taking voltage measurements with 9233

[Nasgul]

I am using an NI 9233 DAQ to read voltages from a resistor in series with a DC fan motor. The motor is 12V, 2W, and the series resistor is 0.1 ohms (the DAQ has a voltage limit of 5V, hence the small resistor). The motor should be pulling 170 mA or so (confirmed with a separate DMM), but the readings are consistently different from the DAQ - more like 150mA. Could the DAQ internal resistance be messing up my measurements? I'm sampling at 50 KHz.

[Now_With_Underscores]

Hey Nasgul,

 There are a few issues with using the 9233 to try to read the voltage across the .1 ohm resistor.  First off, the 9233 is hardware AC coupled, which means the 9233 is specifically trying to remove the more or less DC voltage you are trying to measure across the resistor from the signal.  What's left after we filter should ideally be an AC signal.  With the 9233 we are specifically trying to remove DC components from the signal.

Second, the 9233 is a dynamic signal acquisition device, which means it is aimed at measuring relative changes with very high precision.  The tradeoff here is that we are less accurate at determining the actual real world value, but more precise on the change that has occured per iteration.  This is due to the delta sigma analog to digital converters we use for DSA devices, which are designed to be used in sound and vibration applications, among other things, because the measurement of interest is the relative change from past samples.

In short, you will need to use different hardware.   For your application, I would recommend the NI-9215. Have a great evening.