Common Mode Rejection Ratio (Cmrr) - Rohde & Schwarz R&S RT-ZD01 User Manual

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®
R&S RT-ZD01
The output voltage V
by superimposing the voltages generated from the differential mode input voltage
and from the common mode input voltage:

V A V
out vdm in
In this equation, A
A is the amplification of the common mode input voltage.
vcm
An ideal differential probe is expressed as A
displayed voltage exactly equals to the differential input voltage V
two signal sockets, and the common mode input voltage is totally supressed.
3.1

Common Mode Rejection Ratio (CMRR)

An ideal differential probe outputs a voltage that depends only on the differential
input voltage V
mon mode voltages. This is equivalent to an infinite common mode rejection ratio
(CMRR).
In contrast, real probes have a finite CMRR, resulting in a small part of the com-
mon mode voltage visible in the output signal. The CMRR is defined as the ratio
of the amplifications of differential and common mode input signals:
A
CMRR 
vdm
A
vcm
Example:
If a differential input voltage of 1 V yields an output voltage of 10 mV (A
and a common mode input voltage of 1 V an output voltage of 0.1 mV
(A = 0.0001), the CMRR is 100 or 40 dB.
vcm
A high CMRR is important if significant common mode signals are encountered at
the probe input, for example:
● DC voltages for setting the operating points of active DUTs
● Different ground levels of probe and DUT, e.g. floating DUTs
● An interference that couples equally to both conductors of a differential trans-
mission line
User Manual 1422.0732.02 ─ 02
, which is displayed on the base unit, is generally obtained
out

A V
vcm cm
is the amplification of the differential mode input voltage and
vdm
between positive and negative input, and suppresses the com-
in
Allice Messtechnik GmbH
Characteristics of Differential Probes
Common Mode Rejection Ratio (CMRR)
= 1 and A
vdm vcm
= 0. In this case, the
between the
in
= 0.01)
vdm
23