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

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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 preconditions 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 groundreferenced measurements. This is also what makes DMMs and handheld scopes so useful – they don’t make groundreferenced 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 noise then cut it]. For high speed digital signals, this dramatically improves the tolerance to noise sources like cross talk and powerplane induced noise.

If I’m a receiver, this is going to cause some problems. We’ll get ones where we should have zeros, and zeros where we should have ones. However, if we use a differential probe to probe the same signal, we get this!
As you can see, the sine wave is completely filtered out! Well, that’s not completely true. It’s more or less filtered out depending my probe’s common mode rejection ratio, known as CMRR. CMRR usually is dependent on the frequency of the noise.
My differential probe sees these signals the same way a differential receiver would in a communications system. You would want to use single ended probes to debug cross talk and interference, but a differential probe to analyze your communications data.

If you’re shopping for a differential probe or have the luxury of trying to decide between multiple differential probes you already have, here are a few things to consider:

First is Bandwidth – this one’s obvious, make sure to get adequate headroom in bandwidth. If you don’t have enough, it’ll affect both your measurements accuracy, especially for things like power measurements, as well as your ability to properly see high speed signals.

Second Attenuation – attenuations vary widely, choose one that makes sense from a loading and signal amplitude perspective
Third, Input Range – How large of an input can it take? This applies to both common mode inputs and differential inputs
Finally, Common mode rejection ratio, or CMRR – If you’re expecting to deal with a lot of common mode phenomena, make sure the CMRR is adequate.

Fun fact, you can also use a differential probes as single ended probes – more on that in this video, which also shows how to use two single ended probes to measure differential signals!

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