# Differential Probes for Floating Voltage Measurements

By: Scientech

What is a Floating Voltage Measurement?

A differential Voltage measurement is the voltage difference between two points; hence all voltage measurements are differential measurements.
Voltage measurements are normally carried out with
1 .Ground-reference
2. Non-ground-reference
When the voltage measurements is done without the Ground reference,
It is called Floating Voltage Measurement or A differential voltage measurement
where neither point has ground reference or earth potential is a Floating voltage
Measurement This is very common in power and power electronics measurements.

A general purpose oscilloscope normally has common grounded inputs.
This means that the signal level measurements are done with respect to ground.
This ground is common with common point of measuring circuit and is also
connected to chassis earth point which in turn is connected to power supply earth
connection (the third wire ground in the power cord for AC-powered equipment.)
This is done so that all signals applied to, or supplied from, the oscilloscope have a common connection point. This also means that each input channel reference is tied to a single ground reference. Thus a passive probe cannot be used directly to make floating measurements on a ground reference oscilloscope.
There are various techniques for Floating voltage measurement. These are: Isolated-input Oscilloscopes, Differential Probes, Voltage Isolators, “A minus B” Measurement, and “Floating” the Oscilloscope Techniques etc. Among these differential probe is
Safe and accurate.

Differential Probes for Floating Voltage Measurements
These probes are designed specifically for differential applications.
Active differential probes contain a differential amplifier at
the probe tip Where as Passive differential probes are used with
differential amplifiers and can be calibrated for precisely
matching the DC and AC attenuation in both signal paths

Common terms of Differential Measurement:

Common-Mode Signal
The component of an input signal which is common (identical in amplitude and phase) on both inputs.
Common-Mode Range
The maximum voltage (from ground) of a common-mode signal which a differential amplifier can reject.
Common-Mode Rejection Ratio
The performance measure of a differential amplifier’s ability to reject common-mode signals. Because common-mode rejection generally decreases with increasing frequency, CMRR is usually specified at a particular frequency.Differential Mode
The signal which is a difference between the two inputs of a differential amplifier. The differential-mode signal can be expressed as:
VDM = (V+input)-(V-input)

After understanding these terms we are better prepared to make probe selections to match both, the oscilloscope that you are using and the type of measurements that you need to make. A proper probe selection leads to correct results.

With advances in solid-state electronics, it has become practical to put the differential amplifier directly within the probe head, greatly easing the requirements on the rest of the signal path because it now becomes single-ended rather than differential . A modern differential probe usually has two two input extensions which can be used by the operator to connect two floating points . Very high CMRRs are thereby made possible.
The Differential probe extends the measurement capability of oscilloscopes to electronic power converters, inverters, motor speed controls, switch-mode power supplies and many other applications. Simply plug in the BNC output connector to the vertical input of a general-purpose oscilloscope, and connect the auxiliary grounding terminal to scope ground.

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About Scientech Instruments Scientech Instruments is a global leader in research, development and manufacturing of high performance Oscilloscope , Floating Voltage measurement instruments and other test and measurement devices. www.scientechworld.com