Common-mode rejection ratio and Instrumentation and difference amplifiers

In summary: B is correct.In summary, the common-mode rejection ratio (CMRR) needed to record an ECG of 1mV amplitude with a signal to noise ratio better than 20/1 in the presence of a 100mV common-mode voltage is -66dB. An instrumentation amplifier is better than a simple differential amplifier for biomedical signals due to its high input impedance and improved accuracy and stability.
  • #1
lavster
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Homework Statement



1. a) If we wish to record an ECG of amplitude 1mV in the presence of interference that causes a common-mode voltage of 100mV to appear on the inputs of the amplifier, then what common-mode rejection ratio (CMRR) do we need require if we need a signal to noise ratio better than 20/1 on our recording?

b) give reasons why an instrumentation amplifier is better than a simple differential amplifier for biomedical signals

Homework Equations



dB = 20 log(Vout/Vin)

The Attempt at a Solution



Im doing a crash course in electronics - never have done it before so there is some concepts I am struggling with!
im not sure if I am approaching this correctly, and I am not sure what the answer is. But this is my attempt

a) ECG amplitude = 1mV, common mode voltage = 100mV. Common mode needs to reduce by 1/20 which is equal to 0.05mV

dB=20log(0.05/100) = -66dB

can someone tell me if this is correct?

b) High input impedances of the buffers of the instrumentation amplifiers eliminate the need of input impednace matching. Precise resistor matching of differential amplifiers cause problems and reduce the CMRR. I am assuming instrumentation amplifiers have better accuracy and stability?

Thanks! :)
 
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  • #2
The S/N ratio is currently 1mV/100mV and you need it to be 20/1.

Instrumentation amps have good CMRR but I think your answer on matching is good as well.
 
Last edited:
  • #3
thats what i thought i was doing 1/0.05 = 20/1

or am i misunderstanding it?

thanks
 
  • #4
If the starting S/N was 1/1 then multiplying by 20 gets you to 20/1 however the starting S/N is 1/100.

To get from 1/100 to 20/1 you need to multiply by 2000.
 
  • #5
s youd agree that the answer is -66dB?
 
  • #6
Sorry yes. I'm half asleep here. I should have read your initial post more carefully. The CMRR is normally expressed as a +ve number.
 

1. What is common-mode rejection ratio (CMRR)?

Common-mode rejection ratio (CMRR) is a measure of how well an amplifier can reject signals that are common to both input terminals. It is a ratio of the differential gain (the amplification of the difference between the two input signals) to the common-mode gain (the amplification of the signals that are common to both inputs).

2. Why is CMRR important in instrumentation amplifiers?

CMRR is important in instrumentation amplifiers because it allows for accurate measurement of small differential signals in the presence of large common-mode signals. A high CMRR ensures that the amplifier will reject any noise or interference that is present on both input signals, resulting in a more accurate measurement of the differential signal.

3. What factors affect the CMRR of an amplifier?

The CMRR of an amplifier can be affected by several factors, including component matching, temperature, power supply voltage, and external noise sources. Any imbalances in the amplifier circuit or changes in these factors can decrease the CMRR, leading to a less accurate measurement.

4. How can CMRR be improved in an instrumentation amplifier?

CMRR can be improved in an instrumentation amplifier by using precision resistors that are well matched, using high-quality components, carefully designing the layout of the circuit, and implementing shielding and filtering techniques to reduce external noise sources. Additionally, using a differential input stage with a high CMRR amplifier can also improve the overall CMRR of the instrumentation amplifier.

5. What is the difference between instrumentation amplifiers and difference amplifiers?

Instrumentation amplifiers and difference amplifiers are both used to amplify small differential signals, but they differ in their input configurations. Instrumentation amplifiers typically have a differential input and a single-ended output, while difference amplifiers have both differential inputs and outputs. Additionally, instrumentation amplifiers often have a higher CMRR and are used for more precise measurements, while difference amplifiers are often used for level shifting or filtering applications.

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