Routing the
FPD-Link III signal traces between the DOUT pins and the connector, as well as
connecting the PoC filter to these traces, are the most critical pieces of a
successful DS90UB635-Q1 PCB
layout. The following list provides essential recommendations for routing the
FPD-Link III signal traces between the driver output pins and the FAKRA connector,
as well as connecting the PoC filter.
The routing of the FPD-Link III traces may be all on the
top layer 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 receiver input pins to minimize the length of coupled
differential trace pair between the pins and the capacitors.
Route the DOUT+ 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 DOUT+ trace
through the ferrite bead or an RF inductor. The ferrite bead 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 ferrite bead 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.
When routing DOUT+ on inner layers, length matching for
single-ended traces does not provide a significant benefit. If the user
wants to route the DOUT+ on the top or bottom layer, route the DOUT– trace
loosely coupled to the DOUT+ trace for the length similar to the DOUT+ trace
length. This may 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 signal traces.
Figure 10-1 DS90UB635-Q1 Serializer DOUT+
Signal Traces and PoC Filter PCB Layout Example
Figure 10-2 DS90UB635-Q1 Serializer
Differential Signal Traces PCB Layout Example
Figure 10-3 DS90UB635-Q1 Serializer CSI-2
Traces PCB Layout Example