CompactRio + NI9203 oscillatory signal

[AndrzejJ]

Hi!

I'm having a following problem. I have NI9203 inserted in compactRio. Im measuring room temperature with Aplisens probe. I have it connected in current loop (with 240 ohm resistor) with NI9203 in follwong way: 24V source -> probe -> resistor -> NI9203 input and COM connected back to source. 

I'm getting very high oscillation of the signal and the error of several degrees, when I replace NI9203 in the loop with the standard digital multimeter I'm getting stable and correct temperature.

Do you have an idea what could be a cause of the problem?

Thanks for help!

[Stephen_Moore]

Hi there,

I found that the 9203 is particularly good at picking up noise and ripple from a power supply especially on loop powered transmitter etc. To get around this I try to use a good quality power supply but more importantly I take many samples on the 9203 channels and average the readings. I guess that the smoothing circuit of a DVM has a similar effect which is why you then get a stable and correct temperature.

For averaging I use the FPGA code and sum 1024 samples for each channel then multiply the sum by 10^-10. I update the values every 50 milliseconds. Rather old school I know but I have used the same code since 2007 and it works. This was in the days of using the raw data.

Cheers

Stephen

[AndrzejJ]

Thank you for the answer.

This however does not help 😕 (I tried it in RT mode, can it be that FPGA work differently?)

I'm getting really large oscillations, such that even that averaged measurement oscillates. 

Intrestingly, the multimeter has measurement time of 0.5s but it remains stable for long periods, so it's not only about averaging.

[nanocyte]

I'm not really sure how you expect us to help. Can you give us more details about your DMM? Are you using current measure mode? The DMM is going to probably have a lower input impedance which might be part of the problem.

Are you using example code? If not, maybe show us what you have.

Maybe give us sample data to show us the oscillations (make sure your x axis is accurate).