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S ELT-61306: Basics of Medical Electronics Tavier Grac a March 5, 2014

Name = GA

Calta lado 15 allowed

1. (20 points) Figure 1 represents the interface of a silver-silvercloride (Ag — AgC1) elec-
trode with a saline (Cl7 , Na*) electrolyte.

Electrode Electrolyte

Ag AgC! CI- Na+

kitti
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Figure 1: Interface electrode-electrolyte for a Ag-AgCl electrode in a saline solution.

Describe the processes taking place when a current flows from electrolyte to electrode.

Tip: You should mention at leas: (If an electric current is a flow of electric charges) what
is the particle, charge, and direction that defines the current in the electrode? What is the
particle, charge, and direction that defines the current in the electrolyte. The chemical
reaction in the interface Ag — AgCl only for the mentioned current direction. Is is an
oxidation or reduction reaction?. The chemical reaction in the interface Ag — electrolyte
only for the mentioned current direction.

2. (10 points) Imaging the previous electrode is place on a patient, and the current only
flows in the mentioned direction. If this situation is prolonged for a long time, what will
be the negative effects for the patient?

3. (15 points) Calculate the formula for the gain in the following differential amplifier.

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Figure 2: Differential amplifier.
 

 

ELT-61306: Basics of Medical Electronics March 5, 2014
Name:

 

4. (15 points) Enumerate the four types of EMC coupling and one way of reducing the
effects for each of them.

5. (10 points) The oscilloscope measures the voltage between the two ends of its input
impedance, respectively connected to the tip of the lead and the ground clamp. The
ground terminal is also internally connected to the ground of the mains.

If you touch the tip of the lead with an ungrounded clamp (Fig. 3), caleulate the ap-
proximate amplitude of the signal you will see on the oscilloscope.

Consider the following situation:

Main power source Vinain 220V050Hz
Coupling of patient to mains Coow 30pF
Coupling of patient to ground Ciodo 3pF

Input impedance of the oscilloscope Zinosc 1M O||15pF'

MAINS POWER

 

 

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Figure 3: Schema floating ground oscilloscope capacitive coupling.

6. In the previous situation we replace the oscilloscope for an instrumentation amplifier
(IA) (Fig. 4) in order to measure the ECG signal. The IA is powered by a power supply
connected to the mains.

Consider a the following values:

Main power source n 220V O50H z
Coupling of patient to mains n 30pF
Coupling of patient to ground Cd 3pF

Input impedance IA in 10G9||150pF
Common mode ratio IA at 50Hz and Gg = 100 CMRRO50Hz 120dB
Resistive value interface skin-electrode Za 20k9 + 10%

 

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ELT-61306: Basics of Medical Electronics March 5, 2014
Name:

MAINS POWER

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GROUND

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Figure 4: Schema floating ground oscilloscope capacitive coupling.

(a) (5 points) What is the voltage (amplitude of the coupled signal) at the body due
to the capacitive coupling with the mains?

(b) (5 points) What is the amplitude of the common mode voltage measured at the
inputs of the IA?

(e) (5 points) What is the amplitude of the error at the output of the IA due to the
common mode voltage measured at the inputs of the IA?

(d) (5 points) What is the amplitude of the differential voltage measured at the inputs
of the IA for the worst case?

(e) (5 points) What is the amplitude of the error at the output of the IA due to the
differential voltage measured at the inputs of the TA?

 

(£) (5 points) There are 3 way this error can be reduce, explain them.

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