(Image Source: Analog Devices Inc.)įinally, a footnote in the op-amp’s playbook: The terms CMRR and common-mode rejection (CMR) are used interchangeably, and the amount is expressed in decibels, or dB. Take the example of a thermocouple operating in a noisy environment: The noise from the environment can potentially offset both inputs, leading to the creation of the common-mode voltage signal and inevitably affecting the op-amp’s output voltage.įigure 2: The applications for the LTC 1050 op-amp from Linear Technology include a thermocouple and battery-operated temperature monitor.
An op-amp should be able to suppress the CMI signal at input terminals, thus canceling out any harmonics to reduce distortion. The CMRR becomes even more crucial when differential input is comparable to the CMI signal in an op-amp. That shows why a high CMRR is critical in empowering an op-amp to attenuate any CMI elements. A differential mode of operation at the input side enables the op-amp to reject various frequency components constituting common-mode input (CMI) and, thus, suppress unwanted noise and electromagnetic interference (EMI). However, in practice, CMRR can’t be infinite, and op-amps should be designed to have CMRR as high as possible. That’s because when common-mode elements in input signals appear to be simultaneous and in-phase at op-amp inputs, it negates the op-amp’s basic premise of producing a clean and precise output signal. Here, before we delve into greater details about CMRR’s importance, it’s worth noting that an op-amp should have the infinite CMRR in an ideal scenario. Why? Because it indicates the presence of common-mode signals at the op-amp inputs, which eventually determines the op-amp’s ability to minimize the noise in audio, video and communication designs. The common-mode rejection ratio, or CMRR, is one of the most important specifications in an op-amp offering. It will also present design recommendations on what engineers need to carefully review when selecting op-amps for their projects. This article will chronicle how CMRR and PSRR metrics are intrinsically tied to an op-amp’s performance and efficiency and how they impact developers’ quests to build better consumer and industrial designs. Instead, they are determined by external factors such as frequency spikes and power-supply voltage changes. The significance of parameters like CMRR and PSRR is also tied to the fact that op-amp features like gain and bandwidth have little dependence on temperature coefficients or manufacturing variations. The role of the humble op-amp, for instance, is becoming crucial in linking the sensors to the analog-to-digital converter (ADC) input of a microcontroller as it performs signal conditioning in power-constrained embedded applications, especially when sensors produce a very small voltage and the signal needs to be boosted before being digitized. That, in turn, makes issues like noise and power consumption vital in an op-amp’s performance.įigure 1: The simulated performance of a two-stage op-amp device showing the CMRR and PSRR values at the bottom. Their role is especially critical because analog designs are moving toward higher bandwidths. The anatomy of op-amps brings us to the two most important parameters in their design: CMRR and PSRR. It’s a differential amplifier in which two input terminals, a positive terminal and a negative terminal, are applied at the same point to create a single output. The op-amp is an important building block of analog designs for its greater precision, higher thermal drift and incredible design versatility.
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