, 2023, 12:51:26 amI was able to short the upstream leg of R1 to TP1 - one of those moments you congratulate yourself for getting those "witches hats" jumpers for a couple of bucks from the evil retailer - and the adjustment of R14 goes very well, except that the 0.000000V point on the screen (per p.4-22) and the lowest voltage on TP3 (I had to whip out my 3478A, which has the most sensitive DCuV range of anything I have around here) are not the same point... I see about 15uV on TP3 with zero on screen, while I can go below 1uV if I dial in the lowest point I can on TP3.Shall I adjust per indication of the SM (which is based upon indications on the screen), or to the lowest DC out of the AF TP3/OUT?... [scratching head vigorously]__________________________________________________________________________________________________ I lean on doing it per SM, in case the adjustment is intended to compensate for other offsets elsewhere and is setting a relative zero point. Or other adjustments during calibration shift the zero point intentionally.

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2023, 02:38:12 am
content: You'll see that step 3 of that procedure has a statement that the ADC is required to be "working correctly". It may not actually matter, but my idea was to set the AF output to zero as measured by your external meter and then set the ADC zero. Then all the zeroes add up to zero vs. 3 random values trimmed to zero in total.I did some stability comparisons for you. Last night I grabbed a spare AF and DC SC, plugged them into my 8505A, did a very quick zero and let the meter warm up a while. I set it for 10V, zeroed it and then let it run overnight with the PEAK and AVG enabled. My heat goes down overnight so it saw a temperature cycle of 4-5C, and that resulted in the lowest value overnight of -10 counts and a high of 0 counts. As the room warmed up in the morning it went up towards 0 counts again. So the 24H stability with a 4-5C temperature change is 10 counts or 1ppm, the short-term stability appears to be 1 or 2 counts. I'll do the same thing on the 100mV range tonight. So far it looks like the 1H stability is 3-4 counts or so with no real continuous drift either way.

author: Rax
author_rank: Super Contributor
timestamp: February 10, 2023, 04:04:16 am
content: Quote from: bdunham7 on February 10, 2023, 02:38:12 amWhich is why I started this last go around with a fresh ZERO-ing of the ADC. Not sure that wholly qualifies as "working correctly" but WTW.Quote from: bdunham7 on February 10, 2023, 02:38:12 amI understand your approach with this. I'll think on this some more. Something tells me your approach may serve the linearity adjustments better, including all zeroes being in a more objective place. The inherent non-linearity of these units vs others may be part of it.Quote from: bdunham7 on February 10, 2023, 02:38:12 amThis is very useful as a comp, thank you. My unit seems to be at that point by now - though still collecting data (do I ever stop?...).To give you an example, after I readjusted the AF ZERO, and the ADC zero prior to that - both open case - the zero point was 2.2-2.0uV off. After having the lid on for a couple of hours, my zero has gone back to apparently less than 3 counts from zero (stays between -.00001(-3) to .00002(-3)). Which is the best it's even been. The effort last night to adjust accounting for the "case on" thermal regime may be paying off. So apparently it almost qualifies for the adjustment +-2 counts criteria! I think we've both observed very long warmup times for these units vs. others from the series, so that seems to match.Given yours has gone through a 4-5C cycle may throw off my point here, but maybe I have a unit that's a bit more sensitive to varying environmental conditions?But maybe this is now back on a good track. I'm hopeful.

author: bdunham7
author_rank: Super Contributor
timestamp: February 10, 2023, 07:37:23 pm
content: Quote from: Rax on February 10, 2023, 04:04:16 amSo last night's thermal cycle was in the 100mV range, shorted, etc. I got a superficially similar result--I started at 0.0000mV on a fully warmed up meter, it appeared to drop to -0.0010 overnight and now that it is warming up, it's back near 0.0000. The PEAK LO was 0.0012 and the PEAK HI was 0.0000. Short-term stability looks like 3-4 counts. I'm pretty sure it would be within the 8-count/hour limit even with the temperature cycling. I say superficially similar because the ~10 counts look similar to the previous 10V test, but while the gain difference is 100X, the actual difference in the excursion is ~10X, ~1uV vs ~10uV. It's not obvious that any single source can be identified as the cause--not that we need to since it is within specs.I'm actually reconsidering how I did my original 8505A, which IIRC was not 'triple-zeroed' in the manner I suggest. It works fine, but needs a loooooooong warmup, like 5 hours, to be golden. I put my original AF and DC SC back in and set the newly characterized ones aside in case they are needed. I'll try the same drill again tonight with these original parts for comparison.

