FOCUS Enhancements FS453 - Manual (Page 32)

FS453/4 AND FS455/6 DATA SHEET: HARDWARE REFERENCE
9.2.2 Video Inputs
The digital pixel data and the pixel clock of the FS453 may toggle at speeds up to 150 MHz (depending
on input mode). It is critical that the traces used for these signals be kept as short as possible. They
should be isolated from the analog outputs and analog circuitry. The signals carried on these traces are
single ended high-speed signals and should be routed together as a bus. It is recommended that these
traces be at least 8 mils wide.
9.3 ANALOG SIGNALS
9.3.1 Video Output Filters
9.3.1.1 Analog Signal Interconnect
Analog output traces are susceptible to electrical noise generated by digital signals. Digital traces must
not be routed under or adjacent to the analog output traces. We recommend placing a third order
reconstruction filter between the FS453
outputs and the output connectors. This filter network will smooth
the stepped output of the FS453’s DACs. The output filter network and the output connectors should be
located as close as possible to the FS453
. This will minimize the possibility of picking up noise from
digital signals. It will also reduce the effects of transmission reflections due to impedance mismatches.
To maximize high-frequency power supply noise rejection, the video output signals should overlay the
ground plane. For maximum performance, the analog video output impedance, cable impedance, and
load impedance should be matched. This will reduce signal transmission reflection.
The output DACs of the FS453 may be configured for many different video formats, since the pins have
no fixed video assignments. The FS453 can assign any combination of Y, C, CVBS, and component (Y,
Pr & Pb) signals to its four DACs. The video traces and the attached components should be laid out
carefully in order to avoid signal coupling amongst each other. It is suggested that the video traces be
separated with ground traces. Do not place the capacitors and inductors attached to those outputs too
close to each other. Route the analog video signals with a minimum of 12 mils spacing between each
other. There should be at least 20 mils between the analog video traces and any digital trace. Route the
video traces in an area of the PCB that does not contain any digital traces. Sharp trace direction changes
(e.g. 90deg) are, in effect, trace width irregularities that affect local transmission line impedances. These
cause minute partial reflections in high-bandwidth signals. 150 MHz clocks with fast rise/fall times are
probably more sensitive to this than 6MHz analog traces. Smooth curves are certainly preferred, and
equal-distant traces help maintain time-alignment. The video traces should be kept on the top PCB layer
with the FS453 to ensure that they are short and direct. Leave unused analog outputs open.
JANUARY, 2005, VERSION 3.0 32 COPYRIGHT ©2003-4 FOCUS ENHANCEMENTS, INC.
FOCUS Enhancements Semiconductor