1. One of the most effective methods to minimize EMI in PCB design is through the flexible use of operational amplifiers. Unfortunately, in many applications, the potential of operational amplifiers to mitigate EMI is frequently overlooked.

2. This oversight may stem from the belief that “op amps are vulnerable to EMI, necessitating additional measures to bolster their noise immunity.” While this may have been true for many older components, designers might not be aware that modern operational amplifiers typically exhibit superior noise resistance compared to previous models.

3. Additionally, designers may fail to recognize or appreciate the significant advantages that operational amplifier circuits can offer in enhancing system and PCB designs to reduce noise. This article examines the origins of EMI and explores the characteristics of operational amplifiers that can effectively address near-field EMI in sensitive PCB designs.

4. EMI sources, disrupted circuits, and coupling mechanisms

5. EMI refers to interference caused by sources of electrical noise, which are generally unintended and unwelcome. In various scenarios, the disturbing noise can manifest as voltage, current, and electromagnetic radiation, with the noise source coupling to the disrupted circuit through a combination of these three forms.

6. EMI is not confined to radio frequency interference (RFI). In the “lower” frequency range, significant EMI sources exist below radio frequencies, including switching regulators, LED circuits, and motor drivers that operate in the tens to hundreds of kHz range. A prime example is the 60Hz line noise. The noise source transmits disturbances to the affected circuit through one or more of four coupling mechanisms.

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1. Three of these four methods are categorized as near-field coupling: conductive coupling, electric field coupling, and magnetic field coupling. The fourth mechanism, far-field radiation coupling, involves the emission of electromagnetic energy at various wavelengths.

2. Active operational amplifier filters can significantly diminish EMI and noise on the PCB within the circuit’s bandwidth, yet they remain underutilized in many designs. The desired differential mode (DM) signal can be confined to a specific frequency band, allowing unwanted DM noise to be filtered out. Figure 1 illustrates the DM noise coupled to the input signal through parasitic capacitance (CP). The combined signal and noise are processed by a first-order active low-pass filter. The cutoff frequency of the differential operational amplifier circuit is set slightly above the signal bandwidth determined by R2 and C1.

3. Higher frequencies experience attenuation of 20 dB/decade. For greater attenuation, a higher-order active filter (such as -40 or -60 dB/decade) can be implemented. It’s advisable to use resistors with tolerances of less than 1%. Similarly, capacitors with excellent temperature coefficients (NPO, COG) and tolerances of 5% or less can deliver optimal filter performance. Common-mode (CM) noise is characterized by the noise voltage shared at the inputs of two op amps, which is not part of the expected DM signal the op amp aims to measure or adjust.

4. A significant advantage of an operational amplifier is its differential input stage architecture, which effectively suppresses CM noise when configured as a differential amplifier. While the common-mode rejection ratio (CMRR) can be specified for each op amp, the total CMRR of the circuit must also factor in the effects of input and feedback resistance. Changes in resistance strongly influence CMRR. Thus, matching resistors with tolerances of 0.1%, 0.01%, or better can fulfill the application’s CMRR requirements. Although good performance can be achieved with external resistors, using instruments or differential amplifiers equipped with internal trimming resistors is another viable option.

5. As noted earlier, active filtering and CMRR can reliably minimize circuit noise within the component’s frequency band limits, addressing both DM and CM EMI up to the MHz range. However, exposure to RFI noise exceeding the anticipated operating frequency range may induce non-linear behavior in the component. Operational amplifiers are particularly vulnerable to RFI in their high-impedance differential input stage, as both DM and CM RFI noise can be rectified by internal diodes formed by p-n junctions on silicon. This rectification leads to a small DC voltage or offset that is amplified, potentially manifesting as an erroneous DC offset at the output. Depending on the system’s accuracy and sensitivity, this may adversely affect circuit performance.

6. Fortunately, employing one of two methods can enhance the op amp’s resilience to RFI. The first and preferred option is to utilize an EMI-hardened operational amplifier, which features an internal input filter that mitigates noise across a range from tens of MHz to several GHz. TI currently offers over 80 types of EMI-hardened components, accessible through their operational amplifier parameter search engine. The second option involves adding an external EMI/RFI filter to the op amp’s input. If the design requires components without internal EMI filters, this may be the sole option.

7. Another crucial characteristic of an operational amplifier is its exceptionally low output impedance, typically just a few ohms or less in most configurations. To appreciate how this benefits EMI reduction, it is essential to consider how EMI influences both low-impedance and high-impedance circuits.

8. In practical systems, the I2C serial bus clock operates commonly in the 100-400 kHz range for audio ADCs and circuits. Although the I2C clock is typically driven in bursts, simulations illustrate the potential impacts when the clock is active. In high-density audio and infotainment PCB designs, clock routing may inadvertently intersect sensitive audio traces. Just a few pF of parasitic PCB capacitance can create capacitive coupling, injecting clock noise into the compromised audio signal. Figure 3 exemplifies simulation results using merely 1 pF of parasitic capacitance.

9. How can the audio circuit mitigate noise? Evidence shows that reducing the impedance of the affected circuit is an effective way to lessen its sensitivity to coupling noise. For circuits with higher source impedance (greater than 50 Ω), coupling noise can be minimized by decreasing the source impedance related to the circuit load. In Figure 4, the OPA350 configured in phase is integrated into the circuit to buffer the signal and isolate the source impedance from the load. The operational amplifier’s output impedance is significantly lower compared to 600 Ω, greatly diminishing clock noise.

10. Incorporating a decoupling capacitor at the power supply pin is highly beneficial for filtering high-frequency EMI noise and enhancing the operational amplifier circuit’s interference immunity. All diagrams in this article illustrate the decoupling capacitor CD as part of the circuit. Although exploring decoupling can soon become complex, several ideal “rules of thumb” apply to any design. It is particularly important to choose capacitors with the following characteristics:

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

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

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

(d) Capacitance values typically between 1-100 nF, but criteria (b) and (c) take precedence over (d).

11. The layout of the capacitor and the connections are just as crucial as the capacitor selection. Position 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 feasible, utilizing short traces or via connections.