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Reference
20
P6330 3.5 GHz Differential Probe Instruction Manual
V
out
V
DM
V
CM
+
--
+
+
Differential
mode
Common
mode
Figure 9: Simplified model of a differential amplifier
Common-Mode Rejection Ratio
In reality, differential amplifiers cannot reject all of the common-
mode signal. The ability of a differential amplifier to reject the
common-mode signal is expressed as the Common-Mode Rejection
Ratio (CMRR). The CMRR is the differential-mode gain (A
DM
)
divided by the common-mode gain (A
CM
). It is expressed either as a
ratio or in dB.
CMRR =
A
DM
A
CM
dB = 20 log
A
DM
A
CM
CMRR generally is highest (best) at DC and degrades with
increasing frequency.
Assessing CMRR Error
Figure 11 on page 28 shows the CMRR of the P6330 differential
probe. This derating chart assumes a common-mode signal that is
sinusoidal.