Capacitor Noise / Antenna in combination with audio codec
Elian Terelle , 11-02-2024, 11:55 PM
Hi, i designed some PCBs with an sgtl5000 Audio Codec and i'm having some problems with noise on the mic input and headphone outputs(so all inputs and outputs that are used on the board). I've tracked down the noise to a decoupling capacitor that is connected to the VAG pin of the codec, like recommended in the Datasheet.VAG is the internal voltage reference for the adc and dac, so that totally makes sense.When i touch the capacitor / the side connected to vag with my finger, i hear a buzzing noise on the headphone output, so just my body acting as an antenna i guess.I've swapped the capacitor with one from the teensy audio board, which uses the same codec, and the noise went away almost completely.So my question would be: What capacitor would most likely solve this? Low ESR?This is the one i originally used: https://jlcpcb.com/partdetail/15530-CL10C101JB8NNNC/C14858Unfortunately i have no idea what the teensy board uses...
Elian Terelle , 11-02-2024, 11:56 PM
It seems like i was trying to consolidate some parts, so that's a 100pf cap, while the teensy board uses a 150pf cap according to its schematic: https://www.pjrc.com/store/teensy3_audio.html
Robert Feranec , 11-03-2024, 05:58 AM
I am not expert for this, but maybe it could be something with resonance (capacitors + inductance of power rails)? maybe this video can give some ideas: https://youtu.be/wbYzPwuEHPw?si=xJM9fVau5q1LZnh4 ... but I guess there may be many reasons that can cause the noise
Elian Terelle , 11-03-2024, 12:17 PM
The noise i‘m hearing is something like white noise. The fact that the capacitor is highly sensitive to touching it and act like an antenna basically, makes me think that it isn‘t resonance (does that make sense?). It isn’t sensitive at all with the capacitor i swapped it with. I guess a low esr/esl cap would be beneficial as it would try to resist all changes in voltage / sink them to ground, right?
QDrives , 11-03-2024, 09:14 PM
No, the teensy board uses a 150nF capacitor, that is 1000x more.And for low noise, it is best not to use a ceramic class II or class III capacitor.
Elian Terelle , 11-03-2024, 10:21 PM
No way… you don’t have any idea how often i looked at that schematic and i missed THAT?! Thanks 😂
Elian Terelle , 11-03-2024, 10:48 PM
That's a little embarrassing actually... Thanks for catching that mistake!
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