9215 distorting input signal

[PaulG.]

I am attempting to measure AC current by measuring a voltage drop across a resistor. Using a NI 9215 the signal looks like this and is not accurate.

When I hook up the same signal to a cheap-o USB 6008 the signal  looks fine and I'm measuring the same RMS voltage from my multimeter.

I would prefer the higher sample rate of the 9215 since we are looking for distortions in the AC signal. What am I missing here? Thanks in advance.

[johnsold]

I can think of two possibilities.

1. The lower input impedance of the USB-6008 has some effect on the driving circuit.

2. Ground loops.

My guess is that 2 is more likely than 1, although the funky input circuit of the 6008 may be a factor there as well. The return path to ground from the AC circuit is probably different from the ground paths in the DAQ devices.  OK. I just looked at the 9215 specs and see that it has isolation. What is connected to the other channels? The input voltage range is defined as +/-10.2 V from the average of the AI- inputs for the BNC version.

Lynn

[DsX · Accepted solution]

Hi,

Are both modules connected in differential mode?

With NI 9215, AI- always needs to be connected to COM (with 1MOhm resistor for DIFF or directly for RSE) because it is an isolated module (see specs).

USB-6008 has no isoloation, but still need bias resitors connected between AI an COM.

Maybe this is the cause of what you are seeing.

Best regards

Matej Zorko

[PaulG.]

When all else fails read the bleepin instructions. This makes sense. The current source is just a sensor. And floating. The 1M cleaned up the signal and measurements match those of my benchtop RMS meter. Thank you.