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Preventing ringing in audio mute circuit
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Psi:
I've been trying to solve this for a few days and not getting anywhere so i thought id ask here.

I have a MCU controlling two opto switches, one passes or blocks the audio the other mutes the output to ground. The mcu is clocked by internal RC at 1mhz.
The issue i'm having is that the mute opto always produces a pulse of super fast output ringing when turned on (~60mhz then ~10mhz).

The signal is instrumentation level audio and this burst of ringing produces an audible 'click/pop' when transiting into the mute state.

Does anyone have any good ideas how to stop or suppress this ringing?
I've tried disabling the MCU and driving the optos manually and i still get the problem, so i don't think its a digital noise issue.

Any help appreciated.



Yellow is AC audio and blue is DC opto drive from mcu. (Logic is inverted, so mcu low = opto on = mute)
This is a silent signal being muted.


hamster_nz:
Do the inputs have a dc offset?

Sharply switching off a signal with a DC offset will give a transient.

Maybe a coupling cap on the input to block DC?
DrGeoff:
What kind of optos are these?
Psi:
Opto is this one
https://nz.mouser.com/datasheet/2/408/TLP222A_datasheet_en_20170710-1134823.pdf

I dont see any DC offset, if there was one i would expect to see that on the scope view above.
It would show as a different level before and after the transition into mute state.
hm.. then again, i do have scope in AC mode so it could easily be masking this.
Thanks i will check this more thoroughly tomorrow and post results.
kg4arn:
I don’t have a RIGOL scope but it looks to me like CH1 is AC coupled based on the yellow symbol at the bottom left. This will block the DC level.


***. Oops. I Didn’t read carefully *****
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