3.2.3 - Common-Mode Rejection Ratio (CMRR)
Enroll to start learning
You’ve not yet enrolled in this course. Please enroll for free to listen to audio lessons, classroom podcasts and take practice test.
Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Introduction to CMRR
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Today, we will explore the Common-Mode Rejection Ratio, or CMRR. Can anyone tell me what they think CMRR stands for?
Is it the ratio of the common mode signals rejected versus accepted?
That's partially correct! CMRR specifically measures how well an op-amp rejects signals common to both inputs. It's crucial for distinguishing genuine signals from noise.
Why is this important in amplifiers?
Great question! A high CMRR ensures that the op-amp primarily amplifies the difference between its inputs, enhancing signal integrity. Let's remember this with the acronym 'CLEAN': CMRR Leaves Error Amplified Negligigible.
Understanding CMRR Calculation
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
CMRR is mathematically defined as the ratio of open-loop differential gain to open-loop common-mode gain. Can anyone express this in their own words?
So, it’s like saying how much an op-amp can amplify the difference versus the noise it receives?
Exactly! If $A_{OL}$ is much larger than $A_{CM}$, then the CMRR value will be high, indicating effective noise cancellation. Does anyone want to try calculating an example with hypothetical values?
I can! If $A_{OL} = 200$ and $A_{CM} = 2$, then CMRR = 200/2 = 100!
Exactly! And we can express that in decibels using the formula 20 log(CMRR). What would that be?
Oh, that's 20 log(100), which equals 40 dB!
Importance of High CMRR
🔒 Unlock Audio Lesson
Sign up and enroll to listen to this audio lesson
Let’s delve into why high CMRR is vital for operational amplifiers. Can anyone provide an example of where this would be particularly significant?
In audio equipment where clarity is crucial?
Exactly! In audio applications, background noise versus the actual audio can create problems. A high CMRR helps keep the output signal clear and clean. Remember our mnemonic 'Signal Stays Solid' for this concept!
What would happen if CMRR is low?
A low CMRR means more common-mode signals leak through, leading to distorted outputs. Imagine trying to hear someone talk in a crowded room; you wouldn’t want background chatter to interfere with clear dialogue.
Introduction & Overview
Read summaries of the section's main ideas at different levels of detail.
Quick Overview
Standard
CMRR is a critical performance metric for operational amplifiers, indicating how well the op-amp can distinguish between common-mode signals and true differential inputs. A high CMRR is desired for minimizing noise and improving signal integrity.
Detailed
Common-Mode Rejection Ratio (CMRR)
The Common-Mode Rejection Ratio (CMRR) is a pivotal metric in assessing an operational amplifier's performance. It quantifies an op-amp's capacity to filter out common-mode signals—those signals present on both inputs simultaneously—allowing only the differential component to be amplified. Mathematically, CMRR can be expressed as:
$$\text{CMRR} = \frac{A_{OL}}{A_{CM}}$$
where:
- $A_{OL}$ is the open-loop differential gain (the gain when input signal is the difference between inputs), and
- $A_{CM}$ is the open-loop common-mode gain (the gain when input signal is the same for both inputs).
A high CMRR is indicative of a robust op-amp that effectively minimizes the amplification of noise and interference, which is particularly crucial in precision applications. High CMRR ensures that only relevant signals are processed, which leads to improved accuracy in analog computations.
Youtube Videos
Audio Book
Dive deep into the subject with an immersive audiobook experience.
Definition of CMRR
Chapter 1 of 2
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
The CMRR is the ability of an op-amp to reject common-mode signals, i.e., signals that are present on both inputs simultaneously. A high CMRR ensures that the op-amp amplifies only the difference between the inputs while rejecting noise or interference that is common to both inputs.
Detailed Explanation
CMRR, or Common-Mode Rejection Ratio, is a crucial parameter for operational amplifiers. It measures how well the op-amp can ignore signals that are applied equally to both inputs, known as common-mode signals. For example, if both the non-inverting and inverting inputs receive the same noise, a high CMRR means that the op-amp will minimize the effect of this noise in its output, allowing it to amplify only the actual difference in the signal inputs. This characteristic is vital for ensuring signal integrity in noisy environments.
Examples & Analogies
Imagine a teacher trying to listen to a student in a noisy classroom. If the student speaks while everyone else is talking at the same volume, the teacher may struggle to understand the student’s actual message. A teacher with strong focus (high CMRR) would be able to filter out the background voices and concentrate solely on what the student is saying, thereby amplifying the intended message.
CMRR Calculation
Chapter 2 of 2
🔒 Unlock Audio Chapter
Sign up and enroll to access the full audio experience
Chapter Content
The CMRR is given by:
CMRR=AOLACM
Where:
● AOLA_{OL} is the open-loop differential gain,
● ACMA_{CM} is the open-loop common-mode gain.
Detailed Explanation
The formula for calculating the CMRR expresses the relationship between the op-amp’s differential gain and its common-mode gain. The open-loop differential gain (AOL) is how much the op-amp amplifies the difference between the two input signals without any feedback applied. In contrast, the open-loop common-mode gain (ACM) measures how much the op-amp amplifies signals that are common to both inputs, which ideally should be very low. The CMRR, therefore, tells us how effectively the op-amp can distinguish between signals of interest and unwanted common-mode signals. It’s usually expressed in decibels (dB) using the following conversion: CMRR(dB) = 20 log10(CMRR)
Examples & Analogies
Think of a radio receiver in a city with various radio signals. The ideal radio picks up only the station signal (differential gain) while ignoring the noise from other stations (common-mode gain). The CMRR can be thought of as a measure of the radio's ability to tune into only one signal clearly while minimizing the interference from other channels, effectively giving a clearer output of the desired broadcast.
Key Concepts
-
CMRR: The ratio that describes how well an op-amp can reject common-mode signals compared to differential signals.
-
High CMRR: Indicates better performance in amplifying desired signals while suppressing noise.
Examples & Applications
In audio amplifiers, a high CMRR ensures that background noise is effectively minimized, allowing clear sound output.
In instrumentation, a high CMRR helps accurately capture small signal variations in the presence of noise.
Memory Aids
Interactive tools to help you remember key concepts
Rhymes
CMRR, oh how bright! It filters noise and brings signals to light.
Stories
Imagine a concert where each musician plays their unique tune, while the audience keeps on talking. A clever conductor (the op-amp) ensures the music is heard clearly and not drowned by chatter. This is like a high CMRR in action.
Memory Tools
Use 'NOISE' - Needs Op-Amp to Inhibit Signals Everywhere.
Acronyms
CMRR can stand for 'Clear Measurement, Rejected Residue.'
Flash Cards
Glossary
- CommonMode Rejection Ratio (CMRR)
A performance metric indicating the ability of an op-amp to reject common-mode signals and amplify only the differential input.
- OpenLoop Gain
The gain of an operational amplifier when no feedback is applied.
- Differential Gain
The gain of an operational amplifier in response to the difference between its two input signals.
- CommonMode Gain
The gain of an operational amplifier in response to signals that are common to both its inputs.
Reference links
Supplementary resources to enhance your learning experience.