-
Hello, I have a similar plus also a worse problem with my 7081.
There are two symptoms:
1) Noise level is more or less the same as described here. At 7 digit resolution it matches the same noise level as of a new Rigol 3068 at 6 digits, 100NPL. From what I gathered here, it may even be within specs. But this is not the main problem.
2) The main problem is a sudden drop in the reading after a few minutes of warmup. This is 100% reproducible. This occurs for all measurements and already in the 10V range. There is sudden approx 80uV drop whenever the temperature crosses a certain line. By fine tuning I even got it to oscillate. I will post pics below. White trace is Rigol 3068 and red trace Solartron 7081.
I spend hours on this, to no avail. What I tried was:
a) Monitoring all sorts of PS voltages in sync with the drop - no correlation seen.
b) Exchanging IC201.
c) Using cold spray selectively on all sorts of locations, nothing systematic
d) checking the PLL voltage (2.5V) whether it jumps in sync - no correlation seen.
e) checking +/- 10V reference voltages, whether they jump in sync - no correlation seen.
f) substituting a VRE102CA voltage reference in lieu of Z-diode, but jumps continue to occur. If at a later stage this works better than the built-in Zener, I may keep it there.
3) Then there is a mysterius property which seems unrelated but puzzles me.
Probably it is a trivial mistake on my side, but so far I couldn't figure it out (without disassembling the board). It is about TP305, supposedly 0V in the reference section. It is located on the PCB between IC304 and IC307.
I measure 15Vrms 50Hz sinoidal AC between this and the 0V point TP505! TP505 itself has perfectly stable +/- 10V ref against the positive and negative reference points, TP302 and TP303. But TP 305 is per schematics the proper 0V point, and carries this high AC voltage. How come? When powered off, I measure like 2Ohms between TP305 and TP505. This doesn't make any sense to me - it appears like a transformer winding, low Ohms but a lot of AC (??)
Perhaps someone can enlighten me... and also give a tip about what to do against this voltage jump.
Thanks!!
-
The 15 V AC at TP305 is strange. Is it possible the meter is in ratio mode ?
This might couple something else to this point.
This AC voltage might cause trouble with the ADC for current setting to the reference and thus cause the jump. So it is like a wonder anything works with this.
The solartron7081 is known to be relatively noisy. I don't really know what the noise source is.
Much of the noise is correlated between adjacent readings - so longer integration time really helps. This also points towards things like "noise" coupled from the switched mode supplies as an important noise source. Though there might be still quite some noise from the reference diode.
Is the 80 µV jump also appearing with a short input, or just when measuring 10 V or similar ? So it an additive jump or a change in scale factor ? The voltage at TP301 might be interesting too - as opto-couplers are somewhat prone to aging and might react to temperature.
-
Thanks for quick answer. I will check ratio mode but don't think it is. But the optocoupler is a good idea, I didn't think about it yet.
Actually there is a pre-history, a few years back: I had to exchange a couple of them (ILQ74 or so) before, since the whole instrument stopped working after a few minutes. There were thermal failures all over.
I will check and get back,
thanks!
-
Doesn't the 7081 perform an internal cal every 15 minutes or so? I thought there was mention of it on the volt nuts mailing group. There was a bug in the firmware that was fixed by one of the members. It would add an offset to the measured value. I think it was Mickle T.
If you haven't downloaded his notes from ko4bb, I would recommend looking them over. There is a lot of good info in the large download.
Edit: Added chart showing AutoZero bug from ko4bb download.
-
Hello, thanks for your answers!
Is the 80 µV jump also appearing with a short input, or just when measuring 10 V or similar ? So it an additive jump or a change in scale factor ? The voltage at TP301 might be interesting too - as opto-couplers are somewhat prone to aging and might react to temperature.
At 5V the jump is like 40uV, and in the 1V range at 1V it is about 8uV and no jump for short circuit, so it is proportional to the voltage. Sounds like reference voltage but surely this isn't the cause..
At TP 301 there is no jump or anything peculiar seen when the reading jumps.Doesn't the 7081 perform an internal cal every 15 minutes or so? I thought there was mention of it on the volt nuts mailing group. There was a bug in the firmware that was fixed by one of the members. It would add an offset to the measured value. I think it was Mickle T.
Yes I downloaded this great collection of data and am trying to sift through. Thanks for pointing out!
The chart you posted is most interesting, especially the error is of the order of the jump I see. The jump however is generically not periodic and happens always at about the same temperature (I track the temperature by comparing the offset against another DMM. The offset decreases at a given rate over time, depending on whether the case is closed or open, room temp, etc.).
