scxi 1308 noise

[nasu]

I have a H2Scan hydrogen sensor which outputs a 4-20mA signal connected to the SCXI 1308. When viewing the signal in MAX I get noise from -4 to 10mA making it impossible to use. I have grounded both the sensor and the chasis to the same outlet as the computer. When I use a multimeter to read the current coming from the sensor i get a correct reading of ~4mA since no hydrogen is present. What are some possibilities of where the noise is being generated and what can I do to remove it? It is coming from somewhere between the SCXI 1308 analog input, SCXI 1001 or the computer. Thanks. I have tried switching channels on the 1308 with no change.

[Jason_D]

nasu,

The 1308 is just a terminal block it doesn't actually take measurements.  The 1001 likewise is just a chassis.  You probably have the 1100/1102/1102B/1102C.  Have you seen the Field Wiring Guide?  What are you using to control the chassis? 

[Broken_Arrow]

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nasu wrote:
I get noise from -4 to 10mAs.

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Unless this is a typo, if you're getting Negative 4mA, your cable is intermittent or your bias voltage is the culprit, such as a bad GND reference or GND loop. Even if you meant +4 to 10mA, this is an insane amount of noise - it almost has to be a hardware failure. How does the bias voltage look on the scope?

I have 4-20mA inputs and outputs (both internally and externally biased), and even without trying very hard, noise is 0.01mA.