Tektronix P7313SMA, Technical Reference

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Operating Basics
Common-Mode Rejection Ratio
Differential ampli fi ers cannot reject all of the common-mode signal. The ability of a differential ampli fi er 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
CMRR(dB) = 20 log (A
DM
÷ A
CM
)
CMRR generally is highest (best) at DC and degrades with increasing frequency.
The typical CMRR response of the P7313SMA differential probe over frequency is shown in Speci fi cations . (See Figure 15
on page 27.) High CMRR in a differential probe requires careful matching of the two input paths. Poorly matched signal
source impedances can signi
fi cantly degrade the CMRR of a measurement. Mismatches between the two differential signal
input paths result in an effective conversion of V
CM
to V
DM
, which reduces the CMRR.
Probe Block Diagram (Simpli fi ed)
The SMA inputs and probe termination network provide a high frequency, 50 Ω signal path to the internal probe ampli fi er.
The use of SMA-female connectors provides a reliable, repeatable attachment method for input signals. The symmetry of
the input termination network is designed to reduce skew and maximize CMRR.
A simpli fi ed schematic of the P7313SMA input termination network is shown. (See Figure 2.)
Figure 2: Input termination network
P7313SMA Technical Reference 3