Hackaday Prize 2023: This Differential Scope Probe Is Smarter Than It Looks https://hackaday.com/2023/09/28/hackaday-prize-2023-this-differential-scope-probe-is-smarter-than-it-looks/ #2023HackadayPrize #differentialprobe #TheHackadayPrize #self-adjusting #oscilloscope #ToolHacks

Differential Probe Clocks at 100MHz and $200

[Voltlog] often looks at interesting test equipment and in the video below he reviews something that isn't very common in hobby labs: a differential oscilloscope probe. These are usually pretty expensive, but the Micsig probe in the video costs under $200. The question, of course, is what do you need with a differential probe?

A typical scope probe has a ground lead that connects directly to the actual grounding point. This can cause a problem if you try to measure across some component that has more voltage than you want to short to ground. It might hurt your device under test, your scope, or both.

The probe isn't isolated in the traditional sense, but it does prevent the problem, as he explains in the video. The probe powers from USB, which might seem odd, but [Voltlog] points out that you probably won't use this often, so batteries that will go bad during the months it sits on the shelf aren't really a great idea. Makes sense.

Towards the end, you can see the probe in action, measuring signals in a switching power supply that would be difficult to measure with a conventional probe. If you'd rather build your own probe, start with this entry to the Hackaday Prize. Or, try this older design.

#toolhacks #differentialprobe #micsig #scopeprobe

This Modular Differential Probe Shows Great Attention To Detail

[Petteri Aimonen] presents for us a modular differential probe, as his entry into the 2021 Hackaday Prize.

This project shows a simple and well polished implementation of a differential-to-single-ended preamplifier, which allows a differential signal to be probed and fed to an oscilloscope via a BNC cable.

PCB Spark gap for primary ESD protection

It implements a classic instrumentation amplifier, where we have two amplifier stages. The first gives us the options for a gain of either 1 or 10, if we need it, with the second stage having a gain of 2.

The remaining circuit is a power supply to generate the necessary dual-rail supplies to feed the opamps. There is a lot of filtering on those output rails as well as on the USB power input side to try keep all that switched-mode power supply noise out of the signal path.

There are a couple of interesting design choices including the use of PCB material for the long removable probe arms, that integrate PCB spark gaps to offer a first defence against ESD reaching the more delicate parts of the system.

Why This Is Useful

There are two main classes of signals we electronics engineers care about: single-ended and differential-mode.

With the first kind, the signal is carried on a single wire, which is defined as being referenced to the common system ground. Current flows along the wire and returns to its source along the path of least resistance, at least at low frequencies. At higher frequencies, the path of least inductance is more relevant. This is all well and good, so long as you design the PCB correctly.

Coupling from adjacent wires due to mutual capacitance and inductance, as well as noise in the reference ground all conspire to mangle the signal we want to pass down the wire.

As the frequencies increase, and especially if you're dealing with sharp edges, with all that extra odd-harmonic power, things start to get bad real fast. The way we deal with this is by utilising differential-mode signalling. This is where instead of a single wire, referenced to some notion of ground, we send the signal down a pair of wires, where the voltage difference between the wires forms the signal. Any external noise that leaks into the pair, will (hopefully!) affect both wires equally, forming what we call a common-mode component. When you look at the difference, this common mode noise disappears. (Our own [Bil Herd] covered this some time ago.)

When probing a circuit, it pays to have the right kind of probe as well as an understanding of the effect the probe will have on the circuit in operation. If you have a single-ended signal and you want to view it on your scope, your choice is either a passive or active probe. Usually some kind of passive probe will be most available. These commonly come in 50 Ω and 1 MΩ versions, and you need to be careful to use the correct probe type for your application.

For probing differential signals, it is possible to use a pair of probes, one for each signal wire, and then utilise the scope's math difference function to show the signal. This is quite often a desperate measure, and what you really want is a differential front-end in hardware. You need a differential active probe.

The circuit may be simple, but don't underestimate how much tweaking it needs to have good performance - a little slip with the PCB layout, as the author describes, caused some annoying resonances which can be hard to track down.

The project is still under active development, with the author showing the process as the project progresses, but its looking pretty good already, if you ask us.

Sources can found on his GitHib, which uses all Open Source tools, so its pretty accessible too.

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#hardware #thehackadayprize #differentialprobe #pcb #sparkgap