Differential and floating voltage measurements

Fundamentally, all oscilloscopes measure voltage (in the Y axis) against time (in the X axis) and all voltage measurements are made between two points.

Most oscilloscopes are designed with the expectation that one of these points will be fixed firmly to ground, and so the reference lead of the probe connects electrically to the shell of the BNC input, which connects to the chassis, which is grounded through the mains lead.

This presents a problem that most oscilloscope users encounter regularly: many voltage measurements are not referenced to ground.

In fact differential or floating voltages are everywhere and there are many solutions to the problem, though some are more suitable than others.

Differential and floating voltages are everywhere

We say a voltage is “floating” when it is not directly referenced to ground, and similarly a voltage measurement is "differential" when measured between two points neither of which is ground.

Also signals are often transmitted differentially over two lines, one the inverse of the other, so that any common-mode noise can be rejected by the receiver.

For example, the image opposite shows how a CAN bus differential signal is the difference between CAN High and CAN Low and how any common-mode noise is rejected.

Other examples of where floating or differential signals encountered are:

• Switch mode power supplies
• Three-phase mains
• Current sensing resistors
• Biological signals

A misplaced ground lead here would at best kill the signal, and very likely blow the device under test.

Floating measurement solutions

Float the scope

It can be tempting to take the direct approach and disconnect the ground lead at the mains plug. Now the reference lead is free to float to where it needs to go, but of course so too is the chassis and all the BNC inputs, and this creates a danger of electric shock.

We strongly recommend that you do not adopt this solution.

Pseudo-differential

A much safer and commonly used method is to use two probes, invert one channel and use math to subtract one channel from the other, so now we only display the differential signal, and all perfectly safely.

However, there are also limitations. It requires two channels and one math channel to produce one trace, and any imbalance between the probes will result in skew between channels and low CMRR (common-mode rejection ratio)

So often a more elegant solution is required.

Active differential voltage probes

A good solution with the following advantages:

• True differential measurements
• High CMRR
• One scope channel per measurement

But there are downsides:

• Depending on the specification, two probes can cost more than the oscilloscope they are used with.
• It requires a power source, either battery, mains adaptor or a special interface to obtain power from the scope.

Differential input oscilloscopes

There are several brands of differential input oscilloscopes available providing a choice of bench, handheld and PC-based form factors.

The key advantage is that the differential inputs allow the use of inexpensive passive probes.

Although each brand can boast strong points, there is a lot of variation in features, performance and price between models.

Many lack features that would be expected in even a basic general-purpose oscilloscope, and some are not true differential inputs in that the reference lead presents a different impedance to ground than the signal input.