Op-amp ECG

BE513 Instrumentation Circuit #2: 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 foravoiding/eliminating it

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 (e.g. tachycardia)

Introduction

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 realise that thetwo are not the same.

The ECG is useful for three basic parameters of clinically interest:

The rhythm and rate of the heart: whether it is regular and normal, faster (tachycardia) or slower (bradycardia) or normal
The axis of the heart, which can reveal any enlargement (hypertrophy) dueto, e.g. chronic high blood pressure (hypertension)
The state of the myocardial muscle

Electrical Noise

ECG has an amplitude of only about 1 mV, so to detect it an amplifier is needed. There is a problem, though - electrical noise, or electromagnetic interference (EMI). EMI is generated by many common appliances, such as power lines, fluorescent lights, car ignitions, motors and fans, computers, monitors, printers, TVs and television, and cell phones. When the ECG is amplified, the noise is amplified too, and often swamps the ECG signal. Luckily, the noise is usually of a higher frequency than the ECG. For example, AC hum is 60 Hz, which is above the highest frequency in the ECG (about 20 Hz). So the noise can be reduced by low-pass filtering.

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 polefilter). 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.
Gain = (R1 + R2)/R2 or (1+ R1/R2)

Measure the Gain

Use your function generator to measure the gain at 1 Hz.

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 the filter module using alligator clips.

Watch your EKG!

See... Dr. Kirtley does have a heart!

Questions

What was the gain of your amplifier?
What would happen if we amplified the signal before we filtered it?
What happened when you changed the value of the feedback resistor?
Try changing the value of the filtering capacitor - which value gave the best results? Explain why.
What was the rate and rhythm of your EKG?
What happens if you put the electrodes on your ankles instead of your arms?
Compare the appearance of your EKG with the commercial one shown here .

What are the main differences and what do you think is the cause?

Did you have any other problems seeing the ECG?