author: Rax
author_rank: Super Contributor
timestamp: February 11, 2023, 04:58:23 pm
content: During one more day or running, I think I'm probably within .0012(-3) and .0000(-3)V. Occasional jumps over .0012 may be explained by air movement in the room (I am covering this with a piece of cardboard, so far from enclosed), but overall seems pretty stable. Short terms stability seemed to be within 4-5 counts.This morning I tried a "double-zero" procedure (based on screen readings):10V range, UUT input short - ADC R810V range, UUT input short, short on AF input - AF R14And they seem to chase each other a bit - meaning if I try for a perfect zero, I'll run out of pot range - which I find pretty odd. But the divergence between the two seemed to become smaller with every adjustment, so I essentially settled for under .000001 on both.After that, I re-trimmed the "third zero" (100mV, UUT input short, DC SC R53) seems to stay within .0006(-3), though mostly closer to zero than that sounds like. I'll let it run for a while again and see if it drifts either side.

author: Rax
author_rank: Super Contributor
timestamp: February 11, 2023, 05:36:28 pm
content: Not to jinx it, and not really thinking this will stay exactly so within the next 24hrs+, but after all these tweaks, and maybe 30 minutes for things to settle, I am pretty much seeing zero on all "three zeroes."Will keep on observing and reporting. Hopefully this is a good adjustment point. I'd wager in saying is this doesn't satisfactorily stay within reasonable fluctuations (8 counts on the DC SC zero), I probably need to investigate each module and see where the issue may reside.

author: Rax
author_rank: Super Contributor
timestamp: February 12, 2023, 05:20:28 am
content: More empirical data below:Warming this up takes as much as bdunham7 says, apparently. After taking the DC SC module out and cleaning its board header contacts (IPA) - suspecting that replacing the MB with all the contacts involved may have to do with the issues - I watched it starting at about 8.5uV on zero (stone cold) to inch closer to the expected zero at about 1.8-1.4uV by the 3rd hour of warmup.Some more time later, it settles at around 2.2-2.4uV, so no, this isn't sorted outI think I need to start swapping modules to see where trouble lies.What my data (these "three zeroes" pursuits) seems to indicate is that, while the ADC and AF modules seem to reasonably maintain their zero points (though, notably, their check points are at 10V, so an "easier" bar), the DC SC seems to have a harder time to stay near zero. So my main suspect currently is the DC SC module.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2023, 09:06:39 am
content: The amplifiers in the DC input and low pass filter secion can have offset drift. The offset adjustment also effects the temperature drift. Ideally (moderately good transistor models) zero offset also comes with zero drift (e.g. the offset proportional to the absolute temperature). However details like mechanical stress can still lead to some residual drift (maby in the 0.2 µV/K range) even if the offset is zero. In addition there can be thermal EMF effects leading to a more or less stable offset.So on warmup it is not surprising to see some 5 µV of offset drift, as these old meters consume quite some power and thus run relatively warm.In therory one could consider doing the adjustment of the input stage for low drift and not for lowest offset. However with gain at the front, there is no simple compensation with the offset in the filter stage.Ideally one would adjust the front end offset in the range with highest gain (200 mV ?) and than adjust the filter part with the front end in the 10 V range).I would not really expect to have dirty contacts to have much effect on the drift. It is more just dust or variations in the power supply and even the mains voltage (migher mains leads to more heat in the meter).The filter module contains quite a lot of switching FETs and there are reports of some of these failing and cause extra leakage current. This can also cause some offset and possible drift.

author: Rax
author_rank: Super Contributor
timestamp: February 12, 2023, 03:50:47 pm
content: Quote from: Kleinstein on February 12, 2023, 09:06:39 amI am seeing so much time needed for full warmup and so much drift during that, that I almost wonder if this heat management issue should not be addressed by itself first to make this a more convenient in use instrument. Otherwise, this would need to be an instrument running 24/7 to be useful and then I'd like a way to turn off the screen to preserve it.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2023, 07:03:53 pm
content: The rather large and quite full case and no fan makes the warmup slow. A fan could make the warm-up faster (and a lower temperature), though it can also help collect dust.For the fiter part one could try a test with some 5-10 K in parallel to R3 and this way make leakage of switching JFETs considerably less relevant.A 0 ohm link may allow to much current flow during transients / power on. So the resistor is to be on the safe side with the current.An alternative test would be with some 5-10 K at the amplifier input to ground.

