How to Eliminate AC Noise When Measuring DC Signals

Updated Jan 16, 2019

Reported In

Hardware

  • NI-9208
  • NI-9216
  • NI-9232

Driver

  • NI-DAQmx

Issue Details

  • I am measuring low magnitude DC signals and am seeing noise from AC signals, such as 50 or 60 Hz power lines running in parallel to my signal wires. How can I filter out this noise?
  • I am seeing 60Hz noise when using my sensor, how do I get rid of this? 

Solution

AC noise can be filtered out of DC measurements with a low-pass filter. To build a low-pass filter, simply set up the RC (Resistor-Capacitor) circuit shown below. Choose the resistance and the capacitance so that the cut-off frequency is below the frequency of the noise signal you want to reject. The cut-off frequency is computed by: 

Cut-off Frequency = 1/(2 x pi x R x C)

where:
R = resistor value in Ohms
= capacitor value in Farads
If a hardware filtering solution is not desirable, it's also possible to eliminate noise purely in software.

Additional Information

Another thing to check is that all of the power supplies to your sensors are connected properly and powered on. 

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