Mixed analog/digital PCB

[Beagles]

Hi! I am trying to figure out different design issues that has to be taken when mixing analog/digital signals.

So far I have used the different "basic" knowledge such as decoupling caps., separate gnd for analog and digital circuits etc.

But still some issues remains. Since I am such a rookie it is hard to know exactly what has to be done.

What I have; an A/D converter an microprocessor with a clock speed of maximum 14MHz. A/D is taking in signals from another PCB (pure analog) and I do have op. buffers on the receiving side. Every IC is decoupled with ceramic .1uF caps.


But what about analog gnd and digital gnd?? How should they connect to each other? Should I use a small cap between the different planes?? Or just a thin line of copper? And the rest of the "world"? How should gnd be connected to the analog signal condition pcb mentioned above??

Lots of questions but still very important!!

Very nice if anyone would considering giving me some advice

Kind greetings
P

 

[Optikon]

Hi! I am trying to figure out different design issues that has to be taken when mixing analog/digital signals.

So far I have used the different "basic" knowledge such as decoupling caps., separate gnd for analog and digital circuits etc.

But still some issues remains. Since I am such a rookie it is hard to know exactly what has to be done.

What I have; an A/D converter an microprocessor with a clock speed of maximum 14MHz. A/D is taking in signals from another PCB (pure analog) and I do have op. buffers on the receiving side. Every IC is decoupled with ceramic .1uF caps.


But what about analog gnd and digital gnd?? How should they connect to each other? Should I use a small cap between the different planes?? Or just a thin line of copper? And the rest of the "world"? How should gnd be connected to the analog signal condition pcb mentioned above??

Lots of questions but still very important!!

Very nice if anyone would considering giving me some advice

Kind greetings
P

Keep analog signals far away from any high speed digital signals.

Keep any high current paths aways from sensitive return points in analog circuits.

Connect analog & digital GND planes together only at *ONE* point. Preferably back at the power supply entry.

Only bridge the planes with a capacitor if you must run a digital signal over the analog plane. Otherwise, don't do it.

Treat your digital and analog supplies (+/-VCC, VDD) as gnd returns as well. Because at high frequencies they are "ac gnd" just like your gnd planes.

without knowing more details from you, this is generic advise.