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 DS90UB638-Q1
PCB layout. Figure 10-1 shows an example PCB layout of the DS90UB638-Q1 configured for interface
to remote sensor modules over coaxial cables. The layout example also uses a
footprint of an edge-mount FAKRA connector provided by Rosenberger (P/N:
59S20X-40ML5-Z). For additional PCB layout details of the example, check the
DS90UB954-Q1EVM user's guide. Note that the
DS90UB638-Q1 shares this user guide with other related products such as the
DS90UB954-Q1.
The following list provides
essential recommendations for routing the FPD-Link III signal traces between the
DS90UB638-Q1 receiver input pins (RIN)
and the FAKRA connector, and connecting the PoC filter.
- The routing of the FPD-Link III traces may 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 should be on the top layer and
very close to the DS90UB638-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 50-Ω single-ended micro-strip with tight impedance
control (±10%). Calculate the proper width of the trace for a 50-Ω impedance
based on the PCB stack-up. Ensure that the trace can carry the PoC current
for the maximum load presented by the remote sensor module.
- The PoC filter should be connected to the RIN+ trace
through the first ferrite bead (FB1). The FB1 should
be touching 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 should be 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 50 Ω as possible.
- Route the RIN– trace loosely coupled to the RIN+ trace for
the length similar to the RIN+ trace length when possible. This will help
the differential nature of the receiver to cancel out any common-mode noise
that may be present in the environment that may couple on to the RIN+ and
RIN– signal traces. When routing on inner layers, length matching for
single-ended traces does not provide as significant benefit.