Tektronix P7313SMA, Technical Reference

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Operating Basics
Differential and Single-Ended Signal Measurement
Although designed for differential signal measurement, the P7313SMA probe can be used to make single-ended
measurements when properly con
fi gured. The analysis that follows describes some differential and single-ended
measurements of typical high-speed serial data signals.
Differential Measurement Topology
A typical differential measurement topology using the P7313SMA probe is shown. (See Figure 7.) The termination network
for the probe in this
fi gure includes a termination capacitor. This is intended to show that the termination network provides a
broadband AC ground for common-mode signals.
Figure 7: Differential measurement topology
Although an ideal differential signal is theoretically terminated at the V
T
node due to symmetry, the low impedance V
T
node
terminates any non-ideal, AC common-mode signal components. The input signal source model includes a common-mode
component, V
CM
, and complementary differential mode components, ±V
DM
.
The differential mode signal source models have double the signal amplitude of the measured signal at each input because
of the 50
Ω voltage divider between the source and termination resistance. The common-mode signal source model does
not have double the signal source amplitude because most serial data transmitters are designed to drive a load resistance
terminated with the DC common-mode voltage, not signal ground.
With V
T
set equal to V
CM
in this model topology, the DC common-mode voltage at each probe input should equal V
CM
.
The resulting differential signals at the probe inputs are:
The attenuator and V
CM
compensation network that follows the termination network nulls out the V
CM
signal and attenuates
the V
DM
signals. The resulting differential signals at the probe ampli fi er inputs for a 2.5X attenuation setting are:
The resulting output signal from the probe output is:
P7313SMA Technical Reference 11