Input Impedance Between AI+ and AI-

Updated Sep 10, 2020

Issue Details

I have a PXI Multifunction I/O Module, such as PXI-6225. When referring to its specification, it only mentions about input impedances from both AI+ to AI GND and AI- to AI GND. I would like to know what is the input impedance between AI+ and AI-.

Solution

The signals from both AI+ and AI- are buffered separately rather than being fed directly into a differential op-amp and therefore its specification is prepared in such a way.

The impedance is so high, and there are factors that will contribute to the AI+ to AI- impedance that can only be measured, but given the impedance is so high, there isn't a great way to accurately measure that impedance. For example, the amount of leakage in bypass capacitors can have a measurable effect, so it is impossible to compute an exact number.

In short, the input impedance between AI+ and AI- is extremely high. We would expect the impedance to be of the same order of magnitude as the AI+ to AI GND specification.  if you are using this device in an application where your DUT has an output impedance that would matter with >10GOhms then this may not be the right device for that application and you should reach NI support for further discussion.