(1) Interference source refers to the component, device, or signal that
generates interference, described in mathematical language as follows: du/dt,
where di/dt is large, it is the interference source. For example, lightning,
relays, thyristors, motors, high-frequency clocks, etc. may all become
interference sources.
(2) Propagation path refers to the path or medium through which
interference propagates from an interference source to a sensitive device. The
typical interference propagation path is through the conduction of wires and
spatial radiation.
(3) Sensitive devices refer to objects that are easily disturbed. For
example: A/D, D/A converters, microcontrollers, digital ICs, weak signal
amplifiers, etc. The basic principle of anti-interference design is to suppress
interference sources, cut off interference propagation paths, and improve the
anti-interference performance of sensitive devices.
1. Suppress interference sources
Suppressing interference sources is to minimize the du/dt, di/dt of
interference sources as much as possible. This is a priority and important
principle in anti-interference design, often achieving twice the result with
half the effort. Reducing the du/dt of interference sources is mainly achieved
by connecting capacitors in parallel at both ends of the interference source.
Reducing the di/dt of interference sources is achieved by serializing inductors
or resistors in the interference source circuit and adding a freewheeling
diode.
The common measures to suppress interference sources are as follows:
(1) The relay coil adds a freewheeling diode to eliminate the back
electromotive force interference generated when the coil is disconnected. Adding
only a continuous current diode will cause the disconnection time of the relay
to lag, and adding a voltage regulator diode will allow the relay to operate
more times per unit time.
(2) Connect a spark suppression circuit (usually an RC series circuit, with
a resistance of several to several tens of K and a capacitance of 0.01uF) in
parallel at both ends of the relay contact to reduce the impact of electric
sparks.
(3) Add a filtering circuit to the motor, and ensure that the capacitor and
inductor leads are as short as possible.
(4) Each IC on the circuit board needs to be connected in parallel with a
0.01 μ F~0.1 μ F high-frequency capacitor to reduce the impact of IC on the
power supply. Pay attention to the wiring of high-frequency capacitors, and the
wiring should be close to the power supply end and as thick and short as
possible. Otherwise, it will increase the equivalent series resistance of the
capacitor and affect the filtering effect.
(5) Avoid 90 degree creases during wiring to reduce high-frequency noise
emissions.
(6) The two ends of the thyristor are connected in parallel with an RC
suppression circuit to reduce the noise generated by the thyristor (which may
cause the thyristor to break down when the noise is severe).
According to the propagation path of interference, it can be divided into
two types: conducted interference and radiated interference.
The so-called conducted interference refers to the interference that
propagates through wires to sensitive devices. The frequency bands of
high-frequency interference noise and useful signals are different. The
propagation of high-frequency interference noise can be cut off by adding
filters on wires, and sometimes isolation optocouplers can also be added to
solve the problem. The harm of power noise is significant, and special attention
should be paid to handling it. The so-called radiation interference refers to
the interference transmitted to sensitive devices through space radiation. The
general solution is to increase the distance between the interference source and
the sensitive device, isolate them with a ground wire, and place a mask on the
sensitive device.
The common measures to cut off the interference propagation path are as
follows:
(1) Fully consider the impact of power supply on the microcontroller. If
the power supply is well done, the anti-interference of the entire circuit is
largely solved. Many microcontrollers are sensitive to power noise, and a
filtering circuit or voltage regulator should be added to the microcontroller
power supply to reduce the interference of power noise on the single chip. For
example, a π shaped filtering circuit can be composed of magnetic beads and
capacitors, and of course, a 100 Ω resistor can also be used to replace the
magnetic beads when the conditions are not high.
(2) If the I/O port of the microcontroller is used to control noise devices
such as motors, isolation should be added between the I/O port and the noise
source (adding a π shaped filtering circuit). To control noise devices such as
motors, isolation should be added between the I/O port and the noise source
(adding a π shaped filtering circuit).
(3) Pay attention to crystal oscillator wiring. The crystal oscillator and
microcontroller pins should be as close as possible, and the clock area should
be isolated with a ground wire. The crystal oscillator shell should be grounded
and fixed. This measure can solve many difficult problems.
(4) Reasonable partitioning of circuit boards, such as strong and weak
signals, digital and analog signals. Try to keep interference sources (such as
motors, relays) away from sensitive components (such as microcontrollers) as
much as possible.
(5) Use a ground wire to isolate the digital area from the analog area. The
digital ground should be separated from the analog ground, and then connected to
the power ground at one point. The wiring of A/D and D/A chips is also based on
this principle, and the manufacturer has taken this requirement into account
when allocating the pin arrangement of A/D and D/A chips.
(6) The ground wires of the microcontroller and high-power devices should
be grounded separately to reduce mutual interference. High power devices should
be placed on the edge of the circuit board as much as possible.
(7) The use of anti-interference components such as magnetic beads,
magnetic rings, power filters, and shielding covers in key areas such as
microcontroller I/O ports, power lines, and circuit board connection lines can
significantly improve the anti-interference performance of the circuit.
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