The
DS90UB953-Q1 EVM platform can be used to evaluate DS90UB935-Q1. The board
layout for the DS90UB953-Q1EVM is shown in
Figure 7-11 and Figure 7-12. All EVM layers are included in DS90UB953-Q1EVM user's
guide (SNLU224).
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 DS90UB935-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 can be all on the
top layer or partially embedded in middle layers if EMI is a concern.
- The AC-coupling capacitors must 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 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. Make sure that the trace can carry the
PoC current for the maximum load presented by the remote sensor module.
- The PoC filter can be connected to the DOUT+ trace through
the ferrite bead or an RF inductor. The ferrite bead must 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 must 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
50Ω 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 can help the differential nature of the receiver to cancel out
any common-mode noise that can be present in the environment that can couple
on to the signal traces.