Author Topic: Solartron 7071 / 7081 last digit is jumping all over the place  (Read 71720 times)

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Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #75 on: February 27, 2016, 09:16:49 am »
Seem to be a rather tough challenge to get down the noise.
One thing might be doing a real noise measurement, to know better what noise is there. It looks like the ADC is continuously running and different resolutions / integration times are just a question of reading at different times or doing averaging in software, which would result in the same. So one would get the most information from taking data at a relatively high rate, but still not missing any conversions or too much resolution, and than do FFT on them.

C201 might still be an issue, as the original specified polypropylene type, not MKS. Through I would expect more like minor non-linearity from this.

If we are at caps, C212 might be critical too, if it shows leakage.

With the ICL7650 for IC201 there might still be a problem: this OP is known to show higher than specified noise in several cases. Though the noise from this OP should give something like 2 µV_pp in the 0-10 Hz range only. So for a 400 ms integration time this should be really low, even if several time the specified noise.

IC203/204 might also want local decoupling. Though I doubt they are the noise source, changing then could be relatively easy / cheap. There was not much jitter at the output of IC205, so I doubt this chip is at fault.

Another point might be the reference section. The ADC relies on the positive and negative 10 V refs to be connected. So noise in the ratio will show up as noise of the ADC, even if measuring 0. So IC306 is critical in this way though at nominal specs it only contributes something like 0.2 µV_pp for 0.1-10 Hz. The forcing waveform might give some dynamic load to the references and thus could give minute ringing that might also contribute.

Another point worth looking at might be the cross over of the two OPs in the integrator, I don't know how accurate the design is (likely done before spice simulation became popular). It might be worth simulating that part and see if it might react sensitive to tolerances in the caps or amplify noise from some bands.

p.s.: I did a short similation: the cross over and tolerances are not critical at all, so not problem here.
« Last Edit: February 27, 2016, 10:35:47 am by Kleinstein »
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #76 on: February 27, 2016, 08:08:36 pm »
Seem to be a rather tough challenge to get down the noise.
One thing might be doing a real noise measurement, to know better what noise is there. It looks like the ADC is continuously running and different resolutions / integration times are just a question of reading at different times or doing averaging in software, which would result in the same. So one would get the most information from taking data at a relatively high rate, but still not missing any conversions or too much resolution, and than do FFT on them.

My plan was to measure the GLUG signals not only for a few seconds to possibly see any issues. I would try this first, because the output of my function gen is ground referenced. Therefore, my scope GND is now shorted to the -15V rail. This will restrict some measurements.



C201 might still be an issue, as the original specified polypropylene type, not MKS. Through I would expect more like minor non-linearity from this.

If we are at caps, C212 might be critical too, if it shows leakage.

What do you think about some linearity tests? I could use my Fluke 343A and my KVD (ESI RV722) to do some tests. First, I will set the KVD to 1.0000000 and tune the Fluke to 10.00000V on the Solartron. With some averaging it should be possible to test the linearity of the meter. Is it worth the effort? I think it will take some time to average the signal.


Another point might be the reference section. The ADC relies on the positive and negative 10 V refs to be connected. So noise in the ratio will show up as noise of the ADC, even if measuring 0. So IC306 is critical in this way though at nominal specs it only contributes something like 0.2 µV_pp for 0.1-10 Hz. The forcing waveform might give some dynamic load to the references and thus could give minute ringing that might also contribute.

I will try a ratio measurement with my 3456A and I can try to have a look with my Tek 7A22 (very long unused ;) ).

Another point worth looking at might be the cross over of the two OPs in the integrator, I don't know how accurate the design is (likely done before spice simulation became popular). It might be worth simulating that part and see if it might react sensitive to tolerances in the caps or amplify noise from some bands.

p.s.: I did a short similation: the cross over and tolerances are not critical at all, so not problem here.

Great! Thanks :)

It is really great how much effort some people in this thread will invest to MY problems. And I have to apologize for my bad english.
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #77 on: February 27, 2016, 10:25:48 pm »
I just read the first part of the thread again. The noise does not look that bad from the descriptions, but it's also confusing. Sometimes it is not clear which mode is used. Also peak to peak values are little tricky, as they fluctuate quite a lot. The RMS values over something like 200 readings (the exact number does not matter much, but should be the same to compare) are more repeatable. Also drift during the run is of interest, as it adds to the RMS values.

