DRA790 Datasheet, PDF (371/436 Pages) Texas Instruments – Infotainment Applications Processor

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DRA790, DRA791
DRA793, DRA797
SPRS968A – AUGUST 2016 – REVISED FEBRUARY 2017
BETTER
SPRS906_PCB_EMC_01
Figure 7-11. Trace Examples
• Always route signal traces and their associated ground returns as close to one another as possible to
minimize the loop area enclosed by current flow:
– At high frequencies current follows the path of least inductance.
– At low frequencies current flows through the path of least resistance.
7.2.5.3 ESD Protection System Design Consideration
ESD protection system design consideration is covered in Section 7.5.2.2 of this document. The following
are additional considerations for ESD protection in a system.
• Metallic shielding for both ESD and EMI
• Chassis GND isolation from the board GND
• Air gap designed on board to absorb ESD energy
• Clamping diodes to absorb ESD energy
• Capacitors to divert ESD energy
• The use of external ESD components on the DP/DM lines may affect signal quality and are not
recommended.
7.2.6 EMI / EMC Issues Prevention
All high-speed digital integrated circuits can be sources of unwanted radiation, which can affect nearby
sensitive circuitry and cause the final product to have radiated emissions levels above the limits allowed
by the EMC regulations if some preventative steps are not taken.
Likewise, analog and digital circuits can be susceptible to interference from the outside world and picked
up by the circuitry interconnections.
To minimize the potential for EMI/EMC issues, the following guidelines are recommended to be followed.
7.2.6.1 Signal Bandwidth
To evaluate the frequency of a digital signal, an estimated rule of thumb is to consider its bandwidth f_BW
with respect to its rise time, t_R:
f_BW ≈ 0.35 / t_R
This frequency actually corresponds to the break point in the signal spectrum, where the harmonics start
to decay at 40 dB per decade instead of 20 dB per decade.
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Applications, Implementation, and Layout 371
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