AN901 Datasheet, PDF (4/14 Pages) STMicroelectronics – Electromagnetic compatibility

EMC GUIDELINES FOR MICROCONTROLLER-BASED APPLICATIONS
3 EMC COMPLIANCE
Once the areas involved are identified, EMC performances are improved by decreasing noise
source emissions, increasing EMI immunity in susceptible areas and weakening the capacity
of noise carriers.
3.1 PRINTED CIRCUIT BOARD
For technical reasons, it is best to use a multi-layer printed circuit board (PCB) with a separate
layer dedicated to the ground and another one to the V_DD supply, which results in a good de-
coupling, as well as a good shielding effect. For many applications, economical requirements
prohibit the use of this type of board. In this case, the most important feature is to ensure a
good structure for the ground and power supply.
3.1.1 Component Position
A preliminary layout of the PCB must separate the different circuits according to their EMI con-
tribution in order to reduce cross-coupling on the PCB, i.e. noisy, high-current circuits, low-
voltage circuits, and digital components.
3.1.2 Ground and Power Supply (V_SS, V_DD)
The GROUND should be distributed individually to every block (noisy, low level sensitive, dig-
ital,...) with a single point for gathering all ground returns. Loops must be avoided or have a
minimum surface. The power supply should be implemented close to the ground line to mini-
mize the surface of the supply loop. This is due to the fact that the supply loop acts as an an-
tenna, and is therefore the main emitter and receiver of EMI.
All component-free surfaces of the PCB must be filled with additional grounding to create a
kind of shielding (especially when using single-layer PCBs).
3.1.3 Decoupling
The standard decoupler for microcontrollers is a 100-µF pool capacitor, and in parallel, a
0.1-µF high frequency capacitor (typical values). Aluminium electrolytic capacitors should be
avoided due to their poor performance at high frequencies. These capacitors must physically
be as close as possible to the V_SS/V_DD pins of the component in order to reduce the surface of
the actual loop.
As a general rule, decoupling all sensitive or noisy signals improves EMC performances.
There are 2 types of decouplers:
– Capacitors close to components. Inductive characteristics, which apply to all capacitors be-
yond a certain frequency, must be taken into account. If possible, parallel capacitors with de-
creasing values (0.1, 0.01,... µF) should be used.
– Inductors. Although often ignored, ferrite beads, for example, are excellent inductors due to
their good dissipation of EMI energy and there is no loss of DC voltage (which not the case
when simple resistors are used).
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