Analogue and digital supply/ground coupling

mojo-chan

Member
2008-04-11 4:59 pm
I am working on a hardware based Replay Gain device. It's all up and working with a WinAMP plug-in at the moment, and sounds excellent despite being made on matrix board. Far better than doing the gain adjustment digitally.

Input is via either USB or a rotary encoder for manual adjustment. I'm using a PGA2311 for gain control.

I am trying to come up with a PCB for this schematic:

[IMGDEAD]http://img228.imageshack.us/img228/87/replaygainschemfj8.th.png[/IMGDEAD]

I'm not sure how I should couple the digital and analogue grounds and supply lines. My best guess is to use a simple ferrite bead, but I'm not sure what value would be best or where to place them.

On the digital side there will be some noise from the microcontroller and rotary switch. Things are complicated by the fact that on USB or battery power, I only have one supply and need to generate +5V digital, +5V analogue, -5V analogue, digital GND and analogue GND. I'm using a TPS60403 to invert +5V to -5V at the moment.

Should I put the ferrites between analogue and digital +5V and GND? If so, what about -5V, should that be generated from the analogue +5V presumably... then again, the inverter is PWM based (I'm using the 250kHz version) so probably produces some noise of it's own.

I'm following the reference designs for both ICs, with an RC filter on the output of the TPS60403.
 
Hi

My best guess is to use a simple ferrite bead, but I'm not sure what value would be best or where to place them.
You are right. Since Agnd is present only near input jacks(I think they are) I think they sould be there somewhere

Should I put the ferrites between analogue and digital +5V and GND
If you do that you will have short circuit, but you could use it on 5v rail, say from reg., so that you will filter noise going from your board to PC and other way around. Another good thing would be double sided board, lot of ground plane(I think Agnd)

Make all supplys from Agnd(based on it)

I could be way wrong, so somebody will have to corrent me or say it is ok