## Tuesday, April 4, 2023

Let's build a differential oscilloscope probe! Sure, I could buy one, but where is the fun in that? I'm working on a big project that will require an LC Tank circuit to be tuned, this tank circuit simulates to have voltages exceeding 1000v peak to peak. Furthermore, the point I need to measure the voltage is across a capacitor, with neither side being earth ground referenced. This rules out simply using a high-voltage oscilloscope probe to measure the voltage. To measure this with standard single-ended probes, I would need to measure both sides of the capacitor with reference to ground, then use the math function in my oscilloscope to find the difference between the signals. While that is possible, it does lose resolution and more importantly for my application, use two of my four channels.

In this open source project, I'm going to attempt to build a 250MHz differential oscilloscope probe with a 1200v peak input voltage. The 250MHz bandwidth is very much a “hope for” spec, rather than a hard requirement, my immediate application for this probe only requires 100kHz of bandwidth. I don't have the test equipment to generate a 250MHz waveform, therefore that specification is purely based on simulations. As with all my projects, you can find the design files on GitHub released under the permissive open-source MIT license.

Did you know that Altium has an all new spice simulation engine? If you have not tried it yet, sign up for a free trial of Altium Designer, or talk to an Altium representative about how integrating simulation directly into your simulation can save you from making mistakes!

This probe uses high performance operational amplifiers, which require a dual supply voltage to be able to work with the input signal ranges. In my last project article, we built a separate power supply that runs off a 9v battery specifically for powering op amps like these.

Let's build a differential oscilloscope probe! Sure, I could buy one, but where is the fun in that? I'm working on a big project that will require an LC Tank circuit to be tuned, this tank circuit simulates to have voltages exceeding 1000v peak to peak. Furthermore, the point I need to measure the voltage is across a capacitor, with neither side being earth ground referenced. This rules out simply using a high-voltage oscilloscope probe to measure the voltage. To measure this with standard single-ended probes, I would need to measure both sides of the capacitor with reference to ground, then use the math function in my oscilloscope to find the difference between the signals. While that is possible, it does lose resolution and more importantly for my application, use two of my four channels.

In this open source project, I'm going to attempt to build a 250MHz differential oscilloscope probe with a 1200v peak input voltage. The 250MHz bandwidth is very much a “hope for” spec, rather than a hard requirement, my immediate application for this probe only requires 100kHz of bandwidth. I don't have the test equipment to generate a 250MHz waveform, therefore that specification is purely based on simulations. As with all my projects, you can find the design files on GitHub released under the permissive open-source MIT license.

Did you know that Altium has an all new spice simulation engine? If you have not tried it yet, sign up for a free trial of Altium Designer, or talk to an Altium representative about how integrating simulation directly into your simulation can save you from making mistakes!

This probe uses high performance operational amplifiers, which require a dual supply voltage to be able to work with the input signal ranges. In my last project article, we built a separate power supply that runs off a 9v battery specifically for powering op amps like these.