As technology advances, EMI poses an increasing threat to the normal operation of circuits. This is because electronic applications are shifting to various wireless communications or portable platforms. Therefore, most interfering EMI signals eventually enter the PCB traces in the form of conducted EMI.
 

When you try to design an EMI-resistant circuit, you will find that analog sensor circuits tend to be huge EMI absorbers. This is because sensor circuits often produce low-level signals and have many high-impedance analog ports. In addition, these circuits use more compact component spacing, which makes it easier for the system to intercept and conduct noise interference into the traces.
 

In this EMI situation, the operational amplifier (op amp) becomes a prime target. We saw this effect in Part 1 of this series, “How EMI Can Disturb Circuits Through Dielectrics,” where Figure 1 showed an EMI signal causing an offset voltage error of 1.5 volts!
 

A standard op amp has three low-impedance pins (positive power, negative power, and output) and two high-impedance input pins (see Figure 1a). Although these pins are immune to EMI, the input pins are the most vulnerable.

 

Figure 1 Comparison of EMIRR and EMIRR IN+ measurement methods
 

EMIRR Electromagnetic Interference Rejection Ratio
 

The characteristics of the inverting and noninverting pins of a voltage feedback amplifier are essentially the same. However, the amplifier is easiest to test for EMI immunity at the noninverting input (see Figure 1b).

Equation 1

 

In Equation 1, VRF_PEAK is the peak value of the applied RF voltage, VOS is the DC offset voltage of the amplifier, and 100 mVP is the 100 mVP input signal EMIRR IN+ reference.
 

You can compare amplifier EMI rejection performance using the EMIRR metric. Figure 2 shows the EMIRR IN+ response of the TI OPA333 CMOS op amp. This graph shows that this device does a good job rejecting signals with frequencies above the 300 kHz bandwidth of the device.

 

Figure 2 OPA333, EMRR IN+ vs. frequency
 

There are three benefits to using an EMI filter inside an IC compared to an external RC filter. Potential users can test the performance of an amplifier that includes an integrated filter to ensure EMI suppression over a wide frequency range (2). Passive filter components are not ideal in terms of parasitic capacitance and inductance, which limits the filter's ability to suppress very high frequency noise. In contrast, the electrical characteristics of the IC and on-chip passive components are closely matched. Finally, using an IC with an internal filter can provide other customer benefits such as fewer components, lower cost, and reduced board area.
 

To reduce the EMI sensitivity of a circuit, board designers should always pay attention to using good layout practices. This can be achieved by keeping trace lengths as short as possible, using surface mount components, and using a printed circuit board (PCB) with a dedicated signal return ground plane. Keep the ground plane as intact as possible and keep digital signals away from analog signal paths. In addition, place RF bypass capacitors on all integrated circuit power pins. Keep these capacitors close to the device pins and ensure that their impedance is as close to 0 ohms as possible at the potential EMI frequency.