Question

What should be CMRR of pre-amplifier up to 5 kHz?

• A. \( \text{10 dB} \)
• B. \( \text{30 dB} \)
• C. \( \text{90 dB} \)
• D. \( \text{greater than 90 dB} \)

Hint:

A high Common Mode Rejection Ratio (CMRR) is critical for pre-amplifiers in biomedical applications because biological signals are typically very small and often contaminated by large common-mode noise (e.g., 50/60 Hz power line interference). A higher CMRR means the amplifier is better at rejecting this unwanted noise, allowing for clearer amplification of the true differential biological signal. In practical terms, aim for CMRR>100 dB for sensitive measurements.

Answer 1

The Correct Option is D

CMRR stands for Common Mode Rejection Ratio. It is a measure of the ability of a differential amplifier (like a pre-amplifier used in biological signal acquisition) to reject common-mode signals, while amplifying differential-mode signals. In biological signal acquisition (e.g., ECG, EEG, EMG), common-mode noise, such as power line interference (50/60 Hz) and environmental noise, can be significantly larger than the actual biological signal. A high CMRR is essential to effectively suppress this noise and extract the desired signal. For pre-amplifiers used in biomedical applications, particularly for low-frequency biological signals (which typically range from DC to a few kHz), a very high CMRR is required to minimize common-mode interference. While 90 dB is a good value, for sensitive biological signals, particularly where noise is prevalent (like up to 5 kHz which includes power line frequencies and harmonics), a CMRR greater than 90 dB is generally preferred and often considered necessary for high-quality recordings. Values of 100 dB or more are common for high-performance biomedical pre-amplifiers. Therefore, for a pre-amplifier operating up to 5 kHz, a CMRR greater than 90 dB is desirable.