Eliminating Powerline Noise from DC Measurements in NI Software

Issue Details

• I am measuring low level DC-class signals like from a pressure or strain transducer and see 50 Hz or 60 Hz noise or spike from the power lines nearby. I want to remove the noise without using an external hardware low-pass filters. Can I reduce the noise in software? Are there any preinstalled filters or functions in LabVIEW?
• Is it possible to add 50/60 Hz noise rejection to my module?  Is it possible to add this using LabVIEW software?

Solution

Yes. Remember that power lines deliver voltages in sine waves. Knowing that the arithmetic mean over one period of a sine wave is 0, you can remove power line noise from your measurements by simple averaging.  

In general, the sampling rate must be an integer multiple of the noise frequency. The number of samples averaged must be integer multiples of the number of samples per period.  

If you are in a country where power generators generate power in 60 Hz (e.g. USA), you must sample at a rate that is an integer multiple of 60 Hz.  If you sample at 120 Hz, you will have 2 samples per period. If you average over 60 points (integer multiple of 2) you will be averaging over 30 periods. In this case, you can get effective readings every half a second, without the 60 Hz line noise in your measurements. The more periods that are averaged, the further the interference is reduced. 

If you are also seeing 50 Hz noise, you could sample at the least common denominator of 50 and 60, which is 300 S/s. This way you are averaging a multiple of 5 points for 60 Hz noise and a multiple of 6 points for 50 Hz noise with only one sample rate. 

In some cases it is not possible to average without eradicating the interesting data. In this case, it is possible to use a software filter, such as a Butterworth filter, to remove the 60 Hz signal. Software filters are available in the Advanced Analysis Toolkit for LabVIEW and come standard with the Full and Professional Development versions.