K2001 ADC

[Kleinstein]

The idea of a boostrapped supply workes nice for the CMRR with the input FETs and U341 (AD548). It's not infinite CMRR, but quite some improvement (e.g. 60 dB and a bit more for the JFETs). The PSRR should be good enough with U341 - but needs some care with zeners (some are pretty noisy).

The supplies for U341 and the JFETs are linked, which complicates the circuit quite a bit.

For linearity the cross over distortion of U341 might still be an issue. This might cause some a small step near zero in the 20 V range. It may however bit tricky to see error on the order of 1 µV in the 20 V range with all the noise.

[GigaJoe]

" boostrapped supply workes nice "
not sure how nice it works , don't want to argue , but in my practice  that 4 caps drop down noise almost 3 times , especially nasty peaks up to 200 microvolts P-P ..   U341 as AD548 - never exist such type ,  one unit TL061 , other unit , not remember OP124 or something (but it was ANALOG) .
https://i.ibb.co/PZm01fD5/caps.webp
and many tantalum capacitor on the back board in parallel to bypass caps ...  that alone drop down noise to around 50 μV , and 10-12μV wobbling due to zener. 
how it was:  https://www.eevblog.com/forum/metrology/k2001-adc/msg6084777/#msg6084777

After that was a second wave of opamp replacement , major U341  TL061 -> OPA140 ,  noise decreased a bit became more even .. , replacement LT1007 -> OPA189 - almost non measurable,   but couple μV there and there .. , of course zener adjustment eliminate waves and wandering ... , couple more caps

and last wave adc , itself a bit there and there :   LT1097 -> OPA188 , LM311 -> LM211 , NE5534 -> TL888 (OPA140) + ampl 5K -> 511 , and another resistor.  additional bypass caps, 5V that over divider generate reference voltage floor for comparators 680uF , of course couple for the digital part .. again few μV  there and there ..

and final most headache 1 opamp integrator to 2 opamps  second unit ADA4077 no divider  OPA1655. first still in progress  (second device more noisy btw ...)

that the story about ...

BTW,  replacement NE5534 -> OPA140 made some magic in noise that i can not reproduce, potentially, i assume,  that particular NE5534  wasn't good , it has to be fast as replace to ADA4077 made it RND generator.  for purists NE5534 ->SA5534A , i pick TL888 .

[HairyWombat]

So the LCSC order turned up with a Ti OPA827AIDGKR for U809 and no I didn't notice it was a VSSOP-8 when I ordered it.



Luckily I have a reflow oven as there is no way I could solder that, along with some dual type 8pin DIP Ali boards fits nicely.


Added a machined pin IC socket as I intend to try some of your dual op-amp configurations.
Small signal diode legs soldered as pins for the DIP PCB are just perfect for the socket.
C825 is still the original as Fedex are stuffing around with the Mouser order.


20V range shorted no filter


Ref nulled to compare the bounce of around 10-15uV
You can tell No.2 has had a bit of use based on how bright the last digit is.
I knew No.1 had low use so the ADC must have failed very early on.

Now K2001 No.1 (Single OPAMP repaired ADC) Left - stabilizes within 10 minutes or less but always has the offset as shown, calibration, zero issue?
K2001 No.2 (Dual OPAMP ADC Worked) Right - Takes a good 30 minutes to settle down and really goes nuts during the warm up but settles to low values.
Both eventually bounce around by about the same amount but with the offset shown.
I haven't even isolated the SOP-8 tracks on the back of the adapter PCB and it seems ok.

Both Units have the shield fitted as it makes a huge difference and the cases sitting on top (clip leaded to the frame, also makes a difference) for thermals and shielding.
K2001 No.2 seems very sensitive to thermals which explains the weirdness on warmup, maybe zener issues also.

Yes I know eye-balling it isnt very scientific but I need an AR488 interface, wasnt able to work out how to log with a keysight GPIB adapter easily.

Still plan to replace C825 when they come and after testing them all, still think the FKP3 will be best.

So as I have OPA1655's and more Ali boards I want to build up some dual opamp trials as Giga-Joe has.
Maybe we can come up with an adapter board.
I wouldnt mind doing some circuits, only found one of yours Joe.

Last combo seemed to be ADA4077 + OPA1655 (Instead of OPA827) is this still the consensus?
Can we do some suggestion lists for both single and dual opamp combos, I lost track several pages back.
Alternatives to the OPA827 would be good as they are pricey.

Whats the latest results Joe?

[GigaJoe]

for dual:
ADA4077 + OPA1655  , kinda work , i think not optimal

for single:
OPA205A + OPA197 ;  ideal ... i think OPA205A + OPA1655  but i have difficulties to make it stable so OPA197
here last: so noise around 7-12µV  and wobbling , i give up ...
https://i.ibb.co/ns91Nqg3/voltage-actual-centered.png
so average x7 (NOT 10 ) give a quite reasonable output of few µV floating

ADC a fraction of whole thingi ,  before screw-up ADC ,  add capacitors for filters ( make sure it not start ringing )
replace U341 ( for 20v ramge)
adjust current to zener TC=0 on adc board.
may replace LT1007
and hook GPIB to get data for analyzing .... then try ADC ...

