Noise issues when measuring analog input signal

[bhk]

Hi there,

I was trying to record the current output out of my power supply (Agilent 6674A, 0~60V, 0~35A) using the current monitor (IM) port on the rear panel of the power supply.

The current monitor (IM) port will output -0.25V~7.93V corresponding to 0~35A. The signal source type is a grounded one when one of the power output is grounded.  Following the instructions from some documents an NI AE gave me, I tried both DIFF and NRSE configuration. I supposed to get a  -0.25v signal when the power output isn't connecting to anything (i.e. open loop) while I always got a noisy results (please see the attached file; on the top is the noisy results, and the bottom one is after a low pass filter in LABVIEW) with the largest amplitude plus minus 0.25 v which would depends on the power output(The bigger the voltage of power output, the bigger the amplitude of noise is),  at two different frequency. 

I went through some document and wondering if that's from the ground loop. Is there anyway like isolator or something else can help eliminate the noise?

Or what's the other reasons I had this kind of noise? 

I am using LabVIEW 8.2, PCI 6024E DAQ card, SCB-68 connector box, and twisted wires for IM port and power output.

Any help would be highly appreciated.

Regards,

Bo-Han 

[jschwartz]

Hi Bo-Han,

The differential configuration will be your best bet for noise rejection.  The documentation supplied to you was probably the Field Wiring and Noise Considerations DevZone.   From this document, Table 1 shows how to connect the device in differential mode:

 

If you are wiring to channel 0, you will wire to ai0 and ai8, which corresponds to pin 68 and pin 34.  Because your power supply is a grounded source, you shouldn't need the resistors to ground.  Make sure that your task is configured to read in differential mode.  

Another thing that we can try is looking at the channel in Test Panels in Measurement and Automation Explorer.  To get to Test Panels, find your device in Measurement and Automation Explorer then right click on it and select Test Panels.  This will allow you to directly read the data from your channel to see if you get the same noise.  If you still see the same noise, try using a different channel.  If all of the channels are shoing the same thing, you should measure a known voltage source (like a battery) to see if you can get expected results.  If you can get consistant  results with a known source, it could be an issue with your monitor output.  

Just a tip, if you want to automate your power supply and control it with  a PC, we have a driver built for the 6674A that uses the GPIB port to communicate:

Agilent/HP 6674A Instrument Driver

You could control your device through a USB port using a USB-GPIB connection:

USB-GPIB-HS