This is the design for an active differential probe which has a bandwidth of about 800MHz and which is useful to beyond 1GHz.

I have written some articles about the probe: Part 1, Part 2, and Part 3.

So far I have only built the B variant of the probe, that is, the probe with separate OP amps for the inputs.

On variant B, the input resistors R20/R21, R28/R29 and output resistors R30/R31 are arranged in such a way that one can use just one resistor between the input of the first resistor to the output of the second resistor. That's why it says "0Ω/NF" on one of the resistors in both pairs.

The image in S21ProbeBW_1.png shows a plot of the frequency response for variant B using LMH6702 OP-AMPs, 150Ω on R20 and R28, 220Ω on R26, R27, R37 and R38 and 1.1k on R22 and R32. Note that the 150Ω resistors were mounted across R20/R21 and R28/R29 and the and 50Ω matching resistor for the output was mounted across R30/R31. The power supply with a LMH6702 OP-AMPs should be about 5V and the probe should be able to measure signals up to about +/-3.3V.

If you want the probe input to be AC coupled you can mount the input resistor in its proper position on R20 and R28 and mount a capacitor (0.1µF might be a good starting value) on R21 and R29. You can also AC couple the output by mounting the 50Ω matching resistor on R30 and a another small capacitor on R31.

diff-probe by Christer Weinigel is licensed under a Creative Commons Attribution 4.0 International License.

The intent is to make this PCB available to anyone that finds it useful as long as I get credit for the original design. If you want to use the design and have issues with the above license, please contact me.