Thanks for the plot. Is there any difference in noise using digital data via GPIB or analog output plus a voltmeter? How did you manage to generate those 95nV?
1) No difference. The (isolated) analogue output is generated by a 12-bit DAC, either from the top or from the bottom 3.5 digits. Each step is 1mV and clearly visible on a good meter, like the 34465A on 100mV or 1V range, so 1mV on the output from the bottom end = 1LSB = 1nV input on 2mV range.
2) From a divider - 21M/10R (just an accidental value) mounted in the plug and connected to 200mV output from the Keithley 617. 10 ohm in the divider is probably adding to the noise/drift as it is a simple 1% metal film resistor. I will re-build it with better resistors - probably a metal foil 10R, and a good quality 10M for 1/1000000 divider.
Cheers
Alex