What is common mode rejection ratio
Common Mode Rejection Ratio (CMRR) is a measure of the ability of a differential amplifier or operational amplifier (op-amp) to reject common-mode signals. Common-mode signals are those that appear identically on both input terminals of the amplifier. CMRR quantifies how well the amplifier responds to differential signals (which appear as a voltage difference between the input terminals) compared to common-mode signals (which appear as a voltage common to both input terminals).
Formula:
CMRR is defined as the ratio of the differential voltage gain (Ad) to the common-mode voltage gain (Acm):
CMRR=?AdAcm?\text{CMRR} = \left| \frac{A_d}{A_{cm}} \right|CMRR=?Acm?Ad???
Where:
- AdA_dAd? is the differential voltage gain, which is the ratio of the output voltage to the differential input voltage (the difference between the voltages applied to the two input terminals).
- AcmA_{cm}Acm? is the common-mode voltage gain, which is the ratio of the output voltage to the common-mode input voltage (the voltage that is applied equally to both input terminals).
Interpretation:
- A high CMRR indicates that the amplifier is effective in rejecting common-mode signals. For example, a CMRR of 80 dB means that the differential gain is 100,000 times higher than the common-mode gain.
- CMRR is typically expressed in decibels (dB) for convenience, where 20 dB corresponds to a ratio of 10:1 and 40 dB corresponds to a ratio of 100:1.
Importance:
- In many applications, especially in instrumentation and communication systems, it is crucial to accurately amplify differential signals while rejecting noise and interference that appear as common-mode signals.
- A high CMRR is desirable to ensure accurate and reliable operation of differential amplifiers, reducing the impact of common-mode noise and improving the signal-to-noise ratio (SNR).
Factors Affecting CMRR:
- Component Matching: The matching of components (such as resistors and transistors) in the differential amplifier stage affects CMRR.
- Temperature Stability: Variations in temperature can affect the differential and common-mode gains differently, impacting CMRR.
- Power Supply Variations: Changes in power supply voltages can affect the CMRR of the amplifier.
- Layout and Grounding: Proper circuit layout and grounding techniques can influence CMRR by minimizing parasitic effects and noise pickup.
Conclusion:
CMRR is a critical parameter in differential amplifiers and op-amps, reflecting their ability to accurately amplify differential signals while rejecting common-mode signals. Designers aim to maximize CMRR through careful circuit design, component selection, and layout techniques to achieve optimal performance in applications requiring high fidelity and low noise operation.
