Author Topic: Advantest R6581 8.5 digit DMM mini teardown/repair  (Read 201427 times)

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Offline TiN

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #425 on: April 22, 2018, 12:37:30 pm »
I second that request. Ohm error on my box is horrid.  :-//
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #426 on: April 22, 2018, 01:30:21 pm »
Hi,  Mr. Mickle T.,

Thanks a lot !

The only thing is the U107/U104 changing to TL071 in my case. But it seems that 20ppm transfer error is still not so good . Could you give some advice on how to do the TRANSFER CHECK in the diagnostic mode. I am wondering if it can help to fix the transfer error.

Best Regards,
szszjdb
 
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #427 on: April 22, 2018, 02:12:39 pm »
TRANSFER CHECK in the diag mode has nothing to do with ohms/volts transfer. This is more like a memory card test  :(

U104 already have strong a BW-limiting network C120-R122/R123/R138. I don't fully understand how could a replacement for a faster TL071 be able to get such a result?
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #428 on: April 22, 2018, 03:35:00 pm »
Hi,  Mr. Mickle T.,
Thanks for all!

Reason for the changing U104 is keeping the D103 effective in clamping ,resulting the reducing ringing in guard output ,thus reducing the input current charge for the AZON mode.  All proved in my post before. But it is still not so good comparing to 3458 as lack of precharge circuit.

Best Regards,
szszjdb
« Last Edit: April 22, 2018, 03:36:51 pm by szszjdb »
 
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Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #429 on: April 22, 2018, 09:14:54 pm »
C120, R138, R122, R123 are more limiting the BW of the JFET+cascode stage, by loading the output in addition to the two 40 K resistors.  The change to the faster TL071 should work in 2 aspects:
the first is the higher BW and slew rate. The problem with the OP07 is especially the limited slew rate.

The second difference with a JFET based OP for U104 is that the slew rate limit is reached at a higher input voltage. This allows D103 effectively limit the slew rate before the OP itself does it. This reduces overshoot due to charging C120 too far. Typical BJT based OP reach there slew rate limit with about 50 mV (or 100 mV for a Darlington or similar input).
The TL071 may not be the best choice, it is more like a first easy to get part. The gain of the TL071 is kind of limited - though I am not sure if gain at frequencies much below 5 Hz would really help. The JFETs and the LT1220 at the output add gain - so the overall loop gain might be still sufficient.

The JFET MUX  (with gate supply from the output side) is problematic when switching from a high to a lower voltage. Despite of the C026 for the DCV input it just takes too long for the guard voltage that supplies the gate voltage to drop to the new level. C026 is mainly adding a delay before the new channel is turned on, but it is not very effective in turning on the JFET slow. This can cause the gate to get forward biased and this adds leakage current. This will also effect the 4 wire ohms mode, possibly even more as there is no capacitor to slow down switching. For a large resistor the disturbance from AZ (sense Hi / sense Lo) switching seems to be so large that it does not fully recover before the ADC conversion. The ohms current source is from the negative side, so the voltage at sense_Hi would actually be negative and thus the critical switching case with a large resistor.

The input current measured with the discharge curve still looks rather high. However some of this could be due to the missing pre-charge phase and only so high in 1 PLC mode. Chances are it would be considerably smaller in 10 PLC (or similar) mode (could be measured with a larger capacitor).  The input current still looks like a nonlinear resistance and could thus contribute to the INL, though I don't think it will be much in the slower modes. The 1 PLC with AZ mode might be kind of limited to not so low input current - this looks like a principle limit and not a defect. Not sure if this is mentioned in the specs / instructions.

The INL test from looking at ADin (with the 3458) and the ADC reading is different in two aspects:
it does not include the input amplifier and because there is no AZ phase, the DA will have a much smaller effect. The ADC is converting essentially a constant value and this no carry over to the next (e.g. Zero) conversion.

Looking at the expected average charge in the integrator again, the DA might actually contribute to a square law type INL. The average voltage in the capacitor should be at about half the amplitude of the integrator signal, as the lower end is fixed at 0. So it would follow a kind of curve with it's maximum at 0 V input.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #430 on: April 23, 2018, 01:46:20 pm »
Hi,Mr. Kleinstein and Mr. Mickle T.,
Thanks a lot!

