Have you wondered one thing—that how widely electronics is used? This paper gives you a small-window view of the places where electronics is used and has been sitting silently in our lives over the years. Now switch over to our health needs and how medicine and medical equipment is an inseparable part of modern life. Lifestyle diseases are very prominent among us these days. Along with it comes all the medicine and, of course, the use of medical equipment. Think of any of those and you will get one thing common and that is the presence of electronics in all of them. From the tiny gadget that checks the amount of sugar in blood to the bulky ECG machine which measures your heart operation. We'll try to cover one such application in this paper.
What is this ECG? ECG stands for electrocardiography. We all know that our heart is a muscle formed in a way that allows it to act as a pump for blood. This process is the acquisition of electrical activity of the heart captured over time by an external electrode attached to the skin. Each of the cell membranes that form the outer covering of the heart cell has an associated charge which is depolarized during every heartbeat. These appear as tiny electrical signals on the skin which can be detected and amplified by the ECG. These electrical signals are then given to an ECG pre-amplifier.
We'll see one type of such an amplifier—instrumentation amplifier. Before going into it, let us see the differential amplifier. A differential amplifier amplifies the difference of the two signals given as input to it. This is very important in our application because it rejects common mode voltages while amplifying the differential signal of interest.
Figure 2 Click on image to enlarge
The circuit shown above is of a differential amplifier. You will see that this circuit is a combination of an inverting and a non-inverting amplifier. Run the circuit and see the output is a difference of the inputs.
The main advantage of this circuit is that this circuit can remove the error caused by common mode voltage. What is common mode voltage? These are signals that are common to both inputs of the opamp. Common mode voltages cause a certain error in the output of the opamp: this error is measured as the common mode rejection ratio. For example, suppose equal 60Hz noise* is present on each input and one input is at 5Vdc and the other is at 2Vdc. The common noise is cancelled out in the difference and the 3V difference is amplified.
This circuit has some restrictions also—it being that the input impedance is limited by the resistors R2 and R3. We all know that the input impedance looking into the amplifier should be high. So if a high gain is required, then a high R0/R2 ratio is required, yet practical circuit considerations limit the maximum and minimum values for these resistors.