Common Mode Rejection Ratio - LeCroy AP034 Instruction Manual

COMMON MODE REJECTION RATIO

²
Common Mode Range
Maximum voltage from
either input to ground
Figure 6. Common Mode and Differential Mode Range
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 not perfect, and a small portion of the common
mode voltage component does appear in the output. Common
Mode Rejection Ratio (CMRR) is the measure of how much 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
ISSUED: January 2000
$3 $FWLYH 3UREH
Differential Mode Range
Maximum voltage
between inputs
AP034-OM-E Rev D