Nevertheless it could be due to combination of factors, one of which is autozero. What I can try is to re-establish, by fine-tuning, a periodic pattern and see what the period is - if it would be about 15min, it would tell something!
I had expected to locate any temperature sensitivity by a cooling spray but never could pinpoint it. Of course some components are sensitive, like the chopper amp IC201, but exchanging that didn't have any influence. So if this is indeed software related then no wonder!
Thanks! -
If I understand the problem with the AZ bug right. It is very well possible that this could explain the trouble:
looks like they don't wait long enough for the ADC / amplifier to settle to the 0 value and this way add an error in the AZ measurement, about proportional to the measured voltage.
I would guess one could start the AZ phase also manually or in software - so it should be reasonable easy to check. Though one would only see a difference if the instrument has actually drifted or changes the measured voltage since the last Adjustment otherwise the wrong offset would just persist. -
Today, I burned the IC803 Eprom with the patch v1.1 of the ko4bb download. Alas, the problem persists.
But here is an interesting observation: I checked the data files I had collected over time and indeed, the jump occurs always in the order of 15min! See the attached plot of today, with the patch applied. Horizontal unit is 10s per bin, makes about 800s, gives 13min...and .there was a bit time before the data were taken. So indeed it looks as if it is somehow related to the AZ.
Edit: of course it may be that it just coincidentally, a certain temperature is always reached after ~15min given constant ambient temp. I will run the whole test overnight at low temp to distentangle time vs temperature, once for all.
Edit2:
I added a plot where I re-initialized a few times the 7081 a few moments after a drop. As one can see, the value before the drop is resumed, until the next drop happens. The drops appear typically around 15min after but not precisely, there are variations. I can hear some relays click after 15min, and assume that this is due to AZ. However, I didn't observe any coinicident jump. Moreover, as you see there was a drop to zero which was remedied by itself, and in the end the whole thing crashed and got stuck at output zero.
So there seems to be a circumstantial evidence that AZ is involved but the timing is not 100% accurate.
I feel about giving up -- if the single jump after ~15min is 100% reproducible, as it looks, it can be taken into account with the calibration (which is messed up anyway). From the various thermal defects I encountered so far the 7081 must have has a pretty hot pre-history..
Thanks for all!
-
Send Mickle T a PM with a link to this thread. He has a pretty good understanding of this meter, maybe he can help diagnose your issue?
-
I think there is an odd thing with AZ, but I can't understand what is
-
My guess is, that at the about 15 min intervals there is more going on than just auto zero. It could also include gain adjustments. At least even with AZ off, there are still things happening at those intervals.
If there is an adjustment, I would expect a break, with no new data coming in.
The spikes somewhat suggest that it takes longer than planed to recover from these adjustment measurements. -
Hi Mickle,
thanks for chiming in. The plots that you show, are they as they are supposed to be, or pathological?
To me they seem to make some sense, a periodic adjustment until the whole thing has stabilized. On the other hand, for precision measurements I'd expect a somehat less bumpy behaviour ;-)
My S7081 shows similar behavior after the first jump. Attached is another, long term plot where the red trace is from a Rigol 3068 at 6 1/2 digits, the blue is for the ailing S7081 at 7 1/2 digits. Clearly even after the jump not really a high-precision behavior....
-
3 things come to mind when i see your log having little ticks. when the AZ switches on/off, the shift of "reference" must be measuring part A of circuit then it go to part B, i am thinking maybe the common ground is not on same level? and small 0.2- 0.3ohm difference in 2 parts of the PCB create small difference (this problem appeared in my old K2015, i bridged all major sections with a 1.5mm, K2015 power traces are tiny)? 2nd is maybe bypass capacitors no longer working well? when certain circuit turns on and the draw create a dip (i am purely randomly guessing this 1)? 3rd : can the time base of AZ be changed/hacked to smaller period? ... or the most remote of all suggestions, what if you create an external clock to trick it to do a faster AZ period? --> i have no idea how the solartron works ! so this is likely way off at planet jupiter !
-
The plots that I show, are Ok.
-
It looks like the plots are for 3 different meters. So it is a little hard to compare the effect of have AZ on and AZ off. The main conclusion is that the glitches are a common thing, with an without AZ active.
I am a little confused, as there is a way to do AZ with the input amplifier includes and one without it (e.g. only the ADC). I would expect to the AZ including the input amplifier, though the chopper stabilized amplifier might not drift very much - though not that much drift is expected for the ADC too. So I am wondering how much of the adjustment steps is due to gain and AZ adjustments.
I am just wondering which part of the circuit is responsible for the glitches just after the AZ / adjustment phases. Is this the input amplifier (e.g. chopper stabilization), the dielectric relaxation in the integrating capacitor or something else.
