(a)Describe how the system will detect when electrodes fall off from the patient
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Question
(a)Describe how the system will detect when electrodes fall off from the patient.
(b)Will the 50kHz signal interfere with ECG? Why?
(c) If the Zener diode had a breakdown voltage of 15V, how much should be the amplitude of 50kHz current source to detect increase of electrode-skin resistance more than 100k Ohm.
50-kHz Low-pass ECG filter current amplifier etc 150 Hz Electrode 1 50-kHz Threshold bandpass Electrode 2 filter Alarm Figure 6.25 Block diagram of a system used with cardiac monitors to detect increased electrode impedance, lead wire failure, or electrode falloff.Explanation / Answer
a.) The Electrode is a device that converts physical signals into electrical voltage.The voltage is in the range of 1 mV ~ 5 mV.
Let's take an example: the system uses left leg of pateint as “driven left leg lead”. Using Wilson Electrode system:
A summing network is used to obtain the sum of the voltages from all other electrodes and driving amplifier, the output of which is connected to the left leg of the patient. The effect of this arrangement is to force the reference connection at the left leg of the patient to assume a voltage level equal to the sum of the voltages at the other leads. This arrangement increases the CMMR common mode rejection ratio of the overall system and reduces noise interference. It also has the effect of reducing the current flow in to the left leg electrode.
b.) No teh 50KHz signal will not will interfere with ECG, because of two components;
**Instrumentation Amplifier
**Low pass filter
The purpose of Instrumentation amplifier to condition small signals in the presence of large common-mode voltages and DC potentials, usually analog instrumentation amplifier type is used to amplify the ECG voltage from electrodes, which is in the range of 1mV to 5mV. Therefore, we designed the instrumentation amplifier using op-amp, with a gain of 1000 and power supply is +12V to -12V.
Secondly, the low pass filter is used to remove unwanted signals like noise, usually the frequency range of ECG is 0.04HZ to 150 Hz, and so the low pass filter is designed with the cut off frequency of 150HZ.
Third is the block used for amplifier which is non-inverting type and used to saturate the ECG signals, and output fed to microcontroller for counting of heart-beat]
c.) A mismatch in electrode-skin impedance reduces the CMRR of amplifier and leads to excessive power line imterfernce.The 6nA current peak amplitude(smalles possible) is used in order to not disturb ECG signals recording. for frequency between 50Khz - 125KHz the same amplitude of current would be used in order to detect the increase of electrode skin resistance
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