Common-Mode Rejection Ratio; Assessing Cmrr Error - Tektronix P6246 Mode D'emploi

Differential
mode
Common
mode
Figure 6: 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
divided by the common-mode gain (A
ratio or in dB.
CMRR =
CMRR generally is highest (best) at DC and degrades with
increasing frequency.

Assessing CMRR Error

Figure 8 on page 28 shows the CMRR of the P6246 and P6247
differential probes. This derating chart assumes a common-mode
signal that is sinusoidal.
P6246 & P6247 Instructions
+
V
DM
+
V
CM
A
DM
dB = 20 log
A
CM
+
- -
). It is expressed either as a
CM
V
DM
V
CM
Reference
V
out
)
DM
21