Download ENTC 4350 Laboratory #1

ENTC 4350 Laboratory #1:
Operational Amplifier Electrocardiogram
Objectives
Learn the following concepts:
Amplification & Gain
Know how to set the gain of an op-amp
Noise
Know main sources and characteristics of electrical noise and methods for
avoiding/eliminating it
Filtering & Attenuation
Know expression for calculating the cutoff frequency of an RC filter
Be able to plot the frequency response of a filter
Prototyping
Oscilloscope
Understand basic operation
Know how to adjust vertical shift, gain and frequency (time-base)
Electrocardiogram
Know source of the ECG, its electrical characteristics and major features(PQRST)
Know the 12 leads commonly used clinically
Be familiar with the appearance of a normal ECG
Understand the physiology of an arrhythmia (tachycardia)
Introduction
This a very simple project to get you started. Nevertheless, there is a lot of good
instrumentation understanding to be gained from it!
The electrocardiograph (ECG/EKG) is produced by the heart muscle (myocardium). It is
the timing signal (pacemaker) that causes the muscle cells to contract and pump blood
round the circulation. So, although it is responsible for the pumping action, it's important
to realize that the two are not the same.
The ECG is useful for three basic parameters of clinically interest:
1. The rhythm and rate of the heart: whether it is regular and
normal, faster (tachycardia) or slower (bradycardia) or
normal
2. The axis of the heart, which can reveal any enlargement
(hypertrophy) due to, e.g. chronic high blood pressure
(hypertension)
3. The state of the myocardial muscle
Circuit
The circuit consists of two modules: the filter and the amplifier:
The filter is a simple low-pass RC network (first order or single pole filter). Its job is to
remove 60Hz electrical noise picked up from fluorescent lights, computers and AC power
lines. The amplifier is the common 741,wired in non-inverting mode.
Method
Step 1: Build the Filter
First, calculate a suitable value for the capacitor so that 60Hz is blocked. Remember that
the cutoff frequency (-3dB, at which the input signal power is attenuated by half) for a
first-order filter is simply 1/2RC:
Note that the gain of the first-order filter drops by 20dB/decade after the cutoff. A decade
means a ten-fold ("order of magnitude") increase in frequency. So, (since 20log(Voltage
Gain) = dB), every time the frequency increases by an order of magnitude, the voltage
gain is reduced by a factor of 1020/20 = 10, i.e. a tenth. Another way of stating this is
6dB/octave, where an octave is a doubling of the frequency. In other words, every time
the frequency doubles, the voltage gain is reduced by a factor of 106/20 = 2, i.e. it is
halved.
So, if you choose your cut-off to be 40 Hz, what will be the gain at 60 Hz? Well, at 40 Hz
it is already -3dB by definition. From 60 to 40 is an increase in frequency of 1.5, and we
know that an two-fold increase (octave) causes the gain to drop by 6 dB, so that makes
6x1.5/2 = 4.5, i.e. a total drop of -(3+4.5) = -7.5 dB.
Test the Frequency Response
At low frequency the output is the same amplitude as the input, but...
as frequency is increased, attenuation is apparent.
We want this to happen in order to remove the high frequency (60 Hz) noise.
Plot the frequency response in Excel by recording the Peak-to-Peak value at several
frequencies: e.g.
Frequency (Hz)
1
Vout
5
10
50
100
Step 2: Build the Amplifer
20 Log(Vout/Vin)
Measure the Gain
Note the different multiplication factor on each channel.
What happens if the input voltage is too high?
The output gets clipped
(saturated)by the power supply rails.
Connect the Electrodes
Attach an electrode to the volar (under) surface of your right and left forearms. Connect
them to the input and ground of filter module using alligator clips or press stud snap-on
connectors.
Watch your EKG!
Questions
1.
2.
3.
4.
5.
6.
7.
8.
9.
What value capacitor did you use for your filter? Explain why.
Show the frequency response of your filter
What was the cutoff frequency (-3dB point)?
What was the gain of your filter at 60 Hz?
What was the gain of your amplifier?
What happened when you changed the value of the feedback resistor?
What was the rate and rhythm of your EKG?
What happens if you put the electrodes on your feet instead of your arms?
Compare the appearance of your EKG with the commercial one shown
here
o
.
What is the main difference and what do you think is the cause?