Author Topic: Solartron 7071 / 7081 last digit is jumping all over the place  (Read 71748 times)

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Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #25 on: February 21, 2016, 12:04:21 am »
The next thing is a power supply (+15, -15 and 5 V).

The power suppy was the first thing I have checked. There was no 50Hz Ripple. The noise was about 100mVpp.
 

Offline TheSteve

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #26 on: February 21, 2016, 01:33:30 am »
The next thing is a power supply (+15, -15 and 5 V).

The power suppy was the first thing I have checked. There was no 50Hz Ripple. The noise was about 100mVpp.

Seems like an awful lots of ripple for the power supply in a precision instrument. 
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Offline Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #27 on: February 21, 2016, 06:14:16 am »
7081 Floating Power Supply noise, measured by HP 34401A.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #28 on: February 21, 2016, 09:59:03 am »
7081 Floating Power Supply noise, measured by HP 34401A.

Thank you soo much :) A comparison is extremly helpfull!

It seems, that TheSteve is right.

This time I measured the Rails with my HP 3456A to compare it with Mickles values. Most of the voltage are extremly close to Mickles, but TP901 to TP903 has 0.8mV (I think that could be ok) and the noise of the +/-15V rails is about 10times higher than Mickles.

My next step is to replace all caps with new ones in the floating power supply.

 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #29 on: February 21, 2016, 07:35:50 pm »
I replaced C903 and C904, but that doesn't change anything.

I read this http://www.gellerlabs.com/34401a%20ac%20zero.htm

Perhaps the measurements of the 3456A and the 34401A aren't comparable in the sub mV rms range? I had another look with the scope and it looks quite well. I think the power supply is ok.

Another thing I wonder about is the -20µV Offset in Test0 mode. This offset stays in 6.5, 7.5 and 8.5 digit mode. Is an offset in Test0 mode normal?
 

Offline Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #30 on: February 21, 2016, 07:52:22 pm »
Some µV Offset in Test0 mode is absolutely normal. My 7081#2 have even more offset in "Drift,off" mode.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #31 on: February 21, 2016, 08:20:05 pm »
Ok..

I think your graphic was measured in 8.5 digit mode. Do you have any measurements in 6.5 or 7.5 digit mode?

I've attached another Test0 measurement in 7.5 digit mode. It shows 11µV peak to peak within a few minutes :(

I think in 8.5 digit mode it is hard to see if there is also this around 10 count noise, because there will be real zero noise in the last digits.
 

Offline Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #32 on: February 21, 2016, 08:33:32 pm »
I have only 7.5-digits mode TEST0 statictic.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #33 on: February 22, 2016, 10:56:18 pm »
I did some further measurements..

I fed in a stable voltage and measured the signal path down to the ADC. All outputs (Input Amplifier and DC Ranging) are stable (measured with 3456A and scope). Therefore, I'm sure there is no power supply issue. The reference voltages are also completly stable measured with the 3456A.

Next step was to desolder SP501 (the voltage at this point was also stable before desoldering) and feeding a stable voltage directly into the ADC. This results was the same jumping display as seen before.

Now, I'm completely sure that the ADC is the problem. I already replaced IC201 with a new one. I think it will be hard to track down the error in the last digits. On the scope everything looks fine of course. :(

Any ideas?
« Last Edit: February 22, 2016, 10:58:16 pm by e61_phil »
 

Offline ManateeMafia

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #34 on: February 22, 2016, 11:06:04 pm »
What about the +/- 10v references at TP302 & TP303?
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #35 on: February 22, 2016, 11:09:43 pm »
What about the +/- 10v references at TP302 & TP303?

Absolute stable on the 3456A.
 

Offline ManateeMafia

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #36 on: February 22, 2016, 11:54:40 pm »
I had a Keithley 2001 failing ADC diagnostics due to a leaky FET that I was able to compare to others on the same board. I am pretty sure it was the ADC noise test.

Unfortunately, the VN67AK and 3N163 are not cheap.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #37 on: February 23, 2016, 12:01:01 am »
If only one FET is damaged, the DMM should be stable in the other input polarity or not?
 

Offline ManateeMafia

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #38 on: February 23, 2016, 12:19:03 am »
Good question. I cannot answer that, but it looks like both + and - references are always used as part of the forcing waveform. I am sure someone here can give a better answer.

BTW, I have a couple of the 7081 and have been following your progress. These are nice voltmeters and it is a good complement to the 3456A.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #39 on: February 23, 2016, 12:34:36 am »
Yes, you're right. Changing the polarity doesn't make any difference :(. TP204 is switching between both rails, as you mentioned.

