Phantom Offset Voltage of ~0.6V

[blackburnite]

I'm having an issue with some voltage measurements using a PXI 6224.  Several of the AIs will show readings of ~0.9V, but when the harness pins where these readings are taken are probed with a multimeter, the voltage is closer to 0.3V.  Additionally, when this multimeter reading is taken, the 6224 reading is also ~0.3V while the meter probes are in contact, but returns to ~0.9V when the leads are removed.  Any thoughts?

[johnsold]

High source impedance?

You do not indicate what is connected to those inputs. It sounds as though you may not have anything connected (floating inputs) or the inputs are connected to a signal with a large source impedance.  Either way, the change in loading which occurs when the external mutimeter is connected causes the change in the readings.

Lynn

[Henrik_Volkers]

The physics behind is the bias current of the AI. Each analog input can be seen as a small current source (see bias current in the spec) and the input impedance.  Both together result in a measurable voltage.

According to your readings, the DAQ has about twice the input impedance compared to the DMM  😄