Noisy signal with differential analog signal on USB-6221

[D]

Hi,

I've been trying to troubleshoot a noisy signal that I'm getting.  I'm using a DAQ USB-6221 and the signal is differential analog (4-20mA from a dew point sensor converted to 0-5V with a 200 ohm resistor).  The signal itself is pretty noisy, with some ~1 volt spikes at regular intervals (see attachment) and occassionaly rails at 5V (or whatever the allowable maximum is if I change the voltage range in MAX).  I read the "Field Wiring and Noise Considerations for Analog Signals" paper and I have tried the following fixes:

-bring DAQmx drivers up to the most recent version

-add bias resistors (I tried 20kOhm and 84kOhm, and I tried adding just one resistor to the low side and adding two resistors to the signal, one from each signal wire to AI gnd)

After these two fixes I still got the cyclic noise/spikes and no railing was obvserved (although that might just be coincidence) but bigger spikes (+1 volt).  I have now tried the following as well:

-the 6221 was situated between a computer tower, back up power supply and a small vacuum pump so i moved it away from all of that

-switched out to a brand new USB-6221

AFter these fixes, I don't know if it still rails but the signal spikes are still there. 

The goal is too eliminate these voltage spikes and the railing.  Any thoughts?

Thanks,

Diana

[EEsRULEtheWORLD]

Hi, Diana.

I have some suggestions/questions:

1.  Try inputting this signal on a different channel on the 6221.  Do the spikes and/or noise still appear?  That particular channel may be damaged.

2.  Connect one of your analog output channels to an analog input channel and use test panels in MAX to simultaneously output and measure a DC signal.  Do you still see the noise and/or spikes?

3.  What is your signal source?  This will determine how exactly to configure your 6221.

4.  Have you tried measuring your signal source with an oscilloscope?  It may be the signal source that is the issue.  If this is the case, a filter may of value to you.  Internally, the analog input of your 6221 provides a low-pass filter can be programmatically engaged or disengaged with a cutoff frequency of 40 kHz.

[Henrik_Volkers]

Where is your 200R located? Near the sensor or near the DAQ?

Seen this thread ?