To get the VN67AK seems to be quite difficult. The 3N163 costs about 20$ at farnell, but one can buy it.



It seems to me, that there are a lot of 7081 around :).
 

Offline ManateeMafia

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #40 on: February 23, 2016, 12:52:30 am »
You can get them from here

http://www.americanmicrosemi.com/products/search/index.php

I have never used them so I don't know if they are trustworthy. Most seem to use random number generators for inventory levels. At least they do have pricing and accept PayPal so there should be some protection.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #41 on: February 23, 2016, 01:01:13 am »
You can get them from here

http://www.americanmicrosemi.com/products/search/index.php

I have never used them so I don't know if they are trustworthy. Most seem to use random number generators for inventory levels. At least they do have pricing and accept PayPal so there should be some protection.

Thx :)

I there a way to measure these transistors before I spend 100€ for nothing? (shipping to germany is expensive)

Do you know why they use these 3N163 with only 20fA input current? The gate drive is low impedance anyway.
 

Offline TiN

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #42 on: February 23, 2016, 04:26:24 am »
As with any integrating ADC, +/- reference with related switched resistors are used to provide current for ramp-up/ramp-down steps, so current in ADC is more important than voltage, but it's bit PITA to measure properly.

One thing I can suggest - try relocating 7081 somewhere else in the lab, it could be pickup of some EMI from something nearby. Sounds vague, but you never know.
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Offline Mickle T.

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #43 on: February 23, 2016, 07:40:19 am »
For testing purposes you can swap VN67 and 3N163 from Ratio Circuit TR505,506 and Ohms Circuit TR603,610.
The oldest revision of 7081 have a VN10 instead of VN67 in the ADC, the latest - WM222. Also R222 can be adjusted to set TR201/202 ~30 uS dead time.
« Last Edit: February 23, 2016, 10:53:24 am by Mickle T. »
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #44 on: February 23, 2016, 10:48:21 am »
I swapped the four transistors, but nothing changed :(

(2x 1N163 and 2x WM222 from ratio and ohms)

Later I will have a closer look at the deadtime. There was a deadtime on every edge, but I havn't measured it. Therefore, I don't think deadtime is the problem.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #45 on: February 23, 2016, 04:02:23 pm »
Deadtime is around 38µs. With a discrete Jitter (much smaller than the 38µs)
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #46 on: February 23, 2016, 04:09:39 pm »
Visible jitter at the dead time could be a problem. The jitter would be part of the PWM signal, thus even 0.1 µs jitter could give noise in the ppm level range.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #47 on: February 23, 2016, 04:21:12 pm »
I've attached a measurement of the deadtime jitter. Signal was measured on TP204 (GLUGS)
« Last Edit: February 23, 2016, 04:22:43 pm by e61_phil »
 

Online Kleinstein

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #48 on: February 23, 2016, 05:40:59 pm »
This dead time is the time when essentially no current flows from the reference. The jitter looks like just steps of the ADC clock (seems to be 5 MHz). So it does not look critical, it's about what I expect: mainly one of two values with what looks like a single event of one more off. So far the switching looks OK.

If in a socket, one could try replacing U202 (LM301), as noise from this OP also enters ADC performance.

I would also consider adding local decoupling at the supply of OP IC201 - at least the plan I found does not have it (may be a note somewhere ?), which is a little strange.
 

Offline e61_philTopic starter

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Re: Solartron 7071 / 7081 last digit is jumping all over the place
« Reply #49 on: February 23, 2016, 05:58:11 pm »
This dead time is the time when essentially no current flows from the reference. The jitter looks like just steps of the ADC clock (seems to be 5 MHz). So it does not look critical, it's about what I expect: mainly one of two values with what looks like a single event of one more off. So far the switching looks OK.

If in a socket, one could try replacing U202 (LM301), as noise from this OP also enters ADC performance.

I would also consider adding local decoupling at the supply of OP IC201 - at least the plan I found does not have it (may be a note somewhere ?), which is a little strange.

Ok, I didn't save this picture the first time, because I also thought it should be ok. After your post in you said 100ns could be crucial I saved a picture. Ok, PWM is also fine..

I already installed a 100nF ceramics and a 2.2µF MKS on IC201, that doesn't changed anything.


There ist not much left. IC202 isn't in a socket, but if I found a LM301 in my lab I will replace it. Perhaps I should also try to replace the comparators (IC203 and IC204).


Might it be the case that a leaky integration capacitor cause this problems? I saw a huge offset an much more noise after cleaning the FET soldering, since some IPA went under IC201. It vanished after drying with a hair dryer.
 


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