Routing the FPD-Link III signal traces between the
RIN pins and the connector as well as connecting the PoC filter to
these traces are the most critical pieces of a successful DS90UB964-Q1 PCB layout.
Figure 6-12 shows an example PCB layout of the DS90UB964-Q1 configured for
interface to remote sensor modules over coaxial cables. The layout example also uses
a footprint of an edge-mount Quad Mini-FAKRA connector provided by Rosenberger.
The following list provides
essential recommendations for routing the FPD-Link III signal traces between the
DS90UB964-Q1 receiver input pins (RIN) and the FAKRA connector, and
connecting the PoC filter.
- The routing of the FPD-Link III traces can be all on the
top layer (as shown in the example) or partially embedded in middle layers
if EMI is a concern.
- The AC-coupling capacitors are advised to be on the top
layer and very close to the DS90UB964-Q1 receiver input pins to minimize the
length of coupled differential trace pair between the pins and the
capacitors.
- Route the RIN+ trace between the AC-coupling capacitor and
the FAKRA connector as a 50Ω single-ended micro-strip with tight impedance
control (±10%). Calculate the proper width of the trace for a 50Ω impedance
based on the PCB stack-up. Verify that the trace can carry the PoC current
for the maximum load presented by the remote sensor module.
- The PoC filter must be connected to the RIN+ trace through
the first ferrite bead (FB1). The FB1 is advised to
touch the high-speed trace to minimize the stub length seen by the
transmission line. Create an anti-pad or a moat under the FB1 pad
that touches the trace. The anti-pad is a plane cutout of the ground plane
directly underneath the top layer without cutting out the ground reference
under the trace. The purpose of the anti-pad is to maintain the impedance as
close to 50Ω as possible.
- Route the RIN– trace with minimum coupling to the RIN+
trace (S > 3W).
- Consult with connector manufacturer for optimized
connector footprint. If the connector is mounted on the same side as the IC,
minimize the impact of the thru-hole connector stubs by routing the
high-speed signal traces on the opposite side of the connector mounting
side.
When configured for STP and routing differential
signals to the DS90UB964-Q1 receiver inputs, the traces must maintain a 100Ω
differential impedance routed to the connector. When choosing to implement a common
mode choke for common mode noise reduction, take care to minimize the effect of any
mismatch.