DIY or Buy - Differential Probe?

[LooseJunkHater]

Taking a look, it seems like most differential probes are $100+ (more like $200+) USD. I'm undecided if I should just DIY the probe, such as by ordering a PCB from a fab and following one of many DIY differential probe guides. I'll be primarily using the probe for low voltage (less than 100v) circuits. Ideas?

https://github.com/nostradomus/LabTools_100MHz-Differential-Probe

https://github.com/devEmbedded/differential_probe + https://hackaday.io/project/181065-modular-differential-probe

https://hackaday.io/project/169390-a-10x-100mhz-differential-probe/details

+ https://github.com/issus/DifferentialProbe

[tggzzz]

Without reading all those links, it isn't clear to me why you want a differential probe and hence whether you want an HV differential probe (e.g. to probe mains and SMPSs but not 1000V) or a low-voltage differential probe (e.g. for differential signals).

For an HV differential probe, there are many points that you could get subtly wrong, with "significant" adverse consequences.

Common mode performance and input capacitance(s) can be the limiting factor at higher frequencies.

If you want to see small signals (mV) riding on top of high voltages (kV), then have a look at isolated probes, e.g. Tek A6902 family can be obtained relatively cheaply.

Consider whether a single-ended HV probe might also be suitable for your use cases. Tek P6013/5 family can be obtained relatively cheaply and will be OK for 10kV - but almost certainly not higher.

[Doctorandus_P]

There can be lots of reasons for choosing one path over the other, the reasons can be personal, and you have written nothing about your intentions.

If you are stuck on an important project, need direct and reliable results then just go buy a probe and get on with your project.
If you're curious about electronics, have fun building projects, are on a tight budget or have more time then money, then those can all be good reasons to DIY a differential probe.

And it also depends on what you want to do with it. You write your input voltages are below 100V. That is low enough to probe with normal 1:10 probes, and subtracting two channels on your scope may be good enough. (Especially handy to have a 4 channel scope). The commercial differential probes are also mostly geared to wards very high input voltages. They use a high ratio to divide the signal, and then need to amplify it again, and this is of course bad for signal to noise ratio. If your needs are for only lower voltages, then building a simpler circuit may be good enough for you. You can use a smaller dividing ratio on the input section to get a better signal to noise ratio. Just building some generic instrumentation amplifier circuit from an application note may be good enough for you.

And if you want to build a high voltage differential probe, then watch the parts used very carefully. I just looked at a Yageo datasheet and their 0805 resistors have a rating of just 150V and a 300V as "Max overload voltage" (whatever that means). Ceramic capacitors are often rated for much lower voltages.

It's also not sure if DIY would be much cheaper. LTC6269 is an EUR15 opamp and one of the designs uses two of them, (and a LTC6268 which is another EUR11)

[u666sa]

Perhaps better get galvanic separation between mains and oscilloscope? 

[Solder_Junkie]

I needed to measure a few mV across a current sense resistor and considered building, or buying, a differential probe. The problem was that most were intended for large measurements, not a few mV...

I ran the circuit that I needed to measure with a battery, and it was therefore possible to completely isolate it from ground. I was then able to use an ordinary scope probe across the sense resistor as there wasn't an issue with grounding one side with the probe clip.

Not all circuits are able to be run from a battery, but it easily solved my measurement issue for zero cost.

SJ

[bitwelder]

For an HV differential probe, there are many points that you could get subtly wrong, with "significant" adverse consequences.

I concur. A "DIY or Buy" question in the Beginner section about something to be used with high voltages or high energy circuits should have Buy as default answer.

[LooseJunkHater]

Without reading all those links, it isn't clear to me why you want a differential probe and hence whether you want an HV differential probe (e.g. to probe mains and SMPSs but not 1000V) or a low-voltage differential probe (e.g. for differential signals).

Recently I was wanting to measure the voltage drop across some series-resistors to see the waveform, and to compare the change to a different part of the circuit. I do have a 4-channel scope and could have done a differential measurement using two of my scope probes (following this video - ; only learned about this two weeks ago. I think it's called "math subtraction mode" per ChatGPT, or also "x1-x2"?). Using a dedicated, differential oscilloscope probe would be much easier.

[LooseJunkHater]

BTW, when I said "DIY or Buy", for "DIY" I meant buying a differential probe PCB (I see one on Tindie? https://www.tindie.com/products/land_boards/differential-scope-probe-pcb-only/) and populating it myself. There is sadly no complete-set differential probes on Tindie.

[LooseJunkHater]

Bump?

[Doctorandus_P]

I had a very short peek at that tindy thing, and the schematic does not look good.
Did you look at any other specifications except price?
It may be a usable gadget, but it is certainly not a generic differential probe, and it's also not fit for higher voltages.

[LooseJunkHater]

I had a very short peek at that tindy thing, and the schematic does not look good.
Did you look at any other specifications except price?
It may be a usable gadget, but it is certainly not a generic differential probe, and it's also not fit for higher voltages.

Honestly I don't know much about the general schematic of differential probes. I assume it can't handle high voltages due to the lack of high-impedance input resistors? Why else would the schematic be bad?