DRA790 Datasheet, PDF (372/436 Pages) Texas Instruments – Infotainment Applications Processor

DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
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7.2.6.2 Signal Routing
7.2.6.2.1 Signal Routing—Sensitive Signals and Shielding
Keep radio frequency (RF) sensitive circuitry (like GPS receivers, GSM/WCDMA, Bluetooth/WLAN
transceivers, frequency modulation (FM) radio) away from high-speed ICs (the device, power and audio
manager, chargers, memories, and so forth) and ideally on the opposite side of the PCB. For improved
protection it is recommended to place these emission sources in a shield can. If the shield can have a
removable lid (two-piece shield), ensure there is low contact impedance between the fence and the lid.
Leave some space between the lid and the components under it to limit the high-frequency currents
induced in the lid. Limit the shield size to put any potential shield resonances above the frequencies of
interest; see Figure 7-8, Typical Impedance Profile of a Capacitor.
7.2.6.2.2 Signal Routing—Outer Layer Routing
In case there is a need to use the outer layers for routing outside of shielded areas, it is recommended to
route only static signals and ensure that these static signals do not carry any high-frequency components
(due to parasitic coupling with other signals). In case of long traces, make provision for a bypass capacitor
near the signal source.
Routing of high-frequency clock signals on outer layers, even for a short distance, is discouraged,
because their emissions energy is concentrated at the discrete harmonics and can become significant
even with poor radiators.
Coplanar shielding of traces on outer layers (placing ground near the sides of a track along its length) is
effective only if the distance between the trace sides and the ground is smaller that the trace height above
the ground reference plane. For modern multilayer PCBs this is often not possible, so coplanar shielding
will not be effective. Do not route high-frequency traces near the periphery of the PCB, as the lack of a
ground reference near the trace edges can increase EMI: see Section 7.2.6.3, Ground Guidelines.
7.2.6.3 Ground Guidelines
7.2.6.3.1 PCB Outer Layers
Ideally the areas on the top and bottom layers of the PCB that are not enclosed by a shield should be
filled with ground after the routing is completed and connected with an adequate number of vias to the
ground on the inner ground planes.
7.2.6.3.2 Metallic Frames
Ensure that all metallic parts are well connected to the PCB ground (like LCD screens metallic frames,
antennas reference planes, connector cages, flex cables grounds, and so forth). If using flex PCB ribbon
cables to bring high-frequency signals off the PCB, ensure they are adequately shielded (coaxial cables or
flex ribbons with a solid reference ground).
7.2.6.3.3 Connectors
For high-frequency signals going to connectors choose a fully shielded connector, if possible (for example,
SD card connectors). For signals going to external connectors or which are routed over long distances, it
is recommended to reduce their bandwidth by using low-pass filters (resistor, capacitor (RC) combinations
or lossy ferrite inductors). These filters will help to prevent emissions from the board and can also improve
the immunity from external disturbances.
7.2.6.3.4 Guard Ring on PCB Edges
The major advantage of a multilayer PCB with ground-plane is the ground return path below each and
every signal or power trace.
As shown in Figure 7-12 the field lines of the signal return to PCB ground as long as an infinite ground is
available.
372 Applications, Implementation, and Layout
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