USB-6001 unable to read Keithley 487 output voltage

[nccd99]

I have been losing my mind trying to figure out what's going on here so I would really appreciate some help if anyone has seen this before.

I'm measuring a home developed ion detection instrument which by measuring the small current it generates the ion concentration can be estimated. The current is being measured by a Keithley 487 which I know is old but I think it should work for the job I'm trying to do. The Keithley appears to be working exactly as expected and I can see from the display that it is measuring the current therefore the ion concentration correctly. I can also measure the analog output voltage from the Keithley and it also appears to be correct with a voltage being given in the range of 0 to -1.5V, I am measuring this with a basic multimeter. The issue arises when I try to measure this voltage with the USB-6001 which, I have wired up to the voltage output of the Keithley via analog inputs 0 and 4. I also grounded the DAQ to the Keithley chassis and have tried using RSE and differential modes. The signal I see using the test panels and the Labview software is always an oscillating wave that changes frequency slightly.  I will attach a pdf of what I'm seeing on the voltage graph but the important observation is that when I swap the input to the votlage calibrator at any voltage I choose the DAQ can detect and display this voltage perfectly. So I know that the DAQ is wired up correctly and should be able to read the Keithley output voltage. There are some more changes and responses that I have tried and shown in the pdf. My only remaining hunch is that the Keithley output current is too small to be detected by the USB-6001, does this sound possible? I have tried quite a few potential solutions, like making sure everything is grounded correctly, that there is no noise from other power sources and wires but maybe I have missed something.

I had to to paste the file below and not in PDF format sadly as it wouldn't accept it

If anyone has seen anything like this and can offer anything for me to try that would be great as it has ground my research to a halt.

1 sample 1HZ with a voltage of around -10mV

Same again but at 10hz

Same again at 100hz

Went back down to 1hz and the oscillations changed frequency for some reason

Same as above but I paused the measurement twice throughout the 30 second window. The oscillations seem to continue even when the program is paused and when it is unpasued it will try to catch back up.

TURNING on the isoniser and having a voltage signal of around -0.2V going in seems to have some effect on the wave. Seems to get broader and has these flat spots.

returned back to this after a few seconds but is still broader than pre ionisation?

Switched over to a calibrator at 1V and the DAQ is able to read perfectly

What happens if I turn off the filter on the Keithley

then turning the filter back on again

playing around with switching the filter from analog to digital and back

when zero check in on the voltage goes to zero but the oscillations continue. Ion source has no effect here as expected

TURNING BACK OFF THE ZERO CHECK

Two short peaks are when I turned off the -20V bias off