ECG Amplifier

An ECG is a medical