time to do some work on the preamps and line driver cards now I have learned more !
Originally I tried to avoid using buffers between the ADC and preamps and DAC and line drivers.
not really my best idea ! I think I avoided them since I just didn't really know what they did :)
.. since then I have learned the PGA2500 vcom is in fact an input !!! pretty basic information but something I had completely overlooked.
There is an advantage of having the PGA's audio signals swinging around 0V so I had connected VCOM to ground.
BUT... the ADC has to swing around about 2.5v or near since it just used +5v. The exact bias voltage is available on VQ the pin.
you can't just hook up VQ to VCOM on the the pga or ADC buffer driver ( 10uA maximum current sink for VQ )
OK , I need a VQ buffer so I can send this bias reference across all 8 preamps.
why not just add a buffer / proper filter as recommended in the data sheet and keep the PGA2500 VCOM connected to 0V with decoupling capacitors in between the buffer biased at VQ ( buffered )
the buffered VQ bias ref voltage has to get around all the preamps !!
Staring at the cards and backplane it seemed like VQ was going to make a massive trip all over the design and pick up who knows what EMI.
Ages ago I thought at some stage I will need to add a threaded bar right through all the preamps to stop them flapping about in the header sockets. then I though , just send the VQ voltage down that bar.
then I briefly thought I had a clever idea , followed by another moment when I realised they have very likely been doing tricks like this since the days of vacuum tubes.. followed by another thought of , maybe the massive 2m threaded bar will pick up even more noise than if had just supplied the buffer with 2.5v from voltage dividers of the the 5V rail. The same 5V the ADC uses anyway.
I don't know what's inside the ADC IC , how is VQ derived ? Its ridiculous to think it probably is just voltage dividers !