So you are really after the least bit of noise, possibly even in spec for the 7071 model. So having good data on the noise might help to see if we look for a defect part or more like the difference between guaranteed specs and so called typical performance.
Especially in the high resolution modes it is expected to see some noise in the lower digits - that should be normal.

If the noise is a problem when reading a significant voltage, the reference could still be the problem. So a test with measuring the internal 10 V reference with the meter itself could help.

The ratio measurement with a 3456 meter will not really help - the noise might be below the noise of that meter.
Checking that the sum of the +10 and -10 V refs iis really close to 0 V could be done with the 3456, just with two consecutive measurements. One might also check the offset of IC306 directly. In case you want to upgrade with newer parts - IC306 might be a point where better parts (LTC2057 instead of OP07) are available though it's likely only a small improvement.

Linearity test to the performance of the meter are very difficult. So I would on the long term just change C201 to a PP type capacitor like specified. It's not such a special part to buy - I don't think you need low tolerance. I minimal test would be checking with a external reference signal and reversing the polarity. Still without a direct comparison to a good cap this will not help that much. Changing the capacitor type might influence the calibration at least it does so with high resolution dual slope converters. The linearity test is a different thing from the noise, so one problem at a time.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #78 on: February 27, 2016, 11:03:03 pm »
I just read the first part of the thread again. The noise does not look that bad from the descriptions, but it's also confusing. Sometimes it is not clear which mode is used. Also peak to peak values are little tricky, as they fluctuate quite a lot. The RMS values over something like 200 readings (the exact number does not matter much, but should be the same to compare) are more repeatable. Also drift during the run is of interest, as it adds to the RMS values.

So you are really after the least bit of noise, possibly even in spec for the 7071 model. So having good data on the noise might help to see if we look for a defect part or more like the difference between guaranteed specs and so called typical performance.
Especially in the high resolution modes it is expected to see some noise in the lower digits - that should be normal.

The noise is a factor of 10 higher in 6.5 digit mode. So 11µVpp will become 110µVpp for example. And if that is in spec, than I think it is a nearly useless 6.5 digit DMM. My 3456A is completely stable including the last digit even with only 10 NPLC connected to the Fluke 343A, Knick JS3010 or a LTZ1000. Therefore, I can't believe that this should be in spec.

I will do some RMS noise measurements. My LabVIEW file showed it already, but I didn't noted it...

If the noise is a problem when reading a significant voltage, the reference could still be the problem. So a test with measuring the internal 10 V reference with the meter itself could help.

I will do that. If the reading is stable, I would expect a noisy reference. But, I don't think so. I've measured the references already with the 3456A (silly me: I didn't compare +10V to -10V) and they were stable within the 6.5 digits of the 3456A.

The ratio measurement with a 3456 meter will not really help - the noise might be below the noise of that meter.

I think I should see noise down to nearly 1ppm with the 3456A on the 10V references. So if the noise is below 1ppm shouldn't the meter be stable in 6.5 digit mode? Another question of mine is: Why should a noisy reference influence the reading of 20µV by 50%? Should reference noise not appear in the same relative dimension on the output?
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #79 on: February 28, 2016, 12:07:39 am »
Noise of the reference itself should not influence the reading of a small voltage very much. A difference thing would be noise that shifts the +10 V and -10 V reference signal together. This will essentially appear like in the signal, as the 0 V is mainly a compensation of the +10 V and -10 V.

As the +10 V and -10 V are used one after another, there is also a way that higher frequency (e.g. 160 Hz range) noise of the reference will add to the ADC noise, when measuring signals near zero. As the plan shows essentially no filtering for the zener noise, one might think about adding some (e.g. a capacitor from GND to IC305 pin 2, or across the zener diode).

