Instructions

Operator’s Manual

Figure 13, Common Mode and Differential Mode Range

Common Mode Rejection Ratio

The ideal differential probe or differential amplifier would amplify only the differential

mode voltage component and reject all of the common mode voltage component. Real

differential probes and amplifiers are less than ideal, so a small portion of the common

mode voltage component appears in the output. Common Mode Rejection Ratio (CMRR)

is the measure of how effectively the probe or amplifier rejects the common mode

voltage component. CMRR is equal to the differential mode gain (or normal gain) divided

by the common mode gain. Common mode gain is equal to the output voltage divided by

the input voltage when both inputs are driven by only the common mode signal. CMRR

can be expressed as a ratio (for example, 10,000:1) or implicitly in dB (for example, 80

dB). Higher numbers indicate greater rejection (better performance).

The first order term that determines the CMRR is the relative gain matching between the

and

–

input paths. To obtain high CMRR values, the input attenuators in a differential

probe are precisely matched to each other. The matching includes the DC attenuation as

well as the capacitance that determines the AC attenuation. As the frequency of the

common mode components increases, the effects of stray parasitic capacitance and

inductance in determining the AC attenuation become more pronounced. The CMRR

becomes smaller as the frequency increases. Hence, CMRR is usually specified as a plot

versus common mode frequency.

The common mode frequency in these plots is assumed to be sinusoidal. In real life

applications, the common mode signal is seldom a pure sine wave. Signals with pulse

wave shapes contain frequency components much higher that the repetition rate may

suggest. As such, it is very difficult to predict actual performance in the application for

CMRR versus frequency graphs. The practical application of these graphs is to compare

the relative common mode rejection performance between different probes or

amplifiers.