Rev D 2/17/15 14 1:1 DIFFERENTIAL-TO-LVDS ZERO DELAY CLOCK GENERATOR
8745B-21 DATA SHEET
The following component footprints are used in this layout example.
All the resistors and capacitors are size 0603.
Power and Grounding
Place the decoupling capacitors as close as possible to the power
pins. If space allows, placement of the decoupling capacitor on the
component side is preferred. This can reduce unwanted inductance
between the decoupling capacitor and the power pin caused by the
via.
Maximize the power and ground pad sizes and number of vias
capacitors. This can reduce the inductance between the power and
ground planes and the component power and ground pins.
The RC filter consisting of R7, C11, and C16 should be placed as
close to the V
DDA
pin as possible.
Clock Traces and Termination
Poor signal integrity can degrade the system performance or cause
system failure. In synchronous high-speed digital systems, the clock
signal is less tolerant to poor signal integrity than other signals. Any
ringing on the rising or falling edge or excessive ring back can cause
system failure. The shape of the trace and the trace delay might be
restricted by the available space on the board and the component
location. While routing the traces, the clock signal traces should be
routed first and should be locked prior to routing other signal traces.
• The differential 50 output traces should have the same length.
• Avoid sharp angles on the clock trace. Sharp angle turns cause
the characteristic impedance to change on the transmission
lines.
• Keep the clock traces on the same layer. Whenever
possible, avoid placing vias on the clock traces.
Placement of vias on the traces can affect the trace
characteristic impedance and hence degrade signal
integrity.
• To prevent cross talk, avoid routing other signal traces in
parallel with the clock traces. If running parallel traces is
unavoidable, allow a separation of at least three trace
widths between the differential clock trace and the other
signal trace.
• Make sure no other signal traces are routed between the clock
trace pair.
• The matching termination resistors should be located as close
to the receiver input pins as possible.
Figure 5B. PCB Board Layout for 8745B-21
100 Ohm
Differential
Traces
VDDA
VDD
C2
U1
R7
C16
VDDO
GND
C4
C1
ICS8745B-21
VIA
C11
