Layout of a mixed signal PCB

Hello,

I’m finishing a schematic for an audio gizmo which is some kind of variable filter (analog). I also have a digital circuitry that interacts with the analog filter in switching components using CMOS ICs and changing resistance values using digital potentiometers. The layout should be in 2 layers and this brings up many questions…

Mainly: How should the grounds be connected? Both the analog and digital circuitry shares a ground, however I would like to avoid digital signals injecting noise into the audio path. So how should the grounds be connected and where?

Second, if I had little interaction between the analog and digital parts I could do them far away from each other on the PCB, however there are a few digital components that need to be placed inside the analog parts of the design - digital potentiometers and CMOS ICs used for switching different components into the audio path. How should this be done?

What about ground plane on the bottom of the PCB or perhaps on both sides? I am afraid there will be capacitance between the digital signal to the ground of the analog circuit and that will inject noise.

Comments and suggestions are welcome!

Thanks!

Hi slow_rider,

I suggest taking a look at Texas Instruments “PCB Design Guidelines For Reduced EMI” app note (http://www.ti.com/lit/an/szza009/szza009.pdf). Even though the app note is for reducing EMI it has a lot of good principals for you to follow. At minimum read section 2.1 which starts on page 4 and talks about grounding. Also check out section 2.3 starting on page 12 which talks about component placement.

edit:

Be sure to check out this very well written app note as well (http://www.alternatezone.com/electronic … alRevA.pdf)