User Manual

Characteristics of Differential Probes
R&S
®
RT-ZHD07/15/16/60
29User Manual 1800.2588.02 ─ 01
In this equation, A
vdm
is the amplification of the differential mode input voltage and A
vcm
is the amplification of the common mode input voltage.
An ideal differential probe is expressed as A
vdm
= 1 and A
vcm
= 0. In this case, the dis-
played voltage exactly equals to the differential input voltage V
in
between the two sig-
nal sockets, and the common mode input voltage is suppressed.
3.1 Common Mode Rejection Ratio (CMRR)
An ideal differential probe outputs a voltage that depends only on the differential input
voltage V
in
between positive and negative input, and suppresses the common 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 common
mode voltage visible in the output signal. The CMRR is defined as the ratio of the
amplifications of differential and common mode input signals:
vcm
vdm
A
A
CMRR 
Example:
If a differential input voltage of 1 V yields an output voltage of 10 mV (A
vdm
= 0.01) and
a common mode input voltage of 1 V an output voltage of 0.1 mV (A
vcm
= 0.0001), the
CMRR is 100 (40 dB).
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 transmis-
sion line
●
Probing on ground-referenced signals. In this case, the common mode component
is always equal to half of the input voltage.
3.2 Dynamic Range and Operating Voltage Window
Two separate specifications are necessary to characterize the permissible input volt-
age range of a differential voltage probe:
●
The dynamic range (or "differential mode range") designates the maximum differ-
ential voltage V
in
that may occur between the positive and negative signal pin.
●
At the same time, the two voltage values at each of the two signal pins V
p
and V
n
referenced to the common ground must not exceed a specific limit value. This limi-
Dynamic Range and Operating Voltage Window