What is the concept of CMRR (Common-Mode Rejection Ratio) in op-amp circuits?
1 Answer
CMRR, which stands for Common-Mode Rejection Ratio, is a critical parameter in operational amplifier (op-amp) circuits. It is a measure of how effectively an op-amp rejects unwanted common-mode signals while amplifying the desired differential-mode signals.
In an ideal scenario, an op-amp amplifies the difference between its two input terminals (differential mode) while completely ignoring any voltage that appears at both inputs simultaneously (common mode). However, in real-world applications, there are always some common-mode signals present due to various sources like noise, interference, or coupling between different parts of the circuit.
The CMRR is defined as the ratio of differential-mode gain to common-mode gain and is expressed in decibels (dB). The formula for CMRR is:
CMRR = 20 log (|Ad| / |Ac|)
Where:
Ad is the differential-mode gain (the gain for the desired signal).
Ac is the common-mode gain (the gain for the unwanted common-mode signal).
A higher CMRR value indicates better rejection of common-mode signals, which is desirable in many applications, especially in instrumentation and communication systems. It ensures that the amplifier focuses on amplifying the useful signal while attenuating any common noise present at its inputs.
For instance, if an op-amp has a CMRR of 80 dB, it means the differential-mode gain is 100,000 times higher than the common-mode gain, assuming the gain values are measured in linear terms and not in dB. This level of rejection implies that even if there is a relatively strong common-mode noise, it will be substantially reduced at the output.
In practice, real-world op-amps have finite CMRR values, and they may vary with frequency and temperature. It is essential to choose an op-amp with an adequate CMRR for the specific application to minimize the impact of common-mode interference on the output signal.
In an ideal scenario, an op-amp amplifies the difference between its two input terminals (differential mode) while completely ignoring any voltage that appears at both inputs simultaneously (common mode). However, in real-world applications, there are always some common-mode signals present due to various sources like noise, interference, or coupling between different parts of the circuit.
The CMRR is defined as the ratio of differential-mode gain to common-mode gain and is expressed in decibels (dB). The formula for CMRR is:
CMRR = 20 log (|Ad| / |Ac|)
Where:
Ad is the differential-mode gain (the gain for the desired signal).
Ac is the common-mode gain (the gain for the unwanted common-mode signal).
A higher CMRR value indicates better rejection of common-mode signals, which is desirable in many applications, especially in instrumentation and communication systems. It ensures that the amplifier focuses on amplifying the useful signal while attenuating any common noise present at its inputs.
For instance, if an op-amp has a CMRR of 80 dB, it means the differential-mode gain is 100,000 times higher than the common-mode gain, assuming the gain values are measured in linear terms and not in dB. This level of rejection implies that even if there is a relatively strong common-mode noise, it will be substantially reduced at the output.
In practice, real-world op-amps have finite CMRR values, and they may vary with frequency and temperature. It is essential to choose an op-amp with an adequate CMRR for the specific application to minimize the impact of common-mode interference on the output signal.