The thing I was doing is making a 4-terminal current to voltage converter. When I was done it had 4 ranges with +-10V representing +-100 mA. it was capable of +-10 v biasing. It had a Voc mode. It did fine on the AC performance and reasonably well. It had like a 40 pA offset without any nulling. I could not complete the nulling circuit and that wasn't the interest. No manual nulling was possible in the design. I added +-50 mA of suppression, but that meant that the bias was limited to +-5V. Usually we were interested in +-1.5V max anyway. I used 400 meg resistors so the sense leads could be disconnected.
It was a front end to a DSP lock-in amplifier to measure the spectral response of a solar cell. Monochromatic light was chopped at about 40 Hz and the solar cell was connected to this I-V converter. During bench tests it was fine, but when I attached a calibration 1 cm*2 cell, it went unstable.