Teledyne LeCroy TF-DSQ - Manual (Page 26)

Operator's Manual
26 TF-DSQ-OM-E RevC
In this case, the outer conductor connects the oscilloscope's ground to the circuit's ground, and
the cable is terminated with 50 ohms at the oscilloscope, such that the conductor looks like 50
ohms looking into it at all frequencies.
In the case of single-ended measurements, the oscilloscope is measuring the difference
between the probe tip and ground. Since ground is considered to be zero volts, one can say that
the voltage measured at the probe tip is the absolute voltage.
Differential
A differential probe, in contrast, exposes two probing tips with an optional ground connection
lead. Often, the ground connection lead is left unconnected (more on this later). Each of the
probe tips are connected to two different signals in a circuit. The probe measures only the
difference between the two probing points with no actual notion of the absolute voltages
present. In practice, differential probes have limitations not only on the difference allowed
between the two probing tips, but also the absolute voltage allowed. This absolute voltage is
referenced to the oscilloscope's ground. For this reason, the probe ground lead is sometimes
connected to ground in the circuit to make oscilloscope ground and circuit ground the same.
This is often done only when the circuit is floating, which means that the circuit's ground is free
to move to any voltage, depending on its ground connections.
Despite the differential probe only measuring the difference between the voltages at its probe
tips, its accuracy is sometimes affected by the absolute voltages present.
Differential signaling is used commonly for high-speed signals. In a differential system, two wires
are used to transmit the signal. Often, the signals are the direct opposite of each other, with
both swinging across zero volts. Frequently, these signals have a common offset or bias applied
to them. In this case, the common offset applied to each signal is called the common mode
signal component, and the difference between each signal is called the differential mode signal
component. Typically, the differential signal is the actual information signal being transmitted,
with the common mode signal being present for other physical reasons, such as the biasing of
an ECL gate. An ideal differential probe receives only the differential mode signal. Practical
probes reject the common mode signal to a large extent. The ability of a differential probe to
reject the common mode signal is stipulated by the common mode rejection ratio (CMRR).