question on ground on a small class ab amplifier

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davidfet

Joined May 13, 2020
31
Decided to open up my external computer speaker, because why not?

Inside, besides the electrolytic cap cover coming off, I saw that the audio in ground was separated from the power ground with a 200r resistor and smd cap.
Any reason they did that?

It appears to be an class AB amplifier, with 1 ic for a single 4r/5w speaker, and 1 ic for the smaller 4r/3w dual speakers.
 

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Delta Prime

Joined Nov 15, 2019
1,022
Hello there :)
besides the electrolytic cap cover coming off, I saw that the audio in ground was separated from the power ground with a 200r resistor and smd cap.
Any reason they did that?
It's all about technique in designing mixed signals circuits & PCB’s, very good observation by the way!;)
The analog and digital grounds in a mixed mode architecture or system must be joined at some point to allow signals to be referred to a common potential. This analog/digital common point, is carefully chosen so as not to introduce digital currents into the ground of the analog part of the system—it is often convenient to make the connection at the power supplies.
the logic supply is isolated with a small resistance, and decoupled to analog ground with a local 0.1-μF (100-nF) capacitor, all the fast-edge digital currents of the ones and zeros will return to ground through the capacitor and will not appear in the external ground circuit. If a low-impedance analog ground is maintained—as it should be for adequate analog performance— additional noise due to the external digital ground current rarely present a problem for me.
 
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