原创 How healthy is your heart?

Have you wondered one thing—that how widely electronics is used? This blog 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 blog.

Figure 1

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

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 5V_dc and the other is at 2V_dc. 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.

The solution to both high-gain and high-input impedance problems is the instrumentation amplifier (INA). This circuit uses three opamps.

Figure 3

Use the inverting amplifier equations to derive an equation for the gain of the first stage. The input is the difference between the two points connected to positive of opamp and the output is the difference between the two outputs of the opamps. This gain of the first stage comes to A_v¬ = 2(R₁/R₀) + 1. This is then given to the final stage which in itself is a differential stage. So finally the output from the circuit will take into consideration this too – thus the final gain will be [2(R₁/R₀) + 1] [R₅/R₃].
The main advantages of INA's are: ability to obtain high gain with low resistor values, extremely high input impedance, and superior rejection of common mode signals. Modern INA's are of the monolithic IC form with terminals for the R0 resistor which is usually variable and can be used to control the gain. You can see details about these ICs here: http://www.analog.com/en/specialty-amplifiers/instrumentation-amplifiers/products/index.html#Instrumentation_Amplifiers

As a conclusion it should be noted that there are more complexities involved. There will be a filter connected to this amplifier as well as an isolation amplifier involved mostly to eliminate noise. There will also be circuitry to null out a dc offset introduced by the electrodes that are connected to the patients. We will try to introduce them in the coming weeks. For now this is just an introduction to a simple bioelectric amplifier and its concept as well as its implications in ECG.

Visit the following sites to try designing these circuits:
http://www.docircuits.com/circuit-editor/205
http://www.docircuits.com/circuit-editor/206

*60Hz noise is the noise introduced due to electromagnetic disturbance from other electronic appliances in the room along with the ECG machine.