Reference
20 P6330 3.5 GHz Differential Probe Instruction Manual
Vout
VDM
VCM
+
--
+
+
Differential
mode
Common
mode
Figure9: Simplified model of a differentialamplifier
Common-Mode Rejection Ratio
Inreality,differential amplifiers cannot reject all of the common-
modesignal.Theabilityofadifferentialamplifier to reject the
common-mode signal is expressed as the Common-Mode Rejection
Ratio(CMRR).The CMRRisthedifferential-modegain (ADM)
divided by the common-mode gain (ACM). It is expressedeitherasa
ratioorindB.
CMRR =
ADM
ACM
dB =20log
ADM
ACM
CMRR generally is highest (best) at DC and degrades with
increasingfrequency.
Assessing CMRR Error
Figure 11 on page 28 showsthe CMRR of the P6330 differential
probe. This derating chart assumes a common-mode signal that is
sinusoidal.