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Advantest R6581 8.5 digit DMM mini teardown/repair

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dietert1:
The R6581T has multiple design problems and removing/modding any of them won't show a significant improvement until you solved the last one. As long as one or two of them are left, it remains a 6.5 digit meter. Selecting an OP07 for pA input current would be more like a piece of art and might depend on illumination. The OPA140 is better than needed "by spec" and it has a similar power consumption. I didn't use a TL071 as it has about 20 times higher low frequency noise amplitude.

For U504, maybe one could add offset voltage trim. In the instrument i am working on, its offset voltage is about 70 uV. Or replace it by an OPA189 chopper. That mod probably destroys external calibration of lower voltage ranges.

Regards, Dieter

Kleinstein:
The input bias from the OP07 U107 would add some offset voltage. However this would not be a problem as the offset would be essentially constant and is corrected in the normal auto zero cycle. The point would be more the common mode input resistance, and here the OP07 is quite good (some 160 GOhms typ - may vary with manufacturer) and here the nonlinear part of the resistance could have a small effect. The linear part only changes the internal gain a little and this gets corrected with ACAL.

AFAIR the incentive to change U107 was to make it faster and this way reduce the gate current spike a little. There may however be stability issues.

Another odd point is to use the output of U107 for the guard also for circuit parts at the very front. For some parts I would consider the output Of U009 more suitable.

U504 adds  a little to the noise and offset / drift before AZ. Here the offset and drift of the OP07 is usually considerably better than the main amplifier. The noise is also not that bad, though there are lower noise (the relevant frequency would be something like 2.5 Hz) ones available.
A change of U504 should normally not effect the calibration (gain adjustment for the 10 V range). It would however effect the internal calibration and possible linearity corrections. Besides noise, the ouput cross over distortion of the OP07 could habe some effect on the linearity in the 100 mV range.  A formal calibration with the paperwork would normally be considered void after such a change of cause.
One could check the possible nonlinear effect by loading the output of U504 when reading 0 in the 100 mV range.

dietert1:
After removing the original JFET MUX with the strange feedback loop where switching depends on the switch output the guard driver speed does not matter anymore. For me the point was to remove the offsets caused by input offset currents. Those currents are extremely temperature dependent and they are so easy to remove that one could call the placement of OP07 a design error. There may be historical reasons, though.

Here i have a scope image of fast mode with 1 msec integration time and autozero, showing integrator output and guard driver output. Input voltage is 3.5 V.

Regards, Dieter

Mickle T.:
Please! Can anyone provide the diagnostic data of R6581 True-RMS board?

bobAk:
Amp_off +0.0008 - 0.0005
Vca_off -0.00226
Ac_ref1 -9.81724
Ac_ref2 +9.81230
Abs_dead1 +0.17970
Abs_dead2 +0.17504
Abs_off1 -0.17932
Abs_off2 +0.17510
10vf_1 +1.86881
10vf_2 +1.91763
10vf_3 +1.91175
10vf_4 +1.86854
Some readings are drifting.  Everything related to 10vf_X, ref is stable to the last sign.

Add. You can measure noise at the 100 DC volt range with the input shorted? Config 100 nplc 8.5 digits az on.

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