Teledyne Lecroy HVFO108, Руководство оператора

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HVFO108 High-Voltage Fiber Optically Isolated Probe
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Reference
Common Mode Rejection Ratio
The ideal amplifier would sense and amplify only the desired voltage component and reject the
entire common mode voltage (V
CM
) component. Typically, this is best done with a differential
amplifier, but real differential amplifiers are not perfect, and a small portion of the V
CM
component appears at the output.
Common Mode Rejection Ratio (CMRR) is the measure of how much the amplifier rejects the
common mode voltage component. It is most often used to describe the performance of a
differential probe/amplifier and in that case 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).
In a differential probe/amplifier, the first order term determining the CMRR is the relative gain
matching between the + and – input paths. High CMRR values are obtained 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. In a conventional high attenuation
HV differential probe used to measure a small signal floating on a high common mode voltage,
the differential mode gain must be very high, and the input path matching must be very precise,
limiting how good the CMRR performance can practically be for high common mode voltages.
In the HVFO, the amplifier is single-ended. This is possible since the amplifier is floating and the
signal return path is not to oscilloscope ground. The requirement for precisely matched
impedances between two input leads does not have to be met in order to achieve high CMRR.
Additional techniques are used to separate signal return currents from currents caused by stray
parasitic capacitances, as a result making it easier to achieve high CMRR at high frequencies.
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 represent actual
performance in CMRR-vs-frequency graphs. The practical purpose of such graphs is to compare
the relative common mode rejection performance between different probes and amplifiers.