The perception about the operational amplifier, commonly known as the op-amp, as an old dog has been constantly challenged amid the changing analog design landscape, and the increasing importance of common-mode rejection ratio (CMRR) and power supply rejection ratio (PSRR) in op-amps is a testament of this remarkable technology evolution.
The op-amp is an important building block of analog designs for its greater precision, higher thermal drift and incredible design versatility. 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 anatomy of op-amps brings us to the two most important parameters in their design: CMRR and PSRR. Their role is especially critical because analog designs are moving toward higher bandwidths. That, in turn, makes issues like noise and power consumption vital in an op-amp’s performance.
Figure 1: The simulated performance of a two-stage op-amp device showing the CMRR and PSRR values at the bottom. (Image Source: Research Gate)
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.
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. Instead, they are determined by external factors such as frequency spikes and power-supply voltage changes.
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. It will also present design recommendations on what engineers need to carefully review when selecting op-amps for their projects.
CMRR to Counter Noise and EMI
The common-mode rejection ratio, or CMRR, is one of the most important specifications in an op-amp offering. 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.
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. 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.
However, in practice, CMRR can’t be infinite, and op-amps should be designed to have CMRR as high as possible. 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). That shows why a high CMRR is critical in empowering an op-amp to attenuate any CMI elements.
The CMRR becomes even more crucial when differential input is comparable to the CMI signal in an op-amp. An op-amp should be able to suppress the CMI signal at input terminals, thus canceling out any harmonics to reduce distortion.
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.
Figure 2: The applications for the LTC 1050 op-amp from Linear Technology include a thermocouple and battery-operated temperature monitor. (Image Source: Analog Devices Inc.)
Finally, 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.
CMRR-Related Design Considerations
A precision op-amp mandates a high-accuracy, wide operating voltage range and robust operation. Here, with these op-amp features, a high CMRR eliminates the need for trimming or calibration while ensuring a consistent performance across a wide temperature range.
The creation of CMI is mainly due to the imbalance of resistor values, and one way to improve the CMRR is by balancing the resistor values at input terminals. Furthermore, power supplies and common-mode ranges can be accommodated by changing the input voltages in some design conditions.
Also, it’s best to use current sources or sinks of higher impedance to ensure the differential inputs and not use resistors to bias a differential pair. That keeps the gain constant and leads to higher CMRR values. Many op-amps claim to have the CMRR value as high as 90 dB.
PSRR to measure power supply sensitivity
The power supply rejection ratio, or PSRR, has been steadily becoming more important due to the rising demand for power efficiency in electronic designs large and small. It’s a measure of how much disturbance signals have been injected at the input levels, which, in turn, can impact the regulated output. And it shows how well an op-amp carries out the power delivery while rejecting various frequency elements at the input level.
An op-amp’s PSRR is the ability to reject noise or ripple occurring at the input side due to the change in input offset voltage with respect to the change in power supply voltage. That’s why it plays a vital role in ensuring stability in challenging operating conditions in automotive, industrial and medical designs, especially for op-amps going into battery-powered devices, where designers want to burn minimum power.
Figure 3: A view of the PSRR plots for Linear Technology’s RH108A op-amp. (Image Source: Analog Devices Inc.)
Again, like the case of CMRR and CMR, the terms PSRR, which is dimensionless, and power supply rejection (PSR), expressed in decibels, are used interchangeably in the semiconductor industry literature.
There is another similarity with its CMRR counterpart. In an ideal scenario, an op-amp’s output is equivalent to DC frequency, but some spikes occur at higher frequencies. In other words, PSRR, a measure of frequency response measurement, is a critical requirement in the discrimination of noise coupled with power supply lines.
PSRR-Related Design Considerations
For a start, use a low-voltage current source to lower parasitic gain from the power supply so that there is no detrimental effect on the output swing. Next, use an op-amp that works across a wide range of power supplies. That allows designers to measure the PSRR by changing the supply voltages and see how the voltage offset varies.
Moreover, op-amp datasheets provide details about current flowing into the supply pins and flowing out to the output terminal. That allows designers to calculate the total power dissipation for a given condition. The op-amp datasheets also provide details about thermal resistance and maximum junction temperature ratings, which allow developers to calculate the dissipation limits of an op-amp device.
Why CMRR and PSRR are critical
The op-amp is a widely used chip in a broad array of designs ranging from the interface with systems processing analog signals to voltage-amplifying devices. So it’s imperative that design engineers fully understand its key characteristics, including the CMRR and PSRR values.
A clear understanding of CMRR and PSRR is especially important because electronic designs are moving toward higher bandwidths at which design issues related to noise and EMI become far more critical. Likewise, the drive for more efficient power systems demands smarter power supply designs.
Additionally, when it comes to op-amp parameters like CMRR and PSRR, there is a lot of ambiguity in the trade media content due to redundant terms used to explain these concepts. This article clears the air regarding the plethora of industry acronyms.