What Is The Typical Resistance Value From An Analog Input Channel To Ground For NI-9208?

Issue Details

I am measuring resistance from an AI channel to the ground on an NI-9208, and I am receiving values as low as 73.9, and some as high as 77.3, and I am concerned because of the difference I get.

Using Ohm's law, I can provide a constant 4mA input to every channel and get different V results. Using a DMM, here are the R values I got on a few channels:

AI0 = 73.9
AI1 = 76
AI2 = 74.2
AI3 = 74
AI4 = 77.1
AI5 = 74.7
AI6 = 77.3
What should the R-value be for these channels? And should there be a 3.5 Ohm difference between low and high in the above values?

Solution

What you are measuring is correct and expected. The 9208 input circuitry consists of 2 different resistive elements connected in series

• A precision 34 Ohm resistor. The voltage across this resistor is what is measured to calculate the input current

• A very loose tolerance ~42 Ohm posistor. This has a tolerance of +/-20% or more. The posistor is a temperature-dependent resistor that acts as a protection element. It helps provide overvoltage protection since it will heat up and increase in resistance during overvoltage events

So, it's as expected that you would measure ~76 Ohms typically with a several ohm variation. This does not impact the measurement accuracy at all since that only relies on the precision resistor which you cannot directly measure from the IO terminals.

The published specifications state:

This is calculated as the maximum expected input impedance at room temperature. However, typically it's closer to 76 Ohms.