Noise Countermeasures

What are EMI and EMS?

EMI (Electromagnetic Interference) pertains to an electromagnetic interference regulation that eliminates influences of an electronic system on other systems, by regulating radiation and transfer of electromagnetic (electric wave) noise that leaks from the electronic system. EMI is legally regulated and standardized in Europe and the USA. The regulations in Europe are particularly stringent. In Japan, EMI is voluntarily regulated by industry.
EMS (Electromagnetic Susceptibility) pertains to immunity to electromagnetic interference with which an electronic system can operate without problem even if it receives electromagnetic noise that leaks from other electronic systems.
EMI and EMS are combined and also referred to as "EMC (Electromagnetic Compatibility)".

(2006/11)

What causes noise generation?

Noise is likely to be generated if the voltage or current heavily changes (in quantity and direction) per unit time. Therefore, high-power and high-frequency circuits may become a source of noise.
Noise is transferred via wiring such as power or ground wiring and signal wiring, or radiated into the air as electromagnetic waves (crosstalk).
Some sources that generate noise are the structure of the set and electrical characteristics of the circuit (such as current, operating speed, and inductance). Brush motors, relays and switches using an armature may generate surge noise (similar to lightning) by sparking, or power or ground noise. If the wiring on a board is at right angles or acute angles, noise could be emitted into the air.

(2008/02)

What are the basic concepts of EMS design for semiconductor devices?

Noises are divided into several types according to their path and noise countermeasures differ according to type of noise. Generally, however, the following points are important.
Take the appropriate measures, combining the following, taking into consideration cost and feasibility.

(1) Noise entering from common wiring, such as power and ground lines, can be eliminated by inserting a regulator to suppress external noise, and by installing a ceramic capacitor with good frequency characteristics for use as a bypass capacitor in an appropriate location near the entry of the circuit board. Some laminated ceramic capacitors, especially those having a high capacitance, have poor frequency characteristics because of the influence of dielectrics, so care is needed. The basic rule is to prevent noise at the entry of the board.

(2) Noise superimposed onto input/output signals can be eliminated by using anti-noise components such as ferrite beads or by inserting a noise filter. However, using a capacitor alone may be a problem because the charge of the capacitor may increase the voltage to a higher level than the supply voltage. Measures have to be taken to prevent noise from entering the circuit from the entry of the board, to eliminate noise on the input and output signals. Using shielded lines for signal lines that have to be extended, or using a twisted pair to transfer differential signals is effective.
When installing noise filters, noise flows to GND, so the filters should be connected on parts where GND is sufficiently reinforced.

(3) For noise that is directly induced to wiring, the impedance of the signal lines of a multi-layer board have to be lowered. Putting the board itself into a shielded case is another option.
If it is not necessary to go that far, it is also possible to erect a shielded plate and place the board near it.

(4) It is important to use noise-eliminating components in the circuits. Equally important is the layout of the components and the routing of wiring.

(5) In the case of a microcontroller, another way is to perform a refresh operation (that periodically sets the same value to ports, so that data can be corrected even if it has been changed) whenever possible.

(2006/11)

May it be understood that a 4- or 6-layer board having a power layer and a ground layer has a high noise immunity?

Yes, as long as the capacity of the power supply is high and the bypass capacitor for the major devices is correctly placed. In a power wire and ground wire which are not in line form, but are in solid form, the current does not flow through only the necessary route. As a result, if current density flowing into a device is high, a voltage drop may occur at a device nearby, and a sufficient current may not flow into the device.
For a device having a heavily fluctuating supply current, a voltage fluctuation occurs due to the inductance component (to suppress the changes in the current) and, therefore, placing a bypass capacitor is important. The same applies to a board where the power and ground are in a line form.

(2007/08)

A capacitor should be placed between a ground line and signal line to suppress noise. Is there any limit of the capacitance?

This generally poses a problem to MOS devices.
An output of the device permits a value defined as a load capacitance, including the wiring capacitance and input capacitance of another connected device. If this is exceeded, a circuit that suppresses an inrush current is necessary.
If the capacitance to be added is too high, the signal changes slowly. Therefore, it must be confirmed that the timing of the input of the device is satisfied and that the input is not affected by the through current (the problem of the timing also applies to the analog device).

(2007/08)

Is it effective to connect resistors in series to their pins to protect semiconductor devices from damage, as a countermeasure against surge and static electricity?

No. Application of an overvoltage cannot be avoided by using a dumping resistor, because it only limits the current. To drop the voltage, a ground capacitor must be connected between the resistor and pin. If a large-capacitance capacitor that can absorb surge and static electricity is connected, however, the waveform may attenuate and this must be taken into account. For a MOS device, a diode must be connected at the power supply side to prevent an inrush current that flows when the power is turned off.
Comparing with these circuits, placing a Zener diode as a surge absorber between the signal line and ground is simpler.

Related page:Surge Absorber Device (NSAD series)

(2007/08)

Noise is superimposed onto the clock input to an IC when a relay operates.
It seems that the IC is operating correctly. Is there any problem? If a problem occurs, how does the IC behave?

Since a relay can be a source of electromagnetic waves (surge noise), precautions must be taken to protect the IC from the electromagnetic waves.
The behavior of the IC if the input clock is disturbed cannot be predicted. (The clock waveform does not satisfy the specifications, because a level width and transition time are defined.)
The noise component that exceeds the absolute maximum ratings could promote degradation of the IC and affect its reliability.
Noise affects not only the clock but also the other signals. Therefore, drastic action must be taken to improve the relay circuit itself.

(2008/02)