I have a small (< 100 uA) current from a photomultiplier tube that I would like to measure with my 1608HS DAQ. I have previously successfully measured this current with a 1616HS DAQ, but I purchased the 1608HS because I wanted a DAQ with simultaneous acquisition. When I try using a load resistor (see picture) to convert the current to a voltage, the reading I get floats around and eventually steadies at -5 Volts. AGND is attached to ground and I measured 0 voltage between ground and the PMT current source prior to entering any other circuit. My load resistor is 100 kohms.
The code I use for getting the voltage reading is ul.to_eng_units(board, ai_range, ul.a_in(board_number, input_channel, ai_range))
The circuit diagram is also attached. Basically the same circuit/code did work for the 1616HS DAQ, there just seem to be issues with my new 1608HS one. Any help would be appreciated.