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

To solve this problem, let's analyze the Common Mode Rejection Ratio (CMRR) requirements for a pre-amplifier operating up to 5 kHz.

1. What is CMRR?

Common Mode Rejection Ratio (CMRR) is a key specification for amplifiers that measures their ability to reject common-mode signals (signals that appear simultaneously and in-phase on both inputs). It is typically expressed in decibels (dB). A higher CMRR indicates better noise rejection.

2. Importance of CMRR in Pre-amplifiers

Pre-amplifiers are used to amplify weak signals (e.g., biomedical signals like ECG, EEG) before further processing. Since these signals are often contaminated with noise and interference, a high CMRR is essential to:

• Reject power line interference (50/60 Hz)
• Minimize noise from electromagnetic interference
• Ensure signal integrity in the presence of common-mode voltages

3. CMRR Requirements for Biomedical Applications

For pre-amplifiers in biomedical instrumentation (e.g., ECG amplifiers):

• Minimum acceptable CMRR: Typically 60-80 dB
• Good performance: 80-100 dB
• High-quality designs: >100 dB

The requirement becomes more stringent for wider bandwidths (up to 5 kHz in this case).

4. Analysis of Given Options

- 10 dB: Completely inadequate for any practical application
- 30 dB: Still insufficient for biomedical signal acquisition
- 90 dB: Meets minimum requirements for many applications
- Greater than 90 dB: Ideal for high-quality signal acquisition up to 5 kHz

5. Why Greater Than 90 dB is Necessary

At higher frequencies (approaching 5 kHz):

• CMRR typically degrades with frequency
• More interference sources exist in this range
• Biological signals at these frequencies are very small

Therefore, a CMRR specification must guarantee sufficient rejection across the entire bandwidth.

6. Final Answer

The CMRR of a pre-amplifier up to 5 kHz should be greater than 90 dB to ensure adequate rejection of common-mode interference across the entire frequency range.