Exam text content

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ELT-61306: Basics of Medical Electronics April 14, 2014

Name: Jie Gra Cia Tah Uenca
Rules:
e The use of calculator is allowed during the exam.

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

Electrode Electrolyte
Ag AgCI CI- Na+
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Figure 1: Interface electrode-electrolyte for a Ag-AgCIl electrode in a saline solution.

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

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. We have design the differenential amplifier in Figure 2. For amplifying differential volt-
age.

Differential amplifier

   

Figure 2: Differential amplifier.

(a) (5 points) Calculate the differential gain of the amplifier.
ELT-61306: Basics of Medical Electronics April 14, 2014
Name:

(b) (10 points) Calculate the offset error at the output due to the Input offset voltage of
the Operational Amplifier if, according to the OpAmp's datasheet it is vjo = 1mV.

(e) (10 points) Calculate the the worst common-mode gain if we select resistors with
a +1% margin error.

 

3. (20 points) List the four types of EMC coupling, one example of each type of coupling
we can find a PCB (printed circuit board), and one way of reducing the effects.

4. We are using an instrumentation amplifier (IA) in order to measure the ECG signal.
The IA is powered by a power supply connected to the mains. Therefore, the input
impedances of the IA can be consider to be connected to the main ground (Fig. 3).

Consider the following values:

 

Main power source Vaa 220V G50H z

Coupling of patient to mains (n 30pF"

Coupling of patient to ground Chody 3pF

Input impedance IA Z; 10G9/||150pF

Common mode ratio IA at 50Hz and G; = 100 CMRRG50Hz 1204B

Resistive value interface skin-electrode Ze 20k9 + 10%
MAINS POWER

 

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GROUND

NTI

Figure 3: Schema IA connected to patient.

(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?

(c) (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?

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ELT-61306: Basics of Medical Electronics April 14, 2014
Name:

(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 IA?

In order to reduce this error we connect the patient directly to ground (through and
electrode) as seen in (Fig. 4).

MAINS POWER

 

 

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= A

 

 

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GROUND

Figure 4: Schema IA connected to patient. Patient grounded.

(a) (10 points) What is the new amplitude of the error at the output of the IA due to
the differential voltage measured at the inputs of the IA?

(b) (5 points) Although the error is considerably reduced, this configuration is not a
good idea. What are the safety risks of connecting the patient directly to the main
ground?.

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