A Differential Probe Guide - How & Why To Use a Differential Probe With Your Oscilloscope

Your primer on the how to measure differential signals with an oscilloscope

Click to subscribe! ► http://bit.ly/Scopes_Sub ◄

Questions/comments? Ask me on Twitter: @DanielBogdanoff https://twitter.com/DanielBogdanoff

Also check out the EEs Talk Tech podcast: https://www.youtube.com/playlist?list=PLzHyxysSubUnAMeCIi2-S0Vm7YtSAGqx_

Transcript:

Today we’re going to look at the two main reasons differential probes can save your bacon when you need to make scope measurements that aren’t ground referenced. Hi, thanks for tuning in, I’m Daniel Bogdanoff and today we’re going to look at the two main reasons you’ll want to use a differential probe. Before we get to that, what is a differential probe and how is it different from a standard single ended probe? A single ended probe measures the voltage difference between earth ground and the probe tip. Like a single ended probe, a differential probe also measures the difference between two points. Unlike a single ended probe, a differential probe doesn’t have a ground clip – you can measure the voltage difference between any two points. With a single ended probe, the oscilloscope will find the voltage difference. But for a differential probe, there’s a differential amplifier inside of the probe itself. Essentially, it pre-conditions the signal so that the oscilloscope gets a usable input. The oscilloscope hardware doesn’t care if you’re using a single ended probe or a differential probe.

We need a differential probe. So, let’s hook it up to channel 4 and we’re now able to see what’s actually happening in our system.

These are just two simple examples, but this situation pops up pretty regularly. For example, differential probes are often used for measuring switch mode power supplies, inverters, and motor drivers – really any time you don’t want ground-referenced measurements. This is also what makes DMMs and handheld scopes so useful – they don’t make ground-referenced measurements.

Differential probes are also used to measure differential signals, which are common in for audio equipment and high speed serial communication lines. Audio and PCIe have vastly different signal speeds, they’re pretty much as far apart as you can get. So, why would they use differential signaling? Noise.

Differential signals, and hence differential probes have a tolerance for common mode noise. Common mode noise is noise that occurs on both lines of a differential signal. Differential receivers (and differential probes) only care about the difference between the two signals, not their individual magnitude. So, if both signals jump up a volt or 100 volts simultaneously, the voltage difference doesn’t change. For audio applications, this gets rid of a lot of audible noise[ramp audio ... https://www.youtube.com/watch?v=OZDijMDHmtI