PCB design query

[mattomatto]

Hi all,

Just a couple of basic PCB design queries..

My circuit contains a teensy 3.1, the audio shield and a bunch of pots and buttons. Is it a good idea to have a AGND plane on the bottom and a GND plane on the top? This would help make my routing easier and if I understand correctly, help reduce noise in the circuit. If so, which side would it be preferable to run power, digital and analog signals on to avoid picking up noise on each?

I'm also a little confused about why it's best to keep all traces as short as possible to stop them acting as an antenna and picking up noise, but at the same time it's fine to flood the entire board with a power/ground plane?

Thanks

 

[LenSamuelson]

Ground and power are both low impedance "signals" in order to allow the circuit designer to count on their stability almost axiomatically. Signals are measured and driven with respect to ground, having a common ground reference is A Good Thing. Power is also designed to be a stable and common resource, consistently available to all devices in a circuit. Physical / layout designers often specify that ground (especially) and power (wherever possible) are "poured" to a greater thickness of copper to ensure the required consistency.

Back in the '90's when signal rates surged and EEs started running lots of high-powered parts, we all got into big and very expensive trouble with "ground bounce": When switching wide buses, the chips would effectively short-circuit the signal pathways to either power or ground to charge them to a new state (either low-to-high or high-to-low). The effect on ground was to pull it to a higher voltage, if only for fractional nanoseconds. That caused any signal that was already low, or nearly equal to the original ground, to be negative with respect to the now-raised ground. When signals are below ground, the dreaded CMOS latchup could occur and things got broken in a hurry.

So ground should be thick, heavy and rock-solid in order to serve as a consistent and essentially invariant basis for circuit measurement. Power also should be thick, heavy and common to the extent possible, though the common assumption is that power supplies are not perfect and are expected to have a bit of ripple or occasional distortion. Bypass and decoupling capacitors are intended to control those effects.

Analog power/grounds (and references) are isolated from digital circuitry (often referred to as an "island") because analog measurement tends, by design, to be measuring signals with finer resolution. So the analog sections are isolated from the digital sections for greater stability.