The input amplifier looks a little strange in three aspects:
1) They use a BJT pair for the copper part amplification instead of JFETs.
2) use of OP05 as an AC amplifier - could be already limited by GBW.
3) Cross over from the direct path to the chopper path seems to be at rather low frequency
But I can not see an obvious reason for slow response to to large signal steps. At least one be able to test this independently. -
Well, a few words in support of AZ
1. Drift correction includes only ADC zero and did not touch a full scale constants or an Input Amplifier.
2. If drift correction is disabled via the terminal command "drift off", there is no any glitches in the measurement results. In other words, in known working condition the Solartron 7081 have no any other sources of glitches.
3. The main (input) amplifier is very stable and "silent", and does not require any seft/auto/e.t.c. corrections. The noise floor in 1:1 configuration and short input, measured by N1a nanovoltmeter, is 10...20 nV p-p (DC-1 Hz).
4. It is possible to change AZ interval by swapping one byte in the Parsing_7F procedure. But I think it makes no sense.
-
I very much doubt the input amplifier could reach 20 nV_pp level, even an 20 nV/sqrt(Hz) would be on the low side.
Already the two 15 K filter resistors should give about 22 nV/sqrt(Hz) of unavoidable noise. An BJT amplifier following this should about double the noise power (3 dB NF is already good for BJT). So likely more than 30 nV/Sqrt(Hz). And thus maybe a 200 nV_pp noise range, if there are no large other noise sources. This would be still a good value.
Changing the rate of AZ measurements does not make that much sense. It might be a good idea to increase the delay for the ADC to settle, when changing from normal measurement to the zero measurement. The glitches somewhat suggest that it takes some time for the ADC to settle to its final value. The source could be DA in the integrating capacitor.
Thus too fast a measurement would introduce a small error. One cold test for this by doing and Zero compensation with a high input voltage applied (e.g. 10 V) before, than switch to a shorted input and after something like 1 min do a zero adjustment again. The jump during that last zero adjustment should show the error from the first adjustment (plus some drift).
In principle there could be constants to correct for the fraction carried over from earlier readings. -
The source could be DA in the integrating capacitor.
Thus too fast a measurement would introduce a small error. One cold test for this by doing and Zero compensation with a high input voltage applied (e.g. 10 V) before, than switch to a shorted input and after something like 1 min do a zero adjustment again. The jump during that last zero adjustment should show the error from the first adjustment (plus some drift).
This is exactly what is happening: after a large voltage change (esp. switch-on), the next AZ (also when manually triggered) creates the jump of 80uV or so, the subsequent ones are fine! So this is coming closer to a resolution.
I exchanged the 0.1u integrating cap C201 with a MKP for the time being, there was no change however. Also the noise stayed as before. But at any rate, I am expecting a high precision 0.5% Polystyrene cap and will put it in and see what happens.
This was really helpful and is highy appreciated, thanks!
Bertik -
The integrating cap is already specified as MKP. So changing to another MKP is not changing much. I am not sure than polystyrene is really better in that respect. There is no need for high precision with that cap - 10% tolerance would still be OK.
For the noise it might be interesting to check for the type of noise, by looking at correlations in subsequent readings. The analysis earlier in this thread suggested that much of the noise (in the 6 digit mode) comes from the start and end of integration, so the first and last comparator readings. The nice thing about this type of noise is that it goes down linear in integration time and not just with the square root as white noise would do. So noise should go down with increasing integration time relatively fast. This also applies to taking data and average later on.
I suspect noise from the power supply could be a factor, as the noise is higher than what is expected from the circuit diagram. -
Here is a brief update. Excerpt of my blog .
While investigating there was a serious setback: when checking the current setting for the reference Zener, I inadvertently shorted the test pin 301 with the – input of IC 304 (refers to service manual). There was a sense of smoke, which was surprising, since this test pin is supposedly at ground. I checked and alas, there was 17V AC voltage at it! With source impedance of like 2 Ohm… how could this possibly be? Was it the effect of the short? How can 17V AC go straight into the reference voltage section?
After disassembly it turned out that the test pin was wrongly placed by the manufacturer, namely on a via. And indeed there is 17V AC voltage there, fed across half of the board to a few mm near the reference section (!). It enters IC303 which is close by. So that explained it. As a consequence, IC 301 and IC 303 where blown and possibly more.
I decided to by-pass this problem by substituting an (aged!) +10V/-10V precision reference VRE102CA of which I had salvaged a few from trash. It can be made fit perfectly into the reference section of the 7081, by attaching the outputs to the reference terminals TP 302/TP 303 and opening the base and collector connections of TR 301 and TR 302. This works as good as before, the reference has like 1ppm/C temperature stability and like 3uV pp noise 0.1-10Hz.
