This module is designed to measure currents at relative high speed (50kHz, 10us rise/fall) while still using the long cables that are needed in a cryogenic setup. Total input capacitance including cables (typical 100pF/meter) and filters should not exceed 1nF. If you use a matrixrack, default Pi-filters are 3nF so instead use the low-capacitance lines at pin 21..24 that are equiped with 200pF filters (if it was ordered in standard configuration, check sticker at back of matrixmodule)
The unit has a fixed gain of 10MV/A with a selectable postgain of x1/x10dc/x10ac(dcx1).
When doing lock-in or noise measurements as a function of dc-current the [x10ac] makes it possible to amplify the ac-signal or noise while maintaining a large dc-sweep range.
This unit is implemented with ground-lift, this solves problems with thermovoltages that are usually present in cryogenic setups. The zero reference pin(2) should be connected to a cold-ground-wire or otherwise to a room temperature ground.
Other use is hooking up a small dc-voltage (S3m module) to pin 2 and doing a 1-wire resistance measurement via pin4 on a sample that is grounded with it's other contact.
2=ref-in means that you can (and should) connect pin 2 to a reference ground that can have a difference dc-level (up to some 100mV) e.g. a wire from cold-ground inside the fridge. This helps to remove excess offset by thermovoltages.
Usual procedure to determine offset voltage: If you put your voltagebias on 0V there should be no current, otherwise tune it to a value where you find I=0, the voltage that you have now is the offsetvoltage in the system. You can compensate by taking this into account.