I agree that there has some different condition for system INL under AZON mode and ADIN INL under AZOFF mode, but they are so similar both in the shape and the scale. Please refer to the attached picture.  As proved before that the error introduced by the DC amplifier is around 1uv scale, it is most likely the INL coming from ADC.
What would be the next step? Would like to have your further advice。

Best Regards,
szszjdb   
« Last Edit: April 23, 2018, 02:05:45 pm by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #431 on: April 23, 2018, 03:57:50 pm »
I replaced U104 and U107 with the precision mid-speed AD8675. No luck  :-[
x10 and x100 mode of the input amplifier gives about 8 V p-p sine wave at the output.
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #432 on: April 23, 2018, 04:30:17 pm »
The INL curves measured from the input in AZ mode and AD_in point without Az are in deed rather similar, even more than I have expected.  This is because there are 2 INL contribution that I think I under-stand: One is self heating of R200 (center part) in combination with a TC.  The other is DA to cause charge transfer between conversions and thus an error mainly in the AZ mode, but should have little influence on longer times with a fixed input.
The odd thing is that these two contributions should be different for the two cases: more self heating (e.g nearly twice as much) in the non AZ mode and the DA effect mainly in AZ mode. The voltage dependence is different ((U-2V)³ vs U² ) so it is not that one is exchanged for the other.

This makes it unlikely that these two contributions are causing most of the common curve.

One possible test could be looking at the S.out test-point again, and compare the curves for a few different input voltages (e.g. 0 V, 4 V, 6 V, 10 V) to see if something changes at around +5 V input.  With higher input voltage the curve before the comparator triggers gets increasingly steeper. Its possible for the slope amplifier to just reach positive saturation at about 5 V input. A change in waveform at S.out could effect INL via capacitive coupling towards the current source S1024 (not very likely due to distance) and also through coupling via the supply. Another possibility could be an excursion of the integrator that goes too far at the beginning of run-up.

For the range -1V to +2 V it might be worth to get a few more points for the INL curve(s), to see how wide the dip around 0 really is. Some coupling effects might only effect very small ranges and thus look odd with just a few points.

Having a kind of automated INL test could allow for testing small modifications (like added bias, added decoupling) to see if the INL curve changes. With getting the curve manually this might not be very practical as is would need quite a few curves with preferably more points.

@Mickle T:
U104 needs to be in the right GBW range. The internal compensation of that OP is used for the whole loop too. This OP has to be considerably slower than the LT1220 used for the output and the JFET stage. One might get away with a faster OP when a small capacitor in local feedback (output to inverting input, a little like with the fast amplifier channel). I would more consider an LT1055 or OPA171 for U104, as there is an extra advantage in having a FET type.

My simulation still worked with the LT1056, but it did not include parasitic capacitance, e.g. of the switches. The version with TL071 already showed quite some ringing in real world - so anything faster might get tricky. The noise and drift of U104 should not be that critical as the JFETs in front should have a gain of around 20-40 at low frequencies. So the TL071, LF411, LF351, AD711 and similar could be acceptable. Noise wise the AD8675 is also not that good as the input side of U104 is in the 40 K Ohms range.

For U107 a faster OP should be not problem, but low input bias could be an advantage. The current noise of the AD8675 would already add to the amplifiers noise in the x 10 range. So again a FET based OP is likely a better choice.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #433 on: April 23, 2018, 06:07:59 pm »
Hi,Mr. Kleinstein ,
Thanks a lot!

Checked the Sout with Cout with scope, seems normal in the curve. The odd is there has a decline both in the Sout and Cout at the positive period.

Would like to have your advice!

Best Regards,
szszjdb
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #434 on: April 23, 2018, 08:31:58 pm »
The curves look a little odd:
The very fast ringing on the S-out signal is likely jus coupling via ground to the scope. So I would consider this an artifact from the connection. It is likely not there if C-out is not probed.