As far as I understood the specs are < 20 µV_pp for the 10 V range. For the noise sources I looked at so far I would expect something like:
1 µV_pp from IC201 (ICL7650), possibly more as specs of ICL76650 are rather optimistic
3.5 0.35 µV_pp from IC202 (LM301) - current noise, some variations possible as not all LM301 are the same
0.35 µV_pp from IC305 (OP07)  - this might get critical for 7.5 Digit mode due to 1/f part.
0.4 0.3 µV_pp from the resistors (R221,R201,R202)
e.g. 0.3 µV_pp from 160 Hz band noise of Zener diode (assuming 50 nV/Sqrt(Hz) for the zener), hard to estimate.

Looking at the comparators might also be worth while. Due to the rather large 100 nF integration cap, the slope is rather slow and thus the comparator delay at low overdrive could be tricky.




« Last Edit: February 28, 2016, 09:44:14 am by Kleinstein »
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #80 on: February 28, 2016, 12:23:20 am »
As far as I understood the specs are < 20 µV_pp for the 10 V range.

That sounds really bad :(. But it is "only" 2ppm pp at 10V



I did some further measurements:

+10V Ref: 9,90954 V
-10V Ref: 9,90958 V
That is much better than the printed 0.005% and the Vishay divider. During measurement I swapped the leads in a way, that the 3456A sees only positive voltages, to get a more reliable result. The reading was done with 100NPLC and was stable.


I made 200 Measurements of the positive reference voltage:

6.5 digit mode: 5.05ppm pp and 1.23ppm RMS
7.5 digit mode: 2.42ppm pp and 0.29 ppm RMS

It seems to me like more than the spec allows. The measurement was done with an open case to access the reference voltage. But measurements in the pasts doesn't show any difference with an closed case.


Next, I will replace IC203 and IC204 with new LM311.
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #81 on: February 28, 2016, 09:42:05 am »
The reference readings look good. So no big trouble there. Though there is still a chance that the reference might have quite a lot of noise. Noise specs for the reference are hard to estimate, but zener diodes are known to be somewhat noisy.

The noise reading really look not that good:  for the 6.5 digit mode the RMS value is about 4 times the specs of the 7081. As the 7081 are better tested, a slightly higher noise for a 7071 model is possible.
The noise in the 7.5 Digit mode is about 1/4 from the 6.5 digit mode - this is indicating that the excess noise does not have a strong 1/f character. The only 8 times longer integration time would even expect only a 3 times reduction in noise for a white noise source, though a little may come from less quatisation-noise due to the extra digit.

I checked my calculations for the noise - was a little late yesterday: I found that the noise of R221 should not contribute as much as expected - forgot the AC coupling for it, so a little less there. The cap might also reduce the noise from the 160 Hz range of the reference - have to rethink that.
Also estimate for the current noise of IC202 was wrong by a factor of 10, so much lower contribution there - sorry. Sill some uncertainty there, as the 1/f noise can vary quite a lot between individual chips. Newer LM301 might be better without notice.
So I change the numbers above.

The main result is the same: there are some noise sources, but the seems to be other import ones still not in the list.
So looking at the comparators is a good idea. If the slow slope is a problem for the comarators - there might be the option to use an extra amplification stage between the integrator and comparators - other meters like 3458 or keithly 2000 use this trick.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #82 on: February 28, 2016, 01:29:34 pm »
Today I replaced IC203 and IC204 with new LM311. Still nothing changed. The noise in 6.5 digit mode is still 60µVpp and 23µV RMS with a closed case. (Back to PLL)

Furthermore, I desoldered C212 for a quick leakage test and put it in series on a 40V supply (Knick Calibrator) with my 3456A in 100V Range (to get 10Meg input resistance) the 3456A went to 0 after some time. So less than 100µV over 10Meg -> less than 10pA.

