Open Hardware Power Rail (DC offset) Oscilloscope Probe

[blinken]

For the last 4-5 months I've been working on an open hardware power rail probe: https://github.com/blinken/power-rail-probe. It's a device that sits between your 1X probe and your oscilloscope and applies a fixed DC offset, allowing you to view millivolt ripple and sag on rails up as high as +/- 20V, with a fairly flat frequency response to 500MHz. Introduced noise is simulated at <400nV/sqrt(Hz) below 100kHz, and tailing off at higher frequencies.

It's a little more complex than a simple AC blocking capacitor, because the offset is fixed - so slow DC drift is also measurable, in addition to ripple.

I've just finished version 1.0 - while there are some obvious areas for improvement to make it perfect, it works very well for day-to-day use, and so I'm releasing it in the spirit of "perfect is the enemy of good" 

A commercial version of this probe costs around USD $5000 (eg, https://www.tek.com/en/datasheet/active-power-rail-probes). This open design costs ~£250 (USD $320), fully assembled in a nice case. I have a handful remaining of the first version, and any early support would be very welcome!


My goal is to fully characterise the device and release well-calibrated data around noise and frequency response on the GitHub page, so it can be used for precise measurements. If anyone in the UK has the ability, time and patience to make calibrated noise measurements (down to say -100dBm) I'd love to get in touch.

I'd appreciate any feedback on the design!

-Patrick

[apetersw]

Hi Patrick, the design looks like an interesting project. I took a look at the schematics and I wonder why u do not go all the way to 60V range, so one can use the probe on 48V rails. U did use the OPA322, two stages x1. First try I would go sonething fancy like OPA891. This might allow directly feeding into 50Ohm, even lowering the noise.