Still it is odd that the S-out signal stops rising essentially immediately after the comparator has triggered. I would not expected this, as the control of the current source and response of the integrator should take some time and would need to by synchronized to the main clock (e.g. 12 MHz or slower). So I would expect some delay at least in the 50-100 ns range and a corresponding overshoot of some 100-200 mV. The flat top part looks a little like the slope amplifier would already be nearly at it's upper limit.  So the hysteresis on the comparator seems to be a little on the high side and this might cause the switching to take place rather late, when the slope is already slower again. Together with a slight offset of the comparator this leads to relatively long integrator pulse at +10 V. Still I don't see a reason to cause significant INL by this. It is more like a point that might cause trouble the temperature gets much higher and thus a reduced amplitude at S-out, possibly up to the point of not triggering the comparator in time anymore. When at the edge it might cause some trouble during rundown. Though I would more expect increased noise right at the edge, not a large range with extra INL.

One might consider to reduce the value of R205 a little (e.g. 10 K instead of 12 K - a large change could have quite an influence on rundown timing) to get the switching more in the steeper part, and thus have a little more reserve for a possibly higher temperature. The offset due to R260, R261 seems to be a bit off center and thus much longer pulses only with a positive voltage. It is at about 5 V where the pulse length really gets longer, but I don't see a way how this should cause INL issues.

Another point worth looking at might be the I.out test-point at around the end of the run-up phase and the beginning of the rundown phase. Here things might change with voltage. The comparator switching right before the end of integration time could cause some INL issue at some rather fixed voltage, possibly at around 0 V.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #435 on: April 24, 2018, 06:21:48 pm »
Hi,Mr. Kleinstein ,
Thanks a lot!

Attached the Iout and Cout waveform FYI. All test will under 10PLC, AZOFF mode.
Will find the 12K resistor for test tomorrow.

Would like to have your advice!

Best Regards,
szszjdb
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #436 on: April 24, 2018, 08:03:19 pm »
The I_out curves look normal at around +5 V. At least I can so no real change there. There seem to be smooth transition from 0 V up to 10 V. So I don't see a source of INL here.  There seem to quite a long minimum time for switching before the short brake before rundown.

However the curves look a little unusual for the -4 V and -6 V: here the is change from negative so positive slope before the final cycle, that appears without the comparator to get activated. This might indicate that the feedback during run-up is not producing a stable patter as I would have expected. Still this is the range that is working better. So the ADC is still good for surprises.

As long as everything still works, there might be no real reason to modify R205. Rethinking the effect, it should only effect the very fast initial stage and thus not have a significant effect on INL. The slower slopes are likely less critical as there would be less delay from the comparator.
R205 might be more like a point to address if problems happen at a higher temperature.


I found a note (on a much older Keithley 19x) that the output cross over transition of the integrating OP might have an effect on linearity. This would be the point of the output current of  U206 is low when switching. This might give a different type of settling during switching and this in cause influence the charge injection during switching. Looking at the currents, it is possible that at around 5 V input voltage this critical current range might be hit. It would be relatively easy to add some current load to U206 (e.g. a resistor (e.g. 10 K) from the output ( = Iout) to the -15 V supply of the OP). This extra load should shift the transition to a higher voltage, likely > 10 V.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #437 on: April 25, 2018, 01:58:42 am »
Hi,Mr. Kleinstein ,
Thanks a lot!

To shift the center of the integator is more like the experiment which Mr. Mickle T. have done. I will try it tonight!

Best Regards,
szszjdb
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #438 on: April 25, 2018, 02:46:38 pm »
I suggested adding current to the output of the integrator. So this does not change much with normal operation, so no problem with self test or cal. It only changes the current seen by the output stage of the OP and thus possibly the settling. the change Mickle suggested was adding current to the input side and this way possibly shift the INL to other voltages. This could be still interesting, but it could interfere with ACAL and self-test. So this test would have to be with a very small current or a switchable current enabled only after ACAL. This effect on INL might give a little more information on where it comes from.

Another possible way that might improve settling of the integrator could be adding something like a 100 pF + 50 Ohms series combination to GND (what GND point to use is a difficult questions) to the input of the integrator. This could dampen overshoot seen in some older curves as well. Quite a few other DMMs use such an RC combination at the integrator and the simulation also shows some, usually positive effect.

