Common Mode Rejection

This simple demonstration shows the effect of common mode noise on DC measurements. Common mode noise is the noise, most frequently 50 or 60 Hz hum which is common to both input lines. The better the Common Mode Rejection Ratio of the input circuitry (CMRR) the less the measurement is affected by the noise.
In this demo, the DC voltage is 1 V DC and there is low frequency random noise at 0.1 V on top of the signal.

Experiment #1: Increase the CMRR to 100 dB and notice that the measurement is almost noise free. Now go to 120 dB and notice that only the last digit is affected by noise.

For more information, go to zone.ni.com/glossary and search for CMRR.

Requirements

Filename: cmrr.llb

Software Requirements

Application Software: LabVIEW Full Development System 6.0
Language(s): LabVIEW

Reader Comments | Submit a comment »

A fine example of the effects of common noise but no hints about how to remove it - this example mixes a noisey signal with a desired signal and the only control is to adjust the amplitude of the noise mixed with the desired signal.
- Oct 15, 2005