All of your improvement ideas are really great, but in the first place I want to get the meter in spec with the original circuit.
« Last Edit: February 28, 2016, 01:42:22 pm by e61_phil »
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #83 on: February 28, 2016, 06:43:18 pm »
I was thoroughly reread a notebook with my handwritten notes about 7081's research and modding. One day the 7081#1 suddenly increased noise amplitude with factor of 4-5. Intercomparison with a normal 7081#2 gives nothing: DC modes, waveforms are almost identicals. Also the TR201, 202 swapping with 7081#2, changing IC201 to LTC1052, IC202 to LME49710 or OPA604 gives nothing too. The only thing that suddenly stops the excessive noise is touching a IC307 (DG301) pins with a hot soldering iron  :-//
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #84 on: February 28, 2016, 08:14:34 pm »
I was thoroughly reread a notebook with my handwritten notes about 7081's research and modding. One day the 7081#1 suddenly increased noise amplitude with factor of 4-5. Intercomparison with a normal 7081#2 gives nothing: DC modes, waveforms are almost identicals. Also the TR201, 202 swapping with 7081#2, changing IC201 to LTC1052, IC202 to LME49710 or OPA604 gives nothing too. The only thing that suddenly stops the excessive noise is touching a IC307 (DG301) pins with a hot soldering iron  :-//

Very interesting and simultaneously annoying that even with a second meter side by side the source of the noise wasn't traceable by measurements.

I touched severels pins of IC307 but nothing changed. Afterwards, I resolder the IC and still no change. Perhaps it might be a good idea to order a new DG301.

 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #85 on: February 29, 2016, 08:19:29 pm »
A DG301 in a ceramic package is not that easy to get and expensive. What do you tink? Is it worth to give it a try with a plastic one from vishay or maxim (Farnell)?

Unfortunately, I will cut the pins of the DG301 to get it out without the risk of damaging the PCB. Therefore, I will have a plastic DG301 after this test.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #86 on: February 29, 2016, 09:05:34 pm »
There is no need to break out the old chip: disconnect R310, R311 and connect new chip to its.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #87 on: March 01, 2016, 06:10:20 am »
Some info about ADC:
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #88 on: March 01, 2016, 01:07:31 pm »
The plastic package for the DG301 should be no problem, as properties like leakage or on_resistance are not critical for this part - it's only AC Coupled. The important property is having low jitter from input to output and short time stability of on_resistance only.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #89 on: March 01, 2016, 08:54:25 pm »
A new DG301 doesn't changed anything..
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #90 on: March 01, 2016, 09:11:48 pm »
Well, it's time to isolate an analog part of ADC and change it to some external waveform generator.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #91 on: March 01, 2016, 09:17:05 pm »
Well, it's time to isolate an analog part of ADC and change it to some external waveform generator.

Could you describe a little bit more what you expect to do?
 

Offline wiss

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Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #93 on: March 02, 2016, 06:45:41 am »
Could you describe a little bit more what you expect to do?
The problem may lie in the digital section only.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #94 on: March 02, 2016, 07:11:03 am »
Could you describe a little bit more what you expect to do?
The problem may lie in the digital section only.

And in which section do you think? And is it easy possible to simulate the ADC with an arb function gen?

The section under this shielding PCB gets quite wam.

Next I will have a closer look at IC205 and the signals which goes to the digital section.
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #95 on: March 02, 2016, 08:09:12 am »
Quote
And in which section do you think? And is it easy possible to simulate the ADC with an arb function gen?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #96 on: March 02, 2016, 08:57:07 am »
Are these two different tests? And what kind of result I should see? Sorry for my stupid questions ;)
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #97 on: March 02, 2016, 09:17:11 am »
The first picture only. It is possible to test a counter/interrupt/e.t.c. logic section of ADC with isolated analog one.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #98 on: March 02, 2016, 11:51:11 am »
The first picture only. It is possible to test a counter/interrupt/e.t.c. logic section of ADC with isolated analog one.

I should use a function generator to create a puls with a period < 3 ms and trigger this pulse from "FW PL501/11"?
Do I understand it right?

Should it be 50% dutycycle? And what effect has a change in periodtime or/and dutycylce?
And where can I get this "FW PL501/11" signal to trigger the pulse?
 

Online Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #99 on: March 02, 2016, 12:03:22 pm »
Quote
I should use a function generator to create a puls with a period < 3 ms and trigger this pulse from "FW PL501/11"?
You are absolutely right. The value of dutycycle is not important as you explore the noise/short-term stability.
 


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