FOCUS Enhancements FS453 - Manual (Page 31)

FS453/4 AND FS455/6 DATA SHEET: HARDWARE REFERENCE
Figure 11: Recommended Power Filter Networks
9.1.2 Ground
Connect the analog and digital grounds of the FS453 to separate ground planes. This will insure that
electrical noise from the digital ground does not pollute the analog ground. Connect these two planes
with either a ferrite bead or a very thin trace. This will allow the two planes to maintain an equal voltage
potential. Whenever possible, connect each of the FS453 ground pins directly to its respective
decoupling capacitor ground lead, and then connect to the ground plane through a ground via. Use short
and wide traces to minimize the lead inductance.
9.1.2.1 Special Consideration:
The FS453 is a high quality mixed process device that has excellent DAC Power Supply noise rejection
(40db of rejection). Good PCB layout will result in an acceptably clean power supply.
In the noisiest environments, a dedicated voltage regulator can dramatically improve the quality of the
power to the FS453. A point-of-use 5V to 3.3V 200mA regulator for the V
DD_PA
and V
DD_DA
lines is
recommended in those situations. A single regulator can be used for both V
DD_PA
and V
DD_DA
lines,
provided that those lines each have their own passive filter networks (see Figure 11 above). Placing a
"no-stuff" zero ohm resistor between 3.3V and the regulated node will create the option of not populating
the regulator. This allows the design engineer to save cost if testing shows that the regulator is not
necessary.
9.2 DIGITAL SIGNALS
9.2.1 Digital Signal Routing
Isolate digital inputs to the FS453 from the analog outputs and other analog circuitry. The high-speed
edge transition rates of the digital signals cause signal overshoot, undershoot, and ringing; this noise can
directly couple onto any nearby signals. Do not overlay the analog power plane or analog output traces
with digital signal traces. Using lower speed logic (3-5 ns edge rates) will benefit lower-speed
applications by reducing data-related noise on the analog outputs. Reducing the digital edge transition
rates (rise/fall time), minimizing ringing with damping resistors, and routing the digital traces
perpendicular to any analog traces can prevent coupling the noise from the digital signals.
JANUARY, 2005, VERSION 3.0 31 COPYRIGHT ©2003-4 FOCUS ENHANCEMENTS, INC.
FOCUS Enhancements Semiconductor