Common-mode rejection ratio (CMRR)

Definition

Common-mode rejection ratio (CMRR) is a measure of an amplifier's ability to reject common-mode signals, which are the signals that appear simultaneously and in-phase on both the positive and negative inputs. A high CMRR indicates that the amplifier can effectively eliminate unwanted noise and interference that affects both inputs equally, ensuring that only the differential signal, which is the difference between the two inputs, is amplified. This is particularly important in applications where precise signal processing is required.

5 Must Know Facts For Your Next Test

1. CMRR is typically expressed in decibels (dB), calculated as 20 times the logarithm of the ratio of differential gain to common-mode gain.
2. A high CMRR value is crucial for applications like instrumentation amplifiers, where precision is key and any common noise can distort the output.
3. An ideal CMRR would be infinite, meaning perfect rejection of common-mode signals, but practical amplifiers usually have finite values ranging from 60 dB to 120 dB.
4. CMRR can be affected by mismatches in input impedance and gain in the amplifier's circuitry, emphasizing the need for careful design.
5. In real-world applications, variations in temperature and component tolerances can impact CMRR, leading engineers to account for these factors during design.

Review Questions

• How does a high common-mode rejection ratio improve the performance of an amplifier in noisy environments?
  • A high common-mode rejection ratio improves amplifier performance by ensuring that common-mode signals, which are often noise and interference present on both inputs, are significantly attenuated. This means that the amplifier primarily amplifies the desired differential signal, reducing the impact of unwanted noise on the output. In environments where electrical noise is prevalent, such as industrial settings or when using long cables, a high CMRR ensures cleaner and more accurate signal processing.
• Discuss how mismatches in input impedance might influence the common-mode rejection ratio in a differential amplifier design.
  • Mismatches in input impedance can lead to unequal loading of the input signals in a differential amplifier, which can reduce its common-mode rejection ratio. If one input has a significantly different impedance than the other, it can cause differences in voltage drop across each input, resulting in amplification of common-mode signals rather than their rejection. To maintain a high CMRR, it's essential for designers to ensure that both inputs are well-matched in impedance.
• Evaluate the implications of low common-mode rejection ratio in critical applications such as medical instrumentation or audio equipment.
  • A low common-mode rejection ratio in critical applications like medical instrumentation or audio equipment can lead to significant issues such as inaccurate readings or distorted sound. In medical devices, noise can mask vital signals from patients, leading to misdiagnosis or unsafe conditions. Similarly, in audio equipment, poor CMRR can result in unwanted hums or buzzes from electrical interference being amplified alongside the desired audio signal. Thus, achieving a high CMRR is crucial for reliability and performance in these sensitive applications.

Related terms

Differential Amplifier: An amplifier that amplifies the difference between two input voltages while rejecting any voltage common to both inputs.

Input Impedance: The resistance that an amplifier presents to its input signal, which can affect the CMRR if not properly matched.

Noise Rejection: The ability of a circuit or system to minimize the effects of unwanted signals or noise on its output.