DM504 datasheet(183/211 Pages) TI1

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DM504

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SPRS977A – NOVEMBER 2016 – REVISED JANUARY 2017

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Applications, Implementation, and Layout

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.

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