Analogue PCBs must often have a
specially-tailored earth scheme or use multiple ground planes
on a multi-layer substrate.
Noisy circuitry (switching supplies and digital
lines, digital supplies etc) should be kept well away from
analogue circuitry.
Ground planes must be used to provide a level
of shielding to sensitive signals. If necessary, screening cans
can also be used - or at least included in the PCB design as
optional extras.
The signal path should flow from one side of
the board to the other. This prevents signals near the output
of the circuit from being coupled back into the input stages.
Decoupling capacitors must be placed extremely
close to key ICs and discrete sub-circuits.
Track spacings and lengths must be carefully
managed to avoid forming parasitic inductances and
capacitances. Additional low impedance ground tracks can be
placed between offending signal tracks to break interacting
electric-fields.
Low impedance grounds and supply must be
established by use of multiple ground and power planes.
Substrates are available with closely spaced internal layers to
establish wide-band capacitive coupling between power and
ground planes.
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