"GK" -TI  usually  VSSOP-8

[Kleinstein]

Good candidates for a single OP-amp version would be OPA827, OPA828 and OPA140 (and similar cheaper ones) and ADA4620 (similar amplifier from AD). They should be better than the original OPA602, as they are low LF noise and higher BW.
The LF noise matters for the noise and the speed can matter for the INL.

For my ADC I have / had good success with OPA145, OPA141 or OPA1641 (the OPA140 should work as well, just a bit more expensive) for the precision OP-amp. As a BJT option (less 1/f noise) the ADA4077 and OPA205 should be rather similar with a slight edge for the OPA205.
A BJT amplifier may want a fast OP-amp with low R_out to keep the excursions to below some 25 mV, where a BJT input may start to become nonlinear. It may help to have a little capacitance (ggf. with some series R) to ground at the integrator input for the super faster transient part.
In theory a AZ OP-amp (like OPA182) could also work, but may want some extra filtering to reduce the effect of switching spikes.

I tried these with TLE2071, OPA172, OPA1641 for the fast amplifier (all around 10 MHz GBW). The main parameters are GBW and open loop R_out. Other options would be:
OPA197   : 10 MHz low power, but has higher R_out - could be an issue, especially with BJT based amplifier.
OPA1677 : cheap 16 MHz, 2 mA supply, R_out looks good
TSB951   : 50 MHz, rel. low supply (2.2 mA typ)
OPA1655 : 55 MHz, but a bit high supply current ( 3.9 mA typ)
OPA810   : 70 MHz - but 27 V max supply. R_out looks good.
THS4631 : 210 MHz  - AFAIK used in some newer KS3446x, but high supply current

For the capacitor I measured the lowest dielectric absorbtion with a TDK C0G MLC capacitor - about 1/5 the DA of  PP / PS film and Kemet and Samung C0G I tested.

[GigaJoe]

a bit practical comment about .
going to fast second opamp may simply caused instability due to non optimal original board layers,
OPA1655 - definitely on edge and oscillated when rail capacitors not in place. I see it as a bad sign of instability... So higher may do not getting any benefits but headache. 10nF , and 30Khz around - do not required something fast , in my opinion ..

Regardless of opamps .. somehow it extremely sensitive to divider, as thevenin resistance and divider coeff. a 50 ohm difference and everything screwed ... it getting absolute instable, or increase high frequency noise , or decrease HF noise but increasing wobbling  a minutes period of instability.  so blindly replace opamp that a serous gambling ..
OPA187 680 - 300 (ground)  - OPA1655
OPA205 300 - 150(ground)  - OPA197
that 2 options more less balance between everything....
probably OPA205 + OPA1641 would be interesting to try .. but i really don't see how it improve . unless  it affect scale linearity make it much better ..  i dont know ...

THS4631 look very interesting with integrator cap in 100-200pF and half megahertz integration -def. not for this appl.

[Kleinstein]

A fast OP-amp should have a decoupling capacitor directly on the adapter PCB and not just rely on the original PCB for decoupling.
I agree that with the 10 nF integration capacitor one would not need high high GBW to keep the input voltage small. The point of a reasonable high GBW is that the 2 OP-amp integrator is a bit slower in settling than the 1 OP-amp integrator. The GBW of the fast OP-amp determines how much divider is needed and this way how fast the integrator settles after switching. The point here is not 30 kHz modulation frequency, but the length of the short pulses in the pattern. The difference could be in the linearity, not in the noise.
All proposed solution here are still faster than the OPA602 + OPA177 in the 2 OP-amp version of the K2001.

I see the OPA205 + OPA197 combination a bit critical, because of the relatively large open loop output impedance (some 350 ohm) of the OPA197. This can lead to voltage spikes that exceed the linear range of a BJT based OP-amp (like OPA205). This might cause some nonlinearity. The simple FB pattern is still rather forgiving and the effect might still be linear.
I would combine the OPA205 more with an OPA1677 or OPA1641.

The sensitivity to the divider should not be so large - that points to maybe instablity or ground issues.

[HairyWombat]

A fast OP-amp should have a decoupling capacitor directly on the adapter PCB and not just rely on the original PCB for decoupling.

Is this referring to my Ali adapter board also, was thinking that I would have added some stray inductance so it would benefit from a cap.
Mouser order finally came so checking which of the 10nF caps are best out of the Kemet, WIMA and TDK varieties I got.
What are your preferred methods for leakage testing.

[Kleinstein]

The capacitor directly at the adater can also help with a standard SO8-DIP8 adapter. For an OP-amp with > 10 MHz GBW this would definitely make sense, especially if the decoupling on the PCB may also not be that good.

For capacitor leakage testing it would be with a really stable reference voltage (e.g. 10 V) and than measure the current (pA range) with a TIA. It may well take some time ( a few seconds for a good capacitor, maybe minutes for a poor one) for the current due to dielectric absorbtion to decay. An alternative test for super low leakage would be to use the capacitor with a fA range bias OP-amp (e.g. LMC6482 or similar) as integrator. The tricky part there is still switching at the input to start with a given voltage or discharge. This would need a very low leakage switch or relay (e.g. better than most simple reed relays).

Getting good enough leakage is likely the simpler part. The more critical parameters may well be the dielectric absorbtion.

[GigaJoe]

I see the OPA205 + OPA197 combination a bit critical

Ouch ... I ddin't noticed that ... gosh ... seems another round ...