By Todd Toporski, Texas Instruments
Analog Applications Journal
Introduction
Automotive, industrial, medical, and many other applications use sensitive analog circuits that must perform their function while remaining immune to noise disturbances in their local environment. Many of these disturbances occur on nearby “noisy” circuits located on the same printed circuit board (PCB), while other interference can be picked up by cable interfaces that couple noise onto the PCB and its circuits.
One of the best ways to reduce electromagnetic interference (EMI) on PCB designs is through intelligent use of operational amplifiers (op amps). Unfortunately, op amps are often overlooked as a tool for reducing EMI in many applications. This may be due to the perception that op amps are susceptible to EMI and that extra steps must be taken to enhance their immunity to noise. While this is true of many older devices, designers may not be aware that newer op amps often have superior immunity performance over previous generations. Designers also may not understand or consider the key benefits that an op amp circuit can provide for reducing noise in their system and PCB designs. This article reviews sources of EMI and discusses op amp characteristics that aid in mitigating near-field EMI on sensitive PCB designs.
EMI sources, victim circuits, and coupling mechanisms
EMI is a disturbance caused by a source of electrical noise that impacts a second electrical circuit in an unintentional and often undesirable manner. In all cases, an interfering noise signal is either a voltage, a current, electromagnetic radiation, or some combination of these three coupled from a noise source to a victim circuit.
EMI is not limited to radio frequency interference (RFI). Strong sources of EMI exist below radio bands in “lower” frequency ranges, sources such as switching regulators, LED circuits, and motor drivers operating in the tens to hundreds of kilohertz range. A 60-Hz line noise is another example. Sources transfer noise to victim circuits through one or more of four possible coupling mechanisms. Three of the four are considered near-field coupling, including conducted coupling, electric-field coupling, and magnetic-field coupling. The fourth mechanism is far-field radiated coupling, in which electromagnetic energy is radiated over multiple wavelengths.
Active filtering of differential-mode noise
Active op amp filters can significantly reduce EMI and noise on a PCB within the bandwidth of the circuit, but they are underutilized in many designs. The desired differential-mode (DM) signal can be band-limited while unwanted DM noise is filtered out. Figure 1 demonstrates DM noise coupled into an input signal through parasitic capacitance (CP). The combined signal and noise is received by a first-order active low-pass filter. The differential op amp circuit has its low-pass cutoff frequency set just above the desired signal bandwidth by R2 and C1. Higher frequencies are attenuated by 20 dB per decade. Higher-order active filters (for example, –40 or –60 dB/ decade) can be implemented if more attenuation is needed. Resistor tolerances of one percent or lower are recommended. Likewise, capacitors having very good temperature coefficient (NPO, COG) and tolerances of 5% or lower are preferred for best filter performance.
Reducing input common-mode noise
In Figure 1, common-mode (CM) noise sources also present noise at the circuit’s input. CM noise can be described as a noise voltage that is common (or the same) at both op amp inputs, and is not part of the intended differential mode signal that the op amp is trying to measure or condition. CM noise can occur in a number of ways. One example is a system where the ground reference of one circuit is at a different voltage potential than a second circuit to which it is interfacing. The difference in “ground” voltages may be in millivolts or many volts, and can also occur at many different frequencies. These differences in voltages cause unintended voltage drops and flow of currents that can interfere with the connected circuitry. Cars, aircraft, and large buildings with many circuits are often susceptible to this type of interference.
A key advantage of op amps is their differential input stage architecture, and their ability to reject CM noise when configured as a differential amplifier. Common-mode rejection ratio (CMRR) is specified for every op amp, but total CMRR of the circuit must also include the effects of input and feedback resistors. Resistor variation strongly impacts CMRR. Therefore, matched resistors with tolerances 0.1%, 0.01% or better, are needed to achieve a desired CMRR for the application. While good performance is achievable using external resistors, use of instrumentation or differential amplifiers with internally-trimmed resistors is another option. For example, the INA188 is an instrumentation amp with internally trimmed resistors and high CMRR of 104 dB.
In Figure 1 the CM noise (VCM_NOISE = VCM1 = VCM2) can be rejected by CMRR of the op amp circuit if the noise is within the active bandwidth of the circuit. The level of rejection depends on accurately-matched resistors to be chosen for R2/R1. Equation 1 can be used to determine CMRRTOTAL, which includes the effects of resistor tolerance (RTOL) and op amp CMRR as specified in the data sheet. For example, if the op amp data sheet specifies its CMRR (dB) = 90 dB, then (1/CMRRAMP) = 0.00003. In many circuits, resistor tolerance will be the main limiting factor to achieve a target CMRRTOTAL.
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| Figure 1. | DM and CM input noise applied to active op amp filter. | |
Equation 1 is derived from an equation in Reference 1 for CMRR of an ideal op amp, in which the CMRRAMP term is assumed to be very large (infinity). For an ideal op amp, the (1/CMRRAMP) term is zero and CMRRTOTAL is based only on resistors and AV. CMRRTOTAL can be converted to dB using Equation 2.
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(1) |
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(2) |
where AV = closed-loop gain of the op amp, RTOL = % tolerance of R1 and R2 (for example, 0.1%, 0.01%, 0.001%), and CMRRAMP = data-sheet specification for CMRR in decimal form (not dB).
