analog input channel noise problems

[ASIM_BHATTI]

Hello every one

I have recently purchased  NI-PC6255 DAQ card along with SCB and SCC connectors. I am interested in measuring low level analog voltages(i.e in range of mV).

Problem is that during first two, three days of testing this card every thing worked very fine(i.e. it was measuring analog 2mV of sine wave (by function generator) very fine and noise level was very low) but now suddenly noise level suddenly increases although no change in test setup or equipment has been made.(Now instead of 2mV(as in beginning) no analog signal is detected below 10-15mV).

Kindly tell me what can be the problem since all equipments,the setup and the environment everything is same as in beginning and i have checked it many times.

Note: i am currently using RSE analog input mode .

[Nathan_R]

Hi ASIM_BHATTI,

Since you are measuring such a small signal make sure you are using the smallest input range. In the case of the PCI-6255 the smallest range is +/- 100 mV. The noise you are seeing sounds like the input range is set too high. Also check to make sure your DAQmx Task terminal configuration is set for RSE. The default setting will depend on what channel you are acquiring from and may not be RSE. So if you have your signal wired for RSE I would specify that it is RSE when you create the task. More information on the default input terminal configuration can be found in the NI-DAQmx Help under NI-DAQmx Device Considerations >> Physical Channels >> Default I/O Terminal Configurations.

Another great resource for tip on reducing noise in analog measurements is the Developer Zone article, Field Wiring and Noise Considerations for Analog Signals.

Thanks,