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HVD3102 and HVD3106 High-Voltage Differential Probes
Common Mode Rejection Ratio
The ideal differential probe/amplifier would sense and amplify only the differential mode voltage
component and reject the entire common mode voltage component. Real differential amplifiers
are not perfect, and a small portion of the common mode voltage component appears at the
output.
Common Mode Rejection Ratio (CMRR) is the measure of how much the amplifier rejects the
common mode voltage component that appears at the output. 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 (e.g., 10,000:1) or implicitly in dB (e.g., 80 dB).
Higher numbers indicate greater rejection (better performance).
The first order term determining the CMRR is the relative gain matching between the + and –
input paths. Obtain high CMRR values by precisely matching the input attenuators in a
differential amplifier. The matching includes the DC attenuation and the capacitance which
determines the AC attenuation. As the frequency of the common mode component increases,
the effects of stray parasitic capacitance and inductance in determining the AC component
become more pronounced. The CMRR becomes smaller as the frequency increases. Therefore,
the CMRR is usually specified in a graph of CMRR versus common mode frequency.
The common mode frequency in these graphs 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 than the repetition rate may suggest. This
makes it 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 and amplifiers.
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