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.

12 thoughts on “Differential Probe Clocks At 100MHz And $200

  1. Use two scope channels, A and B. Invert B then use A + B mode. Voilà, differential measurements for $0 added cost.

    But to be fair, one case where you really do need a hardware differential scope probe is when you are working with high voltages. The Micsig DP20003 100MHz 5600V (Wow!) High Voltage Differential Probe is a good example:

    * Micsig DP20003 High Voltage Differential Probe 5600V 100MHz 3.5ns Rise Time 200X/2000X Attenuation Rate 4.7 out of 5 stars 12 ratings $259.99 USD


    Big name-brand differential probes get much more expensive as the specs get better. Take this one as a baseline example:

    * KEYSIGHT N2790A High-Voltage Differential Probe, 100 MHz. Obtain floating signal measurements of up to 1,400 V of differential voltage and up to 1,000 V of common mode voltage. USD $1,803.00


    1. Indeed, great on most 2 channel cros even very arlier ones. Had an oldie HP1742A with the digital multimeter add-on and a couple of 500v 50MHz AC diff probes too – on one occasion I set up those diff probes on each of the two channels on the cro and then also set the A-B function as demo for my youngest son around 9 yrs old at the time:-
      “Hey son, see the difference of the difference, what the heck are we measuring here, lets sketch it ?” ;-)

      Love those old days, bamboozling the kids, glad I did as they’re far less emotionally malleable to idiocy from earliest age to ambit bluster & arbitrary authoritative claim then the various ignorant numb nuts: ie AGW deniers, vaccine refusers, faked moon landing, flerfers and other low iq conspiracy peddlers mindlessly trying for yootube traction clck bait.

      Science wins, assuming or rather I should say demanding it has integrity open to challenge though, puh-lease…

    1. And dangerous.

      Also, sometimes you need a differential measurement in combination with one or more other signals, in those cases a differential probe really helps.

      He says there is just a bunch of resistors inside, without actual isolation; that is clearly nonsense, because it wouldn’t need power if that were true.

      I’ve never understood why all those probes have long separate leads, instead of a BNC connector for the signal input. Mine are 20MHz, with the same leads. The voltage rating might be an issue, but I’d rather have a lower differential voltage rating (but the same high common mode rating), with a proper insulated BNC connector.

      Sometimes you need to measure a small signal with a high common mode voltage, but probes that can do that are rare and really expensive.

  2. Salvaged some out of cal probes from a scrap bin at work years ago. After replacing the batteries and cleaning the terminals, they were back in spec. Only 5MHz, but handy never the less.

    Suprised there aren’t any inexpensive low spec ones avaliable as they’re just a battery, op-amp and a few passives in a wee little box. For non fancy measurements up to a 1 KV, this should cost under £5 for the bits not including enclosure and insulated probes.

  3. There are some cheap portable oscilloscopes that for most typical troubleshooting can do the job and cost half of what a differential probes will cost. You still have the dangers of working with devices connected to mains power, but it will be safer than floating the scope.

  4. I have several of these. They have proven indispensable when I had to design snubbers for an H-Bridge circuit. Being able to see high-side Vds, low-side Vds, and Vgs all at the same time made identifying the problem and tuning the snubber easy.

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