Reference - Teledyne HVD3000 Operating Manual

HVD3000 / HVD3000A High-Voltage Differential Probes

Reference

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. 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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