Long I/O and power cables usually act as good antennas, picking up noise
from the outside world and conducting this into the system. For unshielded systems, long
PCB tracks may also act as antennas. Once inside the system, the noise may be coupled
into other, more sensitive signal lines. It is therefore vital that the amount of RF
energy allowed into the system is kept as low as possible, even if the input lines
themselves are not connected to any sensitive circuit.
This can be done by adding one or more of the following:
- Series inductors or ferrite beads
will reduce the amount of HF noise that reaches the microcontroller pin. They will
have high impedance for HF, while having low impedance for low-frequency
signals.
- Decoupling capacitors on the input
lines will short the HF noise to ground. The capacitors should have low ESR
(equivalent series resistance). This is more important than high capacitance values.
In combination with resistors or inductors, the capacitors will form low-pass
filters. If the system is shielded, the capacitors should be connected directly to
the shield. This will prevent the noise from entering the system at all. Special
feed-through capacitors are designed for this purpose, but these may be
expensive.
- Special EMC filters combining
inductors and capacitors in the same package are now delivered from many
manufacturers in many different shapes and component values.