HP/Agilent/Keysight 34420A NanoVolt/Micro-Ohm Meter repair

[Kleinstein]

The translated BBS38hot text is about a 34410, so a newer version with a completely different meter.

As far as I see it the largest noise part in the ADC is the current noise of the OP27. This competes with the input signal though a 100 K resistor and calculated back to the input the noise would thus be current noise times 100 K.  The 10 PLC AZ mode has an effective BW of 5 Hz because of the 2 conversions and  an effective frequecy of 2.5 Hz for the 1/f noise. The datasheet gives a current noise for the OP27 of close to 1 pA/sqrt(Hz) at 20 Hz that would translate to some 3 pA at 2.5 Hz with a square root of frequency relation.  However the DS says they measure the current noise with the resistor split over both inputs - which in my view is wrong: This suppresses the correlated part of the current noise and the uncorrelated current noise would only add as root. So at best the current noise figure is too small by a factor 1.4, and possibly more. So the current noise from the OP27 in the integrator would correspond to more than 3 pA/sqrt(Hz) *1.4*100 K = 420 nV/sqrt(Hz) of noise at the input. This would be 930 nV of RMS noise for the input (and possibly more from the correlated current noise part).

The 2nd largest noise part would be the auxiliary ADC with it's quatization noise (around 350 nV/sqrt(Hz) for the input). Here it is a bit unclear how noisy the ADC is and especially in how much the contineous changing input may effect the µC internal ADC. So this noise part may be also quite a bit larger.
The big unknown is how good the timing is when the auxilliary ADC is sampling:  so does it get the last run-up step fully included or only partially and may need an extra correction factor for the last step.

The 3rd largest noise I have in my list is the higher frequency ref. noise (could be some 150 nV/sqrt(Hz) for the input). The noise from jitter (likely mainly the 74HC4053) may reach a compable size.

The ADC mixes back some reference noise from the 50-150 kHz band back the the near DC band. There is some parial filtering from the relatively slow AD706 OPs, but not full filtering. There is a good chance that some capacitance or better capacitance with series resistance (similar to the suggested ARD1399 circuit) in prallel to the LM399 reference could reduce the higher frequency noise. So something like 1 µF+5-10 Ohms in parallel to the 7 V from the LM399. There is a chance this would reduce the reference noise on the 100 kHz frequency band. It is not the largest noise source, but also a relatively small change.