Even if these changes would not solve the INL problem, a significant effect on INL (in either direction) would indicate that those points are somewhat sensitive.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #439 on: April 25, 2018, 06:35:36 pm »
Hi,Mr. Kleinstein ,
Thanks a lot!

By added  a  10K resistor from U214 PIN11 to U206 PIN4 , it seems no improvement and even worse after a quick turnover and the transfer test. The turnover error reached -18uv at 10V input ,in where it used to be -8uv. More INL test will conduct tomorrow.

Best Regards,
szszjdb
« Last Edit: April 26, 2018, 07:25:39 am by szszjdb »
 
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Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #440 on: April 25, 2018, 08:41:27 pm »
If I properly understand, 10k must be added to U206 pin 4-6, isn't it?
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #441 on: April 26, 2018, 02:50:51 am »
Hi,Mr. Mickle T. ,
Thanks a lot!

As the switch of U214 were closed in 10plc mode, I just connected the U214 PIN11 to -15v yesterday. I will try as your suggestion tonight.


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szszjdb
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #442 on: April 26, 2018, 06:39:03 am »
Having the 10 K at the other side of the switches could cause trouble when the switches are operated, e.g. during self-test. The switch resistance would also have some effect - in the final rundown it is about 10s of µV. So the likely better position would be like Mickle pointed out.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #443 on: April 26, 2018, 02:34:33 pm »
Hi,Mr.Kleinstein and Mr. Mickle T. ,
Thanks a lot!

Had corrected the connection of 10K as your suggestion, I got nearlly the same result as yestorday. After  removing the 10K, the error of turnover at 10V recover from 16uv to 10uv as before. It seems the resistor added make more INL at very positive range. Attached FYI.
I have to mention that I performed external ZERO and internal DCV calibration each time when change made.

EDIT: After the INL test , the one which added 10k clearly shows the worse INL than the original ,especially in the positive range. So the resistor should be removed.
Further modified with the 100ohm + 50p connected from U205 PIN2 to C207 ground PIN, the INL is also worse than the original. Attached FYI.

Would like to have your advice.

Best Regards,
szszjdb
« Last Edit: April 27, 2018, 12:09:30 am by szszjdb »
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #444 on: April 27, 2018, 09:42:55 am »
I checked my integrator simulation again, and from that it is no big surprise that the 50 pF +100 Ohms did not improve much. It would likely take considerably higher capacitance to have a significant effect on integrator settling. The extra current source makes is even more complicated.
The negative effect could be due to more coupling from the extra parts.

For the extra current via the 10 K resistor the simulation / model of the LT1056 is not that detailed to include loading effects. So it does not help here. It is interesting and kind of a surprise that the extra current essentially amplifies the existing INL. Normally the positive range is giving less current load to the LT1056 compared to more negative voltages. So I don't think the extra current would have such a bad influence the output stage of the LT1056.

So my guess would be more like more capacitive coupling from the extra parts (one might be able to check that with just dummy wires to get about as much, or a little more capacitive pickup as the resistor) or possibly a not so perfect decoupling of the +-15 V supply.

p.s.
Testing extra decoupling at U206, U207 would be my next try. I is really odd to have a fast OP like the LT1220 without local decoupling.

« Last Edit: April 27, 2018, 10:03:46 am by Kleinstein »
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #445 on: April 27, 2018, 12:30:55 pm »
Hi,Mr.Kleinstein ,
Thanks a lot!

I had used the SMD chip for 100ohm and 50p, directly sorldering on PIN2 of U205 ,then connecting to C207 ground pin with a short wire. So the coupling effect should be small introduced by the parts added.

I will check the decoupling of U205/206/207 as your advice.

EDIT:
After adding the decoupling cap 4.7uf on both PIN7 and PIN4 of U205/206/207 to ground PIN of C207 , found slightly improved in turnover and transfer error test. But INL become worse than before. Attached FYI.

Would like to have your advice.