Incidentally, does anybody know what the corresponding specs are for the 7081 Zener?
For the time being I’ll leave the reference section like that, since the excessive noise problem is the next pressing one. I will report on any progress in the future. -
The good thing is that chances are good that only IC301 and IC304 (741 OP) are blown and there should be replacement for the DAC available. This part is only for selecting the exact current for minimal TC. Even a direct replacement could influence calibration a little. Worst case, or for a first test one could replace that circuit part (optocouplers, DAC and OP) with a trim pot.
A 3 µV_pp noise spec is actually quite good - depending on the frequency range.
The LTZ1000 is specified at 1.8 µV_pp typ (3 µV max) if scaled to 10 V.
With the noise specs of the zeners, there is not much data and the value scatters a lot. The nominal value can be quite poor, but good instruments usually use selected ones. So even if one has a data-sheet that gives typical noise figures this is of limited use. It would likely be more useful to actually measure the zener's noise.
Reference noise is not that important for the noise with a shortet input. In this case one the noise densitiy in the 160 Hz (forcing signal) range should enter. If one really wants one could filter this. -
Thanks for your answers!
The good thing is that chances are good that only IC301 and IC304 (741 OP) are blown and there should be replacement for the DAC available. This part is only for selecting the exact current for minimal TC. Even a direct replacement could influence calibration a little. Worst case, or for a first test one could replace that circuit part (optocouplers, DAC and OP) with a trim pot.
The smoke actually came from the resistor array R302 feeding into the optocoupler IC303.
Despite a burn mark, the resistors are still fine. But the driving IC 830 might have been damaged as well, I didnt yet fully check. All the chips are easily available, so far I just replaced the 741. But it is a pain to replace IC's, when PCB traces tend to peel off etc.
Thus indeed the trim pot is a good idea. All what the DAC plus periphery is doing is to supply a programmable voltage resp current into the Zener. From the setup procedure sheet a numerical code "064" corresponds to zero and "124" to -9.18V (why not 128??) at TP301. From the originall NVRAM settings I know the stored value was "104". Thus it is an easy exercise to compute the necessary voltage.A 3 µV_pp noise spec is actually quite good - depending on the frequency range.
Normally the VRE102CA are spec'd to 6uVpp. I measured a few of them, and they went from below 3uV to 5uV plus a stinker with 30uV+. Two in parallel went down to 1.9uVpp at 10V. See attachement (100mV corresponds to 1uV input). For this the DIY low noise amplifier of the other thread was very useful.
The LTZ1000 is specified at 1.8 µV_pp typ (3 µV max) if scaled to 10 V.With the noise specs of the zeners, there is not much data and the value scatters a lot. The nominal value can be quite poor, but good instruments usually use selected ones. So even if one has a data-sheet that gives typical noise figures this is of limited use. It would likely be more useful to actually measure the zener's noise.
I may give this a try, which is a challenge in the noisy environment of the DMM.
-
Selected 1N829 with R305 & R306 dividers provide a 6.4 ppm/y of long-term stability of the S7081. Guaranteed!
VRE102CA have a much worse specs, so downgrades a S7081 perfomance to HP 34401A level. -
My 7081 is showing stability (drift) issues as well, maybe this also sheds some light on the original issue that was brought up here (and not resolved).
I traced my issue down to an input leakage current, which causes a voltage drop accross the protection resistors R5 and R6 at the input. The leakage varies, so does the voltage drop and hence the voltage read. It is about 25mV too low (10V input). I removed D101 and C103, so these can be eliminated as a cause.
So seems the input amplifier has a leakage issue. So my thoughts are it could be TR401, less so TR412. Since I have not found these, hard to just replace them.
Would not think TR406 to 409. What would the Solartron experts here think, anything else? D401/408?
And what replacement part could be used for TR401? Dont find the original or its data sheet anywhere. Maybe IFN401/U401? None of the parts btw. are cold-spray sensitive.
Any other thoughts? -
The not really solved original problem is something at the ADC itself: Somehow influencing the single comparator triggering time and thus the error of readings in sequence is anti correlated. Leakage / drift of the input amplifier is a completely different thing.
How large is the leakage current about ? One could measure it with something like 1 M or 10 M at the input.
There are quite a few parts that could contribute to leakage. It might be possible to separate the chopper part by unsoldering R407/R408 and maybe R406. This would isolate possible leakage and variations in charge injection from that area. -
The not really solved original problem is something at the ADC itself:
My meter is still waiting to be fixed, but I have to finish my phd thesis first.
Drift was fine, noise is my problem.
@acbern: Where do you come from? Perhaps it can help to swap parts between the meters.