3.0 PC Board Considerations
CX25870/871
3.5 Signal Interconnect
Flicker-Free Video Encoder with Ultrascale Technology
3.5 Signal Interconnect
3.5.1 Digital Signal Interconnect
The digital inputs to the CX25870/871 should be isolated as much as possible
from the analog outputs and other analog circuitry. Also, these input signals
should not overlay the analog power plane or analog output signals.
Most of the noise on the analog outputs will be caused by fast transitioning
clock edges, data edges (less than 3 ns), and overshoot, undershoot, and ringing
on the digital inputs.
The digital edge rates should not be faster than necessary because feedthrough
noise is proportional to the digital edge rates. Lower-speed applications will
benefit from using lower-speed logic (3–5 ns edge rates) to reduce data-related
noise on the analog outputs.
Transmission lines will mismatch if the lines do not match the source and
destination impedance. This will degrade signal fidelity if the line length
reflection time is greater than one-fourth the signal edge time. Line termination or
line-length reduction is the solution. For example, logic edge rates of 2 ns require
line lengths of less than 4 inches without use of termination. Ringing can be
reduced by damping the line with a series resistor (30–50 Ω).
Radiation of digital signals can also be picked up by the analog circuitry. This
is prevented by reducing the digital edge rates (rise/fall time), minimizing ringing
with damping resistors, and minimizing coupling through PC board capacitance
by routing the digital signals at a 90-degree angle to any analog signals.
The clock driver and all other digital devices must be adequately decoupled to
prevent noise generated by the digital devices from coupling into the analog
circuitry.
3.5.2 Analog Signal Interconnect
The CX25870/871 analog output traces should be located as close as possible to
the output connectors and be of equal length to minimize noise pickup and
reflections caused by impedance mismatch.
The analog outputs are susceptible to crosstalk from digital lines; therefore
digital traces must not be routed under or adjacent to the analog output traces.
To maximize the high-frequency power supply 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 the same. The load resistor connection
between the video outputs and GND should be as close as possible to the
CX25870/871 to minimize reflections. Unused DAC analog outputs should be left
floating.
3-10
Conexant
100381B
CONEXANT [ CONEXANT SYSTEMS, INC ]
