Teledyne LeCroy AP034 - Manual (Page 54)

AP034 Active Differential Probe
48 922261-00 Rev A
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.