Active differential power rail probe

Dominic Chan · 2022-05-04T03:48:04+00:00

I've been doing some power rail noise/ripple measurements lately. It seems like the generally agreed best practice is to solder a coax lead as close to the load as possible, taking care not to probe directly across backside decoupling caps to avoid underestimating HF noise. This coax lead is then connected to a power rail probe / 50R scope input. Since the boards I'm working with have 0V tied to Earth, there is a ground loop formed when making these measurements. I've found that if I power the scope and board off the same power strip the environmental noise coupling into this loop is sufficiently small for good measurements.This has got me wondering though, why isn't an active differential probe optimized for power rail measurements a thing? Perhaps it does exist? - If so please let me know!Cheers, Dominic

May 04, 2022 Last reply: 4 years ago 22 Replies

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Phil Hobbs 4 years ago

Sure, they're used a fair amount for low-Z resistive stuff. I have a bunch of old mu-metal shielded transformers for ribbon mic preamps that I've never actually used for anything. ;)

Win and Paul made a pretty cool low-noise front end for a ribbon mic in AOE3.

Cheers

Phil Hobbs

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Phil Hobbs 4 years ago

With a "feedbeside" network, i.e. you DC couple the DC and AC couple the AC.

Cheers

Phil Hobbs

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John Larkin 4 years ago

Er... use a capacitor.

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