What is the concept of CMRR (Common-Mode Rejection Ratio) in instrumentation amplifiers?
1 Answer
CMRR, which stands for Common-Mode Rejection Ratio, is an essential parameter in the design and analysis of instrumentation amplifiers (in-amps). Instrumentation amplifiers are specialized amplifiers used to amplify small differential signals while rejecting common-mode noise.
In electronic systems, signals can be classified into two types:
Differential mode signal: This is the desired signal that appears across the two input terminals of the amplifier.
Common-mode signal: This is any signal that appears with the same amplitude and phase on both input terminals of the amplifier. Common-mode signals are typically noise or interference picked up by the signal cables or transmission lines.
The CMRR is a measure of the amplifier's ability to reject common-mode signals and respond only to the differential mode signals. It indicates how effectively the in-amp can suppress unwanted noise and interference that appears equally on both inputs.
Mathematically, the Common-Mode Rejection Ratio (CMRR) is defined as the ratio of the differential-mode gain (A_d) to the common-mode gain (A_c) of the instrumentation amplifier:
CMRR = |A_d| / |A_c|
Where:
|A_d| is the magnitude of the differential-mode gain (output voltage in response to differential input).
|A_c| is the magnitude of the common-mode gain (output voltage in response to common-mode input).
A high CMRR value is desirable because it indicates that the amplifier is effective at amplifying the differential signal while attenuating the common-mode noise. The CMRR is typically expressed in decibels (dB).
For example, if an instrumentation amplifier has a CMRR of 80 dB, it means that the differential-mode signal will be amplified 80 dB more than the common-mode signal.
It's important to note that perfect common-mode rejection is challenging to achieve in practice, and various factors can affect the CMRR, such as mismatches in the amplifier's components, tolerance variations, and temperature effects. Designers aim to select components and use layout techniques that minimize these effects to achieve the best possible CMRR for their application.
In electronic systems, signals can be classified into two types:
Differential mode signal: This is the desired signal that appears across the two input terminals of the amplifier.
Common-mode signal: This is any signal that appears with the same amplitude and phase on both input terminals of the amplifier. Common-mode signals are typically noise or interference picked up by the signal cables or transmission lines.
The CMRR is a measure of the amplifier's ability to reject common-mode signals and respond only to the differential mode signals. It indicates how effectively the in-amp can suppress unwanted noise and interference that appears equally on both inputs.
Mathematically, the Common-Mode Rejection Ratio (CMRR) is defined as the ratio of the differential-mode gain (A_d) to the common-mode gain (A_c) of the instrumentation amplifier:
CMRR = |A_d| / |A_c|
Where:
|A_d| is the magnitude of the differential-mode gain (output voltage in response to differential input).
|A_c| is the magnitude of the common-mode gain (output voltage in response to common-mode input).
A high CMRR value is desirable because it indicates that the amplifier is effective at amplifying the differential signal while attenuating the common-mode noise. The CMRR is typically expressed in decibels (dB).
For example, if an instrumentation amplifier has a CMRR of 80 dB, it means that the differential-mode signal will be amplified 80 dB more than the common-mode signal.
It's important to note that perfect common-mode rejection is challenging to achieve in practice, and various factors can affect the CMRR, such as mismatches in the amplifier's components, tolerance variations, and temperature effects. Designers aim to select components and use layout techniques that minimize these effects to achieve the best possible CMRR for their application.