Question about ADC frontends...

Let me try:

1. Dedicated ground planes is an easy way to avoid tracks having to share both analog and digital currents and also to avoid ground loops. As a simple rule, if a circuit has analog and digital circuit on the board, then no track should share analog and digital current.

Star type grounding can be used on two layer boards to achieve this, but one needs to route the board manually and should have some experience on this to keep the critical trace lengths short.

Dedicated power planes also allow for short traces especially the de-cap traces, where the trace inductance will affect the EMC performance.

One thing, however, can be debated: Do we really need a seperate analog ground and digital ground planes? In my opinion, no. I cannot justify the two planes connected by a tie that is properly located. My question is: Will the tie offer lower impedance than a plane? If the answer is no, which is correct answer, then, we should be in a good position to get as good noise performance using a single plane layer for analog and digital ground signals.

What I am saying is that, provided we have a dedicated layer for ground, the analog and digital grounds need not be seperate.

2. Apart for the capacity/ value and tolerance, we need to consider the effective series resistance the capacitor will offer and the life of the capacitor. Lower the ESR, the better. I would avoid series electrolytic caps as they would introduce noise once the electrolyte dries-up with time.

My preferred choice would be ceramic caps for de-caps and filters and and poly-caps for coupling.

3. Coupling capacitors serve a primary function of keeping the signal free from and DC. Audio signal riding on DC can saturate the succeeding stages.

The value of coupling caps should be correctly calculated since it is likely for form a high pass filter with the input impedance of the next element in the signal chain. This high pass filter could become a culprit of reducing your low frequency response.

Whether you want to have a passive ADC front end or a buffered front end depends on the source. If the source is unknown, a buffered front end will be preferred as it will enable maximum signal tansfer from source to load and can also avoid loading of the source.

Note: My views may differ from those who are more learned and experienced. I would encourage you to kindly express your reactions as design comes more with experience than from the books. Agreement will help all of us to follow certain guideline while it could otherwise better it.

KKSL.

Outstanding!

This is my first mixed signal type design so it appears that the considerations are growing exponentially.

I think that I am going to take a "modular" approach to the mixed signal layout. I think that I will use analog and digital "daughter" boards to help with isolation. I figure that if I isolate the CODEC on one board, the only digital signals on the entire board would be the outputs to the digital board containing the SigmaDSP and the supervisory microcontroller. That way, the digital board will be pure digital signals.

What do think about this approach, instead of using the multilayer board with dedicated plane?

Thanks very much KKSL. I agree with all of the points you listed.

mdot, with a modular approach you have to be aware that there are still considerations for grounding that need to be addressed. For example, if you have multiple boards in the unit and there are wires connecting signals from one board to another, there will be a return current for that signal on the chassis of the unit.

A star grounding scheme is a good idea for multiple PCB units. You basically need to pick a singular ground point and make everything referenced to that.

BrettG and KKSL,

You guys have me regretting my choice to pursue embedded software instead of electrical engineering after college less and less...*laffs*

I'm still learning a lot about PCB design. I'm reading a book called "The Circuit Designer's Companion" by Tim Williams and it has been a good reference so far.