Since electromagnetic interference (EMI) can affect most electronic devices, including aviation and medical equipment, modern devices incorporate EMI filters to ensure proper operation in harsh EMI environments. An EMI filter is often used to eliminate conducted interference that is present on any power or signal line. They can be used to eliminate interference generated by the device itself, as well as to eliminate interference generated by other equipment, to improve a device's immunity to EMI signals present in its electromagnetic environment. The impedance of an EMI filter has a highly reactive component. This means that the filter provides much higher resistance to high frequency signals. This high impedance attenuates or reduces the intensity of these signals, in such a way that they affect other devices less. EMI filters are made up of mostly discrete components; however, the latest trend is to integrate EMI filters into the integrated circuit. For example, Microchip Technology has begun to design amplifiers
operational and other linear devices with input EMI filters. Thus, the MCP642x family has enhanced shielding to reduce any EMI from external sources such as power lines, radio stations, and mobile communications.
coupling mechanisms
The most important EMI classification for electronic and system designers is the coupling mechanism. Inductive couplings occur when an EMI source has the same mass as the EMI victim. Any current produced by the EMI source is fed into the ground connection and generates a stray voltage at the input of the EMI victim. High frequency, high di/dt signals at the output of the EMI source will couple more efficiently than the EMI victim because the ground plane appears as an inductance for these signals. If there is a feedback path between these two circuits, the parasitic signals can cause oscillations. To stop it, the ground connections of both circuits should be separated, thus preventing a common impedance.
rejection factor
The primary response of an op amp to RF EMI is an offset error voltage or offset voltage variation. This error is reflected at the output of the operational amplifier and causes a degradation of system performance. The offset voltage variation is due to a non-linear conversion of AC EMI to a DC signal. The non-linear behavior appears because of the internal pn junctions, which form diodes and rectify EMI signals, usually at the inputs of ESD diodes. The error signal caused by EMI is superimposed on the existing DC offset voltage. The parameter that describes the robustness against EMI of an operational amplifier is the rejection factor of electromagnetic interference (electromagnetic interference rejection ratio, EMIRR). This factor quantitatively describes the effect of an RF interference signal on the performance of an operational amplifier. New devices with internal passive filters have improved EMIRR over older devices without internal filters. This means that with good board layout techniques, EMC will be better.
current sensors
The common-mode input range of the MCP6421/2/4 op amps, up to 0,3V above both power rails, can be used in current sensing applications on the high and low potential (high) side. -side and low-side, respectively). The low quiescent current helps prolong battery autonomy and the output between rails allows low currents to be detected. Figure 1 shows a battery current sensing circuit on the high potential side. The 10 Ω resistor has been sized to reduce power losses. The battery current (IDD) flowing through the 10 Ω resistor causes its upper terminal to be more negative than its lower terminal. This keeps the common mode input voltage of the op amp below VDD, ie within the allowable range. The output of the op amp will also be below VDD, well within its maximum output voltage swing specification. Low consumption current sensing is often used, even for automotive applications. Due to parasitic signals, an op amp that has not been EMI-enhanced may provide an incorrect value of output current. The traditional way to reduce RF stray signals, or prevent them from entering the input stage of the op amp, is to use a low-pass filter close to the input. For the inverting op amp of Figure 2, the filter capacitor C is located between two resistors of equal value. Note that C cannot be connected directly to the inverting input of the op amp because it could cause instability. To reduce signal losses, the filter bandwidth should be at least 20 to 30 times the signal bandwidth. For the noninverting op amp in Figure 3, capacitor C can be connected directly to the input of the op amp, as shown, and an input resistor with a value R provides the cutoff frequency that the op amp will invert. In both cases, chip-type capacitors with a low inductance must be used. The capacitor must be free of resistive losses or voltage coefficient problems. A ferrite core can be used instead of the resistor. However, the impedance of the ferrite core is not well controlled, is non-linear, and generally does not exceed 100 Ω between 10 and 100 MHz. This requires a large value capacitor to attenuate the lower frequencies. Precision instrumentation amplifiers are especially sensitive to DC offset errors due to the presence of common mode EMI and RFI. This is very similar to the problem experienced by op amps and, like op amps, the sensitivity to EMI and RFI is more pronounced with low power input amplifier devices. Amplifier outputs must also be protected against EMI and RFI, especially if there are long cable runs, acting as antennas. RF signals received on an output line are recoupled to the amplifier input, where they are rectified and appear again at the output as an offset variation. The most common op amp response to EMI is a change in the DC offset voltage that appears at the output of the op amp. The conversion of a high frequency EMI signal into DC is the result of non-linear behavior of the internal diodes, formed by the silicon pn junctions inside the device, especially in the ESD diode. This behavior is called rectification because an AC signal is converted to DC. The rectification of the RF signal generates a small DC voltage in the op amp circuitry. When this rectification occurs in the op amp signal path, the effect is amplified and appears as a DC offset at the op amp output.
Tips and tricks
Normal mode EMI is propagated by loop antennas that are accidentally grown inside circuitry. The amount of current, the EMI frequency, and the loop area determine the effectiveness of the antenna. The induced current of EMI is proportional to the area of the loop. Most common mode EMI is generated from capacitively coupled (conducted) normal mode EMI. The higher the frequency of the parasitic signal, the greater the coupling between adjacent conductors on the board. Thus, adjacent conductors can act as antennas. Board traces and wiring that contain loop currents can act as antennas and couple EMI and RFI into or out of circuitry. Balanced lines and balanced on-board signal traces can help prevent common-mode EMI, either conducted or induced, from becoming a differential signal. If the circuit behind the line shows common mode rejection (CMR) at the EMI frequency, the common mode EMI will be suppressed up to the available CMR level. The balanced line is formed by two identical and separate conductors, equidistant from each other, and with dielectric characteristics such that their impedance is identical and the EMI voltage and current is the same for each conductor. In an unbalanced line circuit, each non-identical conductor sees a different electrical environment when exposed to common-mode EMI. The impedance to ground of each conductor is different and the voltage generated between them is also different. When the EMI reaches the next circuit in the line, it appears as a differential voltage. If an active circuit is used and has sufficient bandwidth, it could amplify the EMI and pass it on to the signal path that follows. A capacitance exists between any two conductors separated by a dielectric; air and vacuum, as well as all solid or liquid insulators, are dielectric. If there is a change in voltage in one conductor, there will be a change in charge in the other and a displacement of current that will flow through the dielectric. If the magnetic flux that varies due to the current flowing in one circuit is coupled in another circuit, an electromagnetic field will be generated in the second circuit. This mutual inductance can be a problematic source of coupled noise from circuits with high di/dt values. To eliminate or reduce noise caused by impedance shared conduction path or common impedance noise, you must first decouple the low and high frequency op amp power terminals. Reduce common impedance, eliminate shared paths, use low impedance (low frequency) electrolytics and low inductance (high frequency) shunts, power and ground planes, and optimize system design. In applications where low level signals and high levels of noise of common impedance coincide, interference cannot be avoided and the system architecture may need to be changed. Potential changes include transmitting signals in differential format, amplifying signals to higher levels to improve signal-to-noise ratio, converting signals to streams for transmission, and converting signals directly to digital format. Crosstalk is the second most common form of interference. In the vicinity of a noise source, near-field interference is not transmitted as an electromagnetic wave, and the term crosstalk could be applied to both inductively and capacitively coupled signals. Capacitively coupled noise could be reduced by lowering the coupled capacitance (by increasing the conductor separation) but is more easily achieved by screening. A grounded conductive shield (known as a Faraday shield) between the signal source and the affected node will eliminate this noise by moving current directly to ground. It is always essential that the Faraday shield is connected to ground.
Conclusion
EMI is a real problem today and can affect most devices, including aviation and medical equipment. Among the modern devices are EMI filters to ensure the correct operation of equipment in harsh environments due to EMI. EMI-resistant op amps are more efficient at rejecting high-frequency EMI than standard op amps, but standard op amps can also reject EMI using external filters.
