We are using a SCXI-1520 with a SCXI-1314 accessory with the intention of using it with some full bridge and half bridge strain gauges.
We have been seeing a offset in the readings (~2.5%) which we have been trying to identify the cause of.
As part of this I have connected a calibrated mV source and have noticed that if I have the excitation of the SCXI-1520 set to none, external or internal with a very low value the card reads the correct voltage.
If I set the excitation to internal with a reasonable voltage (10V in my case) my voltage reading is 0.5mV too low. this is with the strain gauge attached to the excitation pins to ensure that there is current flowing and there aren't any weird effects of effectively having an infinite resistance across the internal voltage supply.
Any suggestions as to why I might be seeing this shift in voltage?
My current work around is to power the strain gauges with an external PSU which isn't ideal. Another work around could be to adjust my scaling to take account of the 0.5mV offset but I don't really want to do that until I understand what is going on.
