the motherboard in my wife's PC has the absolute worst decoupling on the audio DAC. I can hear everything the CPU does at about 30% the volume of the actual sound, TEMPEST style. it's so bad that I wondered if it had faulty caps, but I looked and there's like one 0603 cap near the I²S DAC IC. awful design.
they also skimped on the decoupling on the USB ports. the voltage goes out of spec all the time.
@gsuberland
So a check box ticking afterthought only?
@ftg even that would be cutting them slack, this shouldn't have even checked a box given that it's audible over everything continuously.
@gsuberland
*has 3.5mm audio connector ✅
"has codec laid out and piped to connector ✅
*had decoupling ✅
@ftg yeah :/
I suspect part of it is the layout. given that the EPS 12V connector is north of the IO region, I'm guessing they've got a heap of dI/dt in the planes under the DAC which is coupling into the analogue rail. which *might* be acceptably quiet with a ton of bypass capacitance and a very high PSRR LDO driving the DAC's AVCC rail, but they have neither of those things.
@ftg if it wasn't a huge pain in the ass to remove the board from the case, I'd bodge a bunch of extra caps on there to fix it.