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2023, 11:28:05 pm
content: Quote from: Rax on February 12, 2023, 03:50:47 pmUsually they're good enough that you can zero them during warmup and get an OK reading, you just need the full warmup for the last 1-2 digits (not counting the 7.5 mode) and the best stability. They are probably 'in spec' well before they are perfect, or at least as good as they can be. I think these were mainly intended to be powered on 24/7 and I think the red LED displays last nearly forever. They run at a pretty low power level. I thought about trying a fan, but it would require some real effort. If you removed all the module cases and ran bare boards, you could use an internal fan or two to reduce temperature gradients--that might be a big help.

author: Rax
author_rank: Super Contributor
timestamp: February 17, 2023, 07:21:48 pm
content: I'm still not sure what to conclude, but it seems now, after a few days running idle, being also off for a couple of days, and then being turned back on and let warm up for a few hours the offset seems to have stabilized very nearly dead on zero (all three). The DC SG zero is actually almost dead on .0000(-3)V, with a bit of fluctuation between the + and the - polarity of that (and when straying slightly from it, it'd occasionally go to up to -.0002(-3)V, which I have no issues with).The other two "zeroes" are less than 5-7 counts off (.000005-.000007V).If this stays this way, I think I can go ahead and readjust the ADC ladders and the rest of the adjustments. Not sure what I should be doing if after that I get an offset in the uV range again.

author: Rax
author_rank: Super Contributor
timestamp: February 22, 2023, 05:39:54 am
content: Maybe I just did something very stupid.The unit was on the 1V range, and was measuring either 1V or 2V from my calibrator. Then, without thinking, I switched the calibrator to 10V output. Now I seem to see one order of magnitude higher divergencies from the expected value than before on all values I'm looking at. After the last adjustments, it was showing so close to the actual values I wasn't even sure it'd benefit from software calibration.I read in the SM that during cal mode, some overvoltage protection mechanisms are disabled. I am not sure what the AVG mode was switched to, but when I first pressed it after this has happened, the first value shown was "Err off." So I assume error correction (overvoltage protection?) was on beforehand, and during the excessive voltage exposure?Did I just break something?

author: Rax
author_rank: Super Contributor
timestamp: February 22, 2023, 05:11:46 pm
content: Quote from: Rax on February 22, 2023, 05:39:54 amMaybe I didn't break anything (though I'd still like to hear experience with the effects of breaching events against the warning at 7-8.3). I think I'm fighting some competing offsets from both the 8505A and the DP8200 used as the source for the reference voltages.I just (re)adjusted both and the voltages seems to fall closer in line to before. I guess it's what happens where all instruments one's using are "work in progress..." (or most, as my 8502A does have brand spanking new stickers...).

author: Kleinstein
author_rank: Super Contributor
timestamp: February 22, 2023, 05:29:57 pm
content: I don't know the details of the cal procedure, but the normal input should still be save up to some 50 V. So I don't hink it shold do some real damage - maybe stress some the the BJTs used as a Zener for clamping. Things could be different if the voltage is applied to some more internal nodes.The calibrator offset should not be such a big issue, as a zero should be a true zero and not from the calibrator.There is already a chances to get a few µV of offset from handling cables and stacking instruments.

author: Rax
author_rank: Super Contributor
timestamp: February 23, 2023, 03:03:26 am
content: Quote from: Kleinstein on February 22, 2023, 05:29:57 pmThank you very much, Kleinstein, for providing some peace of mind... There's these points during a project where uncertainty (to borrow a term...) is so high, and thinking on it with the... anxious metrologist is extremely useful and appreciated. Thank you.I am pretty certain (that again!...) there has been no damage to the instrument, but it's been a pretty weird 24 hrs of monitoring and assessing.

author: Rax
author_rank: Super Contributor
timestamp: February 23, 2023, 03:06:14 am
content: Quote from: Kleinstein on February 22, 2023, 05:29:57 pmI am starting to consider shielded cables and paying close attention to the GUARD wiring and other finicky setup aspects. I am looking at ppms, literally, so likely my "noise" and weird shifts and offsets need to be managed at that level.

author: Rax
author_rank: Super Contributor
timestamp: February 24, 2023, 09:07:07 pm
content: Back to the original topic of this - reading through the the datasheet, I see that it mentions that "Zero Stability" - though not sure if it only has to do with software zero or all zeroes - is "less than 5uV," so that's not so off from what I've seen through this.