Best Regards,
szszjdb
« Last Edit: April 27, 2018, 05:53:41 pm by szszjdb »
 

Offline Mickle T.

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #446 on: April 28, 2018, 10:30:07 am »
EPROM content of my R6581T (2006 yr):
U103 (SIS004246) is the same as in other DMMs.
U102 (SIS007342B) differs only in 2 bytes  :-\
 
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Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #447 on: April 28, 2018, 12:10:23 pm »
So decoupling did have an effect on the INL, though in the first try the extra caps did not reduce INL. So I would not give up on this option, it more looks like a hint that decoupling is important.

There are a few options on how decoupling can be done. Going from the positive to GND and the negative side to ground is one option.  Another way of doing decoupling is have a capacitor directly from the +15 V to the -15 V and a separate capacitor from only one side (with most OP preferably the negative side) to ground. A big question here is which ground point to pick - this is especially important when going to GND only.

4.7 µF suggest electrolytic caps - these may help, but they may not really help with the highest frequencies. At least between pins 7 and 4 on U207 is would make sense to have a MLCC cap (e.g. 100 nF), as this OP is really fast and normally requires good decoupling.

There is quite a bit of current circulating between U206 and U207, through R219/R220.
So a relatively tight coupling between these OPs supply can be good.

U205 should not cause much high frequency noise, it is more like the OP that can be sensitive to ripple, especially on the negative supply.

The other mayor current flow would be between U206, thought the caps and Q204 alternating to Q205. So the critical path is from source of Q205 (= GND) through Q205-Q204 C210/ U206 and back to ground.

One could also try to look at the supplies with the scope to see how much "noise" ringing in on the supply. However I would not expect much signal there, despite of the not so perfect layout.
 
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Offline szszjdb

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #448 on: April 28, 2018, 05:54:10 pm »
Hi,Mr.Kleinstein ,
Thanks a lot!

I should make more description on the detail what I had modified yestoday.  The decoupling cap 4.7uf is the MLCC type and I directly soldered then on both PIN7 and PIN4 of U205/206/207 and connected them to ground PIN of C207 by a wire. I also replaced all the electrolytic capacitor 10uf to the larger one 100uf . Also recovered the original 20nf SOSHIN cap . After burn in for a whole day,  found slightly improved in turnover ,transfer error  and INL test , especially in INL of negative range.   Attached FYI.   
But the shape of the curve is still nearlly the same with the original. I suppose that we still have not touch on the key parts, which leading to the bad INL of the positive range above +5v.

The noise on the power rail are difficult to view by the scope. It seems ok so far.

Would like to have your advice.

Best Regards,
szszjdb
« Last Edit: April 28, 2018, 06:11:19 pm by szszjdb »
 

Offline Kleinstein

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Re: Advantest R6581 8.5 digit DMM mini teardown/repair
« Reply #449 on: April 29, 2018, 07:34:38 am »
For decoupling the connection to ground is a difficult point. The point at C207 may not be the best choice, as this goes to one of the OPs inputs and can be thus a more sensitive point.
The main modulated current flow should be from U206, through the integration caps, Q204 and than as the alternative path through Q205 to ground. So the logical ground point would be at the source of Q205. However this point is quite far away. So it is really difficult to find a good ground.

Even without connection to ground, a capacitor just across the supply of the OPs could help, especially with U207. This at least helps with the supply current variations of the OPs and also the current flowing from U206 to U207 (slower, but it can be quite some current). So essentially the same caps, but without the connection to ground.  I am not sure if it is good to couple U205 closer to U206/207 - U205 is not producing current peaks, but it can be sensitive. So a cap across U205 pins 4 and 7 is likely good, but an extra connection to the decoupling of U206/207 might not be that good.

The shape looks really reproducible and so far worse with the extra caps. However as the general shape stays the same, it points to something like poor ground or decoupling as a factor. From the pictures the decoupling of the OPs looks poor - up  to the point of violating the usual design rule that fast OPs need local decoupling at there supply (I would consider the LT1220 with 45 MHz GBW fast). So I would consider a cap across U207 as a minimum - a capacitor to ground may not be needed, if there is not much current towards ground.
 
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