K2001 ADC

[Kleinstein]

For the single OP-amp version the OPA1655 is not that good. It has more low frequency noise. So the OPA140 is already a good choice for a single OP-amp.
The OPA140 is already quite a bit faster than the OPA602 used before.
For the very low frequencies, like from the window up to 50 seconds due to the zero averaging the BJT based OP-amps like ADA4077, OPA207, OPA205 can be slightly lower noise than the OPA140, even when one includes the current noise. However the difference is not really large.

With OPA1655 one could consider removing the resistor to the -15 V. I don't think they would need the biasing for class A. The actual precision is anyway set by the slower OP-amp that sees essentially no load.

[GigaJoe]

so both gettin 200.3 fail for diagnostic ,   but even as i assume, preliminary test  as "Front panel comprehensive calibration"  pass even without j-fet that responsible for fast ADC.  Some how if fail at first time, on open connectors tests, ( passing DC and ohms )  and i missed to write the error , and test completed second time no errors ... i saved constants as it better then before ..

But transistor need back , as the test simply hang on that 200.3, and everything freeze, and only power switch works  ...

in term of stability ... i would say if average the result it still floating up and down on 2-3 microvolts .... but it ok ..

upd: 2nd pass "Front panel comprehensive calibration" , was error again :  363 - 200 µA gain out of spec.    Then i realize improper 20V , was set as 19.998XX more effort an it finish without error ....

lab next step ...  probably will do linearisation report for both ... hope wouldnt be a disaster   as i heavy change ADC ..


so yeah  that was with some filters ... and it already was good ,  comparing to original state ...
https://i.ibb.co/Hfh691m7/Figure-1.png
but it looks so miserable .. comparing to now:
(dont look at spikes ... especially at end , that my activity of on\off some stuff .. it picking everything )
https://i.ibb.co/vCKRZ9VS/voltage-deviation.png
(just a note ,  10PLC, no filter )
 

[GigaJoe]

this is what was measured for me ,  this is 2001M 2-opamp adc.
not really impressive ...  still wonder for class-A output ... but i cant play , unless i have tools

[GigaJoe]

and i have a second 2001 - single opamp ADC  ( OPA140) :

it was rejected, due to this:  ( said fix it before do anything )


so that changes in ADC kinda flop

I'm mulling idea of daughter board and [something -1 ] + [something -2 ]
probably:
something -1  - analog OP27 (proven) ,
something -2 OPA1655 ; and brief look at  ADA4898-1 it looks like fit better  ( unity gain stable , powerful output ,  BW 50 slew 50 ) ...  with ver -2  ( 2 opamps in use )

[Kleinstein]

For a single OP-amp integrator I would consider the OPA140, ADA4625, OPA827 or OPA828. A BJT based type is not really working because of the current noise. So the OP27 is poor, though maybe not as bad as in the 34401 as the impedance is a little lower (some 11 K vs 15 K). The ADA4898 even more noisy.
The OPA1655 is not good with low frequency noise. 10 PLC with good AZ loop would be down to 3 Hz (with 60 Hz mains).
With the poor AZ mode the Keithly meters are somewhat sensitive to even lower frequencies (maybe 0.02 Hz - though likely less gain) can matter.

The 2 OP-amp version has some advantags when it comes to INL. So a daughterboard with 2 OP-amps, the divider and decoupling could be an option.

There are more tings that can cause an INL error than just the OP-amp at the integrator. For my ADC I had little issues with a OPA1641 as single OP-amp integrator. There was a known DNL issue due to non constant impedance, but that applies to all single OP-amp integrators.

[GigaJoe]

that above :  OPA140

in speaking about composite like:
[something]  +  ADA4898-1
in this case current noise not so important , or :
[something]  +  ADA4898-2 + ADA4898-2

i cant figureout of first precision opamp type

what would your ideal combination for composite ?

and , what would be your opinion on this:
https://zenodo.org/records/18300606
same OPA1655

[Kleinstein]

The OPA1655 is good as the fast amplifier in a compound integrator, but it needs support for the low frequencies.
The main downside with the OPA1655 is the rather high power consumption. A powerful output stage naturally comes with higher power consumption.

So for 2 OP-amp combination one could pair it would an OPA140 (JFET) or OPA205 (BJT) or OPA387 (AZ type).

I think the linked article is missing a part for the integrator input voltage. The leave out the part from limited BW and rate of change.
I initially only looked at that half and missed on the Z_out part they are considering. Both are relevant for different time scale: the Z-out part very fast (speed of the fast OP-amp) and the GBW limit with the speed of the slower OP-amp. It depends on the parts which part is larger / more relevant.

They look at the noise for the whole frequency range and leave it to the reader which frequencies really matter. With a slow DMM this are usually the low frequencies, like < 100 Hz. Only with faster conversions (e.g. < 1 PLC) also the high frequenies (up to some 1-5 MHz) have some effect from the run-down / residual charge part.  How much and which frequencies depends on the integration time and design. I consider the importance of the higher frequency noise less relevant. The K2001 uses a rather large integration capacitors and the residual charge noise may thus matter at 1 PLC.

edit:
The current noise does matter, unless you add an extra JFET buffer, like in the HP3458.

Sensible precision amplifiers are
OPA140, ADA4620 as JFET types  - also high BW for very fast integrator (especially with fast OP-amp > 16 MHz)
ADA4077, OPA205 as BJT based types with low current noise and still good BW
OPA387, OPA388, AD8628 as auto zero amplifiers - likely with extra filter and maybe a 3 OP-amp combination (e.g. extra OPA1641)
The old OPA177 as used in quite some older meters is a bit on the slow side and the current noise is not that great.

Sensible fast FET based OP-amps are:
OPA197 (low power), TLV9361, OPA1641   (10 MHz)
OPA1677  (16 MHz - low cost, rel. low power)
TSB 952  (45 MHz low power)
OPA1655
OPA810  ( 70 MHz)
AD8065
TSH4631     ( AFAIK used in newer KS3446x), older ones used JFET + BJT based.

[GigaJoe]

i kinda like OPA387 ,388 but they 5Volt supply , i'm not certain if that would be limit for me ,and space on daughter board ..

so it sound like:  OPA140 - ( div 2K: 200 ) - OPA1655 ?

[Kleinstein]

An OPA140 + OPA1655 is sensible. The GBW ratio is already close to 1:5 and thus not much divider needed. Maybe 1K:1K.
I would plan with space for a series RC to ground (like 100 ohm 100 pF - need to try best value and if needed at all).

The AZ Op-amp would need a 5 V supply and extra filtering, which usually makes them so slow that one would need a 3rd OP-amp.