User's Manual

Silicon Laboratories Finland Oy
Page 19 of 45
5 Layout Guidelines
Use good layout practices to avoid excessive noise coupling to supply voltage traces or sensitive analog
signal traces, such as analog audio signals. If using overlapping ground planes use stitching vias separated
by max 3 mm to avoid emission from the edges of the PCB. Connect all the GND pins directly to a solid GND
plane and make sure that there is a low impedance path for the return current following the signal and supply
traces all the way from start to the end.
A good practice is to dedicate one of the inner layers to a solid GND plane and one of the inner layers to
supply voltage planes and traces and route all the signals on top and bottom layers of the PCB. This
arrangement will make sure that any return current follows the forward current as close as possible and any
loops are minimized.
Layout
• Supply voltage
– If possible use solid power plane
– Make sure that solid GND plane follows the traces all the way
– Do not route supply voltage traces across separated GND regions so that the
path for the return current is cut
• MIC input
– Place LC filtering and DC coupling capacitors symmetrically as close to audio
pins as possible
– Place MIC biasing resistors symmetrically as close to microhone as possible.
– Make sure that the bias trace does not cross separated GND regions (DGND ->
AGND) so that the path for the return current is cut. If this is not possible the do
not separate GND regions but keep one solid GND plane.
– Keep the trace as short as possible
Signals
GND
Power
Signals
Recommended PCB layer configuration
Figure 9: Typical 4-layer PCB construction
Overlapping GND layers without
GND stitching vias
Overlapping GND layers with
GND stitching vias shielding the
RF energy
Figure 10: Use of stitching vias to avoid emissions from the edges of the PCB