Layout related issues: ground scheme, bias current

Hi team,
I am working on the layout of a mixed signal circuit. I have finished the layout of analog part (basically some amplifiers, and a bias current generator, etc) and digital part (basically clock generator) separately. Can I please ask 3 questions?

Q1: I understand that for substrate noise isolation, I should make sure the supply voltage of my analog circuit (AVDD AGND) separate from digital supply (DVDD DGND). So in my layout, should I have 2 separate continents (one for analog supply and one for digital supply) completely unattached? Or should they be just a little bit connected (connected through just one trace)?

Q2: My digital clock generator needs a bias current. Can I generate the bias current along with other analog bias currents in my analog domain? Or do I have to separate the bias current generation for digital noise isolation?

Q3: I have a second set of VDD2/VSS2(2V to -0.5V) for output buffer amplifiers. I am wondering about their bias current design. Should I have a new set of bias current generator? Or can I just use the same bias current generator in my analog domain (which is from 1.2V to 0V)?

Thanks
Allen

you do not need to ask permission to ask questions - just ask

when you say substrate, do you mean your designing an integrated circuit, or do you mean a printed wiring board?

unless i completely misunderstand, when you say bias current, i assume you mean local power for the circuit, Vdd and Vss.

the digital and analog Vdd and Vss should be separated
somewhere along the line, there must be some sort of power input to your board. the grounds get tied together there.
use a big and fat a trace (copper pour / ground plane) as you can - this keeps the current density down and any radiated EM down.

you might want to move all of your bias current generation to a third section with its own ground plane.
again, tie them all together at one point.

Q1: I understand that for substrate noise isolation, I should make sure the supply voltage of my analog circuit (AVDD AGND) separate from digital supply (DVDD DGND). So in my layout, should I have 2 separate continents (one for analog supply and one for digital supply) completely unattached? Or should they be just a little bit connected (connected through just one trace)?

Click to expand...

Substrate noise isolation has nothing to do with separate supply. Substrate noise is passed on from digital side to analog side through the low ohmic substrate. If you think you will have problems with substrate noise, place ground cells in between that will serve as a better isolation.

Q2: My digital clock generator needs a bias current. Can I generate the bias current along with other analog bias currents in my analog domain? Or do I have to separate the bias current generation for digital noise isolation?

Click to expand...

Yes... Put large decoupling capacitors in the bias nodes (to Vdd if they are supply referred to Gnd if they are Gnd referred) if there is a chance of digital signals coupling to those nodes.

Q3: I have a second set of VDD2/VSS2(2V to -0.5V) for output buffer amplifiers. I am wondering about their bias current design. Should I have a new set of bias current generator? Or can I just use the same bias current generator in my analog domain (which is from 1.2V to 0V)?

Click to expand...

Same as above. You can re-use your bias blocks.