I am designing a mixed signal PCB. I have to sample an analog signal with an ADC and send samples to FPGA for processing. I read a lot about mixed signal PCB layout guidelines on internet and came out with a power supply and grounding scheme. Can you please check and tell
whether my layout (attached below) will be good enough to prevent high frequency digital currents from the digital ground plane to corrupt sensitive analog signals over analog ground plane.
I have made sure:
1. No trace crosses the split (MOT) between digital and analog ground planes.
2. There is no secondary ground plane and chassis contact point except at the input supply point.
I have one doubt:
Should I put input supply connector (+12V, Chassis ground) on digital ground side or analog ground side?
Did you provide another isolated supply too? Cos from your block diagram I don't see any isolation supply provided. If your ADC shares the same voltage with the digital portion, I mean come from the same source, then you really have to pay attention on the pinout of the IC on you should place your decoupling capacitors next to your ADC since you opened a moat here for isolation purpose.
There's a lot of article online showing that it is not necessary to split the ground plane into two as long as you adhere to strict placement rules (digital components on digital portion, analog on its own area), there shouldn't be any adverse effect by having digital noise coupled to analog portion. In fact by deliberately isolating, you might end up having uncontrolled large return loop and high impedance return path. Please refer to these good articles:
THANX FOR THE RESPONSE. I had already gone through those documents. They are advising using a MOT in between AGND and DGND. I have followed all routing disciplines but I have few doubts.
1. Which side should I put my input power supply conector i.e. on digital or analog ground side.
2. I have used separate supplies for analog and digital ICs but I am sharing Analog supply of ADC with a 3.3 to 5 volt level converter which is feeding sampling clock to ADC. Can I get away with this if I use a ferrite bead between power pin and supply of level translator?
1. Power supply should be placed near digital circuits, otherwise the return path of digital circuit might pollute the analog circuit return path inadvertently.
2. You have to check the pinout and datasheet of your ADC carefully and refer to their decoupling advice. Ferrite bead is not really necessary from what I've known unless your circuit suffers serious high frequency noise from the supply.
Most authoritys on planes advice not to split the grounds and not to put splits in them.I would also look at the slots being created in the ground plane by you vias.