Active Probe vs Differential Probe

D from BC · 2007-12-27T06:53:06+00:00

Not sure if I have this right..An active probe is single end amplification and cable driving.A differential probe is differential amplification and cable driving.Only the differential probe can provide attenuation to CM noise due tothe Earth ground loop. Probe -----------| Line poweredDUT --------------| scope -----------| | + DUT com | ~ | ~ | Coupling Earth to EarthFig: Passive 1x probe showing earth loop.But, if a scope is battery powered, an active probe is good for bestperformance.D from BC

Dec 27, 2007 21 Replies

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D from BC 18 years ago

[snip]

Hopefully I got this right. It's somewhat new ground for me. (Hey! A pun :) )

1) Battery powered scope: I say no problem with CM noise. The scope probe and scope form only 1 loop. Same with a USB scope +battery powered laptop. It all floats.

2) Scope without ground plug:

Electric Box -----L--------[ ] Scope---probe--->[PCB] +------------N--------[ ] |----------->[ ] |------------G----X X----+ ~ + capacitive | coupling | to +------------------------- earth

Note: Risky...

There are 2 loops. The probe coax is one.. The earth path is the other. Since G and N meet at the electric box, it's up to the scope's power supply to not couple noise. I'd expect more CM noise compared to a battery powered scope.

D from BC

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John Larkin 18 years ago

Sure, as long as you don't get zapped. A rig like that could work kilovolts off ground. "Optocouple" the waveforms back to ground by photographing the screen!

The last case, regular scope without a ground plug, will still have some capacitive coupling back to the AC line, so some hf ground-loop noise can still happen.

But, in all cases, only one floating common exists. The Tek TPS allows four separate folating signals to be scoped simultaneously.

John

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