Noteworthy Oscilloscope Measurement Terms

Common-mode signal: The component of an input signal common — which is to say identical in both amplitude and phase — to both inputs of an oscilloscope.

Common-mode range: The maximum voltage from ground of common-mode signal (see above) that can be rejected by a differential amplifier.

Common-mode rejection ratio: The measured performance of a differential amplifier’s capacity to discard common-mode signals. Common-mode rejection ratio is typically specified at a particular frequency since it usually decreases with increasing frequency.

Differential mode: The signal that is different between the differential amplifier’s two inputs, expressed as VDM=(V+input)-(V-input).

Differential-mode signal: The signal that is different between two inputs.

Differential measurement: The difference in voltage between two points in a circuit.

Differential probe: An oscilloscope probe specifically designed for differential applications. Passive differential probes, which are used with differential amplifiers and can be calibrated for accurately matching both the AC and DC attenuation in both signals’ paths, including the reference lead. Active differential probes have a differential amplifier at the oscilloscope probe’s tip.

Floating measurement: A differential measurement in which neither point is referenced to the ground (earth potential rise).

Ground loops: A ground loop is the result of two or more separate ground paths tied together at two or more points, resulting in a loop of conductor. Acting as a shorted turn, this loop of conductor becomes the secondary of an oscilloscope’s transformer in the presence of a shifting magnetic field. The magnetic field that excites the transformer can potentially be created by any nearby conductor carrying a non-DC current: AC line voltage in mains wiring or the output lead of a digital integrated circuit can create this excitement. Current circulating in the loop will develop a voltage across any of the impedance in the loop. Different points in a ground loop will not have the same AC potential.

By connecting the ground lead of an oscilloscope probe to the circuit-under-test’s ground one grounds the loop, if the circuit is grounded to an earth ground. Voltage potential is developed in the probe’s ground path as a result of the circulating current acting on the impedance in the path. The ground potential at the input BNC connector of the oscilloscope is not the same as the ground in the circuit being measured, and this difference can range from microvolts up to hundreds of millivolts. The waveform displayed on the display of the oscilloscope might not represent the real signal at the probe’s input, because the oscilloscope references the measurement from the input BNC connector’s shell. This error becomes more noticeable when the amplitude of the signal decreases.


George Leger has a Masters in Electrical Engineering from Stanford University, worked in private industry pioneering surface-mount technology and in government research labs for twenty years, published several papers on surface-mount technology, co-authored papers published in national symposiums on accelerator technology, was past president of SMTA and an adjunct professor at the community college level, holds a patent, and is a certified microchip design partner, serving as a consultant to many companies developing electronic circuits.

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