Question

Exam question 8: Instrumentation amplifier - Input coupling

8.a): What is the purpose of implementation of an input coupling on an instrumentation amplifier to measure the electrophysiological signals?

(Hint: This is the first component of our electronic system; i.e. the component to which the electrodes are connected. This means that it is necessary to consider safety aspects and that it is the part of the instrumentation system that is most sensitive to noise.)

8.b): Draw an example of an AC-coupled instrumentation amplifier and explain the diagram.

(Hint: What does AC coupling mean when translated into filter terminology (high-pass, low-pass, band-pass, etc.))

8.c): Which type of noise can you reduce by an implementation of the things you described in 8.b)?

(Hint: What noise typically occurs in this frequency range? (according to the answer to 8b))

Answer 1

8 (a) answer: to eliminate any kind of voltages other than the voltage being measured.

Explanation: firstly, the electrodes placed on our body parts are used to get the potentials which our body cells generate. generally in hospitals ,after the electrodes are attached to the body, the doctors advice the patient not to move much.

Since the movement of the body can cause the electric potential being generated to change. also when the body moves the electrodes attached also move causing frictional voltages of 0.1 to 0.2v. so due to these extra potentials , the original reading might change. also external noise can change the original reading.

to eliminate these errors, we use AC coupling which includes capacitor.

capacitor eliminates any voltages which have lower frequencies.

8(b) answer :

Working: AC coupling allows only AC signals to pass through. any kind of DC offset will be attenuated. this AC coupling can be done by connecting a capacitor In series with the one in which we want to eliminate DC offset. The DC offset being introduced by electrode is suppressed by the Instrumentation amplifier. ( we have to use passive elements like resistors and capacitors to suppress DC offset.)

the Vac in the above circuit is the wanted voltage. Vdc is generated by electrode.

1st opamp in the above circuit will allow both signals and amplify them. then the second opamp containing the capacitor and resistor will act as high pass filter to allow only high frequency signals.

the final output will be accurate and precise without any errors. (final output accuracy will depend on the reference voltage Vref we have given to the circuit.)

since coupling capacitor allows only high frequency signals but not low frequency signals, we call AC coupling to be as acting high pass filter.

8(c) answer:

DC offset or signals which do not have any frequency occurs. also external noises such as gaussian noise can occur. we can eliminate any type of noise which has LOWER frequencies.

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