# Biomedical Instrumentation Questions and Answers – Sources of Noise in Low Level Measurements

This set of Biomedical Instrumentation Interview Questions and Answers for Experienced people focuses on “Sources of Noise in Low Level Measurements”.

1. Bio potential amplifiers have ____________ input terminals.
a) 3
b) 4
c) 5
d) 6

Explanation: Bio potential has three input terminals. Out of the three one is arranges at the reference potential. The other two are live terminals. Bio potential amplifiers are also known as differential amplifiers. The differential amplifier is employed when it is necessary to measure the voltage difference between two points, both of them varying in amplitude at different rates and in different patterns.

2. The ability of the amplifier to reject common voltages on its two input leads is known as ________
a) common mode rejection rate
b) coupled mode rejection rate
c) common mode rejection ratio
d) coupled mode rejection ratio

Explanation: The ability of the amplifier to reject common voltages on its two input leads is known as common-mode rejection. It is specified as the ratio of common-mode input to differential input to elicit the same response. It is abbreviated as CMRR (Common-mode rejection ratio).

3. CMRR is measured in ____________
a) V/s
b) dB
c) dB/s
d) dB/ms

Explanation: CMRR is an important specification referred to the differential amplifier and is normally expressed as decibels. The ability of the amplifier to reject common voltages on its two input leads is known as common-mode rejection. It is specified as the ratio of common-mode input to differential input to elicit the same response.

4. CMRR of the preamplifiers should be as high as possible.
a) True
b) False

Explanation: CMRR of the preamplifiers should be as high as possible so that only the wanted signals find a way through the amplifier and all unwanted signals get rejected in the preamplifier stage. The ability of the amplifier to reject these common voltages on its two input leads is known as common-mode rejection and is specified as the ratio of common-mode input to differential input to elicit the same response. CMRR is an important specification referred to the differential amplifier and is normally expressed as decibels.

5. The common mode rejection for most op-amps is typically between __________
a) 10-50dB
b) 20-40dB
c) 60-90dB
d) 100-120dB

Explanation: The common mode rejection for most op-amps is typically between 60 dB and 90 dB. This may not be sufficient to reject common mode noise generally encountered in biomedical measurements. Also, the input impedance is not very high to handle signals from high impedance sources.

6. The output of differential gain is given by _________
a) (difference of the two input voltage)*(feedback resistance/input resistance)
b) (sum of the two input voltage)*(feedback resistance/input resistance)
c) (difference of the two input voltage)*(input resistance/feedback resistance)
d) (sum of the two input voltage)*(input resistance/feedback resistance)

Explanation: The output of differential gain is given by (difference of the two input voltage)*(feedback resistance/input resistance). gain is given by (feedback resistance/input resistance). The input resistances of but the inputs are the same.

7. In order to be able to minimize the effects of changes occurring in the electrode impedances, it is necessary to employ a preamplifier having a high input impedance.
a) True
b) False

Explanation: True. In order to be able to minimize the effects of changes occurring in the electrode impedances, it is necessary to employ a preamplifier having a high input impedance. It has been found that a low value of input impedance gives rise to considerable distortion of the recordings.

8. The impedance of the input should be ________ in order to obtain high CMRR in the differential amplifier.
a) low
b) High
c) Does not matter
d) Very low

Explanation: This shows that high input impedance is very necessary in order to obtain a high CMRR. Also, the electrode skin resistance should be low and as nearly equal as possible. In order to be able to minimize the effects of changes occurring in the electrode impedances, it is necessary to employ a preamplifier having a high input impedance.

Sanfoundry Global Education & Learning Series – Biomedical Instrumentation.