PCB Tech - Use operational amplifiers to reduce PCB near-field EMI

One of the best ways to reduce EMI for PCB design is to use operational amplifiers flexibly. Unfortunately, in many applications, the role of operational amplifiers for reducing EMI is usually ignored...

One of the best ways to reduce electromagnetic interference (EMI) for PCB design is to use operational amplifiers (OP Amp) flexibly. Unfortunately, in many applications, the role of operational amplifiers for reducing EMI is often overlooked. This may be due to the prejudice that "op amps are susceptible to EMI, and more measures must be taken to enhance the anti-interference ability against noise." Although this is true for many previously produced components, designers may not realize that recent operational amplifiers usually have better anti-interference performance than previous generations. Designers may also not understand or consider the key advantages that operational amplifier circuits can provide for system and PCB design to reduce noise. This article reviews the sources of EMI and discusses the characteristics of operational amplifiers that can help mitigate near-field EMI for sensitive PCB designs.

EMI source, disturbed circuit and coupling mechanism

EMI is interference caused by sources of electrical noise, which are usually unintentional and undesirable. In various situations, the disturbing noise signal is one of voltage, current, and electromagnetic radiation, or the noise source is coupled to the disturbed circuit in some combination of these three forms.

EMI is not limited to radio frequency interference (RFI). In the "lower" frequency range, there are powerful EMI sources in the frequency band below the radio frequency, such as switching regulators, LED circuits, and motor drivers operating in the range of tens to hundreds of KHz. 60Hz line noise is another example. The noise source transmits the noise to the disturbed circuit through one or more of the four coupling mechanisms.

Three of these four methods are considered to be near-field coupling, including conductive coupling, electric field coupling, and magnetic field coupling. The fourth mechanism is far-field radiation coupling, in which electromagnetic energy can be radiated at multiple wavelengths.

Active filtering of differential mode noise

Active operational amplifier filters can significantly reduce EMI and noise on the PCB within the circuit bandwidth, but they are not fully utilized in many designs. The desired differential mode (DM) signal can be limited by the frequency band, and unnecessary DM noise can be filtered out. Figure 1 shows the DM noise coupled to the input signal through the parasitic capacitance (CP). The combined signal and noise are received by a first-order active low-pass filter. The low-pass cutoff frequency of the differential operational amplifier circuit is set to be only higher than the signal bandwidth required by R2 and C1.

Higher frequencies are attenuated by 20dB/decade. If greater attenuation is required, a higher order active filter (such as -40 or -60dB/decade) can be used. It is recommended to use a resistor with a tolerance of <1%. Similarly, capacitors with excellent temperature coefficient (NPO, COG) and 5% (or <5%) tolerance can obtain the best filter performance. CM noise can be described as the noise voltage shared (or the same) at the input of two op amps, and it is not part of the expected DM signal that the op amp is trying to measure or adjust.

An important advantage of an operational amplifier lies in its differential input stage architecture and its ability to suppress CM noise when configured as a differential amplifier. Although the common-mode rejection ratio (CMRR) can be specified for each op amp, the total CMRR of the circuit must also include the effects of input and feedback resistance. Resistance changes strongly affect CMRR. Therefore, matching resistors with tolerances of 0.1%, 0.01% or better can achieve the CMRR required by the application. Although good performance can be achieved by using external resistors, using instruments or differential amplifiers with internal trimming resistors is another option.

As mentioned earlier, active filtering and CMRR can reliably reduce the circuit noise within the frequency band limit of the component, including DM and CM EMI up to the MHz range. However, exposure to RFI noise above the expected operating frequency range may cause non-linear behavior of the component. Operational amplifiers are most susceptible to RFI in their high-impedance differential input stage, because DM and CM RFI noise can be rectified by internal diodes (formed by p-n junctions on silicon). After rectification, a small direct current (DC) voltage or offset is generated, which is amplified and may appear as an erroneous DC offset at the output. Depending on the accuracy and sensitivity of the system, this may result in poor circuit performance or behavior.

Fortunately, using one of two methods can improve the anti-interference ability (or reduce the sensitivity) of the op amp to RFI. The first and best option is to use an EMI-hardened operational amplifier, which includes an internal input filter that can suppress noise in the range of tens of MHz to as high as GHz. TI currently provides more than 80 kinds of EMI hardening components. You can search for EMI hardening through the TI operational amplifier parameter search engine. The second option is to add an external EMI/RFI filter to the input of the op amp. If the design only requires components that do not include internal EMI filters, this may be the only option.

Another important characteristic of an operational amplifier is its extremely low output impedance, which is usually a few ohms (Ω) or less in most configurations. To understand how it is beneficial to reduce EMI, you must first consider how EMI affects low-impedance and high-impedance circuits.

In actual systems, the I2C serial bus clock in the range of 100-400kHz is very common in audio ADCs and circuits. Although the I2C clock is usually driven in a burst (discontinuous) manner, the simulation shows the possible effects when the clock is driven. In high-density audio and infotainment PCB design, clock routing may indeed appear near sensitive audio traces. Only a few pF of parasitic PCB capacitance can occur capacitive coupling and inject clock noise current into the disturbed audio signal. Figure 3 is an example of simulation using only 1pF parasitic capacitance.

How does the audio circuit reduce noise? Facts have proved that reducing the impedance of the disturbed circuit is a way to reduce its sensitivity to coupling noise. For circuits with higher source impedance (> 50Ω), the coupling noise can be reduced by minimizing the source impedance related to the circuit load. In Figure 4, the OPA350 in the in-phase configuration is added to the circuit to buffer the signal and isolate the source impedance from the load. Compared to 600Ω, the output impedance of the operational amplifier is very low, which significantly reduces the clock noise.

Adding a decoupling capacitor to the power supply pin is very useful for filtering high-frequency EMI noise and enhancing the anti-interference of the operational amplifier circuit. All the diagrams in this article show that the decoupling capacitor CD is a part of the circuit. Although the problem of exploring decoupling will soon become very complicated, there are some ideal "rules of thumb" that apply to any design. Especially choose capacitors with the following characteristics:

(a) Excellent temperature coefficient, such as X7R, NPO or COG;

(b) Very low equivalent series inductance (ESL);

(c) The lowest impedance within the required spectrum range;

(d) Capacitance values in the range of 1-100 nF are generally applicable, but the above criteria (b) and (c) are more important than the capacitance value (d).

The layout of the capacitor and the wiring connection are as important as the selected capacitor. Place the capacitor as close as possible to the power supply pin. The connection between the capacitor and the PCB power/ground should be as short as possible, and short traces or via connections can be used.