Common-mode rejection

Definition

Common-mode rejection refers to the ability of a signal conditioning or readout circuit to eliminate unwanted noise or interference that is present equally on both input lines, while preserving the desired differential signal. This feature is crucial in applications involving MEMS and NEMS sensors, where accurate measurement of small signals is essential amidst various environmental and electrical noise. A high common-mode rejection ratio (CMRR) indicates better performance in distinguishing between the actual signal and common noise.

5 Must Know Facts For Your Next Test

1. Common-mode rejection is typically quantified using the Common-Mode Rejection Ratio (CMRR), expressed in decibels (dB), which indicates how well a circuit can reject common-mode signals.
2. In MEMS/NEMS applications, achieving a high CMRR is critical for accurate readings, as sensors often operate in noisy environments with various interference sources.
3. Common-mode signals may arise from environmental factors like electromagnetic interference (EMI) or power supply noise, making their rejection essential for reliable sensor data.
4. The design of differential amplifiers is central to enhancing common-mode rejection in signal conditioning circuits used with MEMS/NEMS sensors.
5. Improving CMRR can be achieved through circuit design techniques such as using matched resistors and careful layout to minimize parasitic capacitance.

Review Questions

• How does common-mode rejection impact the performance of differential amplifiers used in MEMS/NEMS sensors?
  • Common-mode rejection significantly enhances the performance of differential amplifiers by allowing them to effectively filter out unwanted noise that affects both inputs equally. This capability ensures that only the true differential signal, which represents the desired measurement from the MEMS or NEMS sensor, is amplified. As a result, devices with higher CMRR can provide more accurate and reliable readings, which is particularly important in precision applications.
• Discuss the implications of a low Common-Mode Rejection Ratio (CMRR) on sensor data accuracy and reliability.
  • A low Common-Mode Rejection Ratio (CMRR) can severely compromise sensor data accuracy by allowing common noise signals to be amplified along with the desired differential signals. This results in distorted readings that may lead to incorrect interpretations of the sensor outputs. In applications where precise measurements are crucial, such as medical devices or automotive sensors, inadequate CMRR could lead to failure in performance or safety issues due to misleading data.
• Evaluate the strategies that can be employed to enhance common-mode rejection in signal conditioning circuits for MEMS/NEMS sensors.
  • To enhance common-mode rejection in signal conditioning circuits, several strategies can be employed. These include using instrumentation amplifiers designed specifically for high CMRR, ensuring that components are closely matched to minimize differences in resistance and capacitance. Additionally, careful PCB layout techniques should be utilized to reduce parasitic effects and improve shielding against external noise sources. By implementing these methods, engineers can significantly improve the reliability and accuracy of sensor readings in challenging environments.

Related terms

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

Signal-to-Noise Ratio (SNR): A measure that compares the level of a desired signal to the level of background noise, important for assessing signal quality.

Instrumentation Amplifier: A specialized amplifier designed to amplify low-level signals while providing high CMRR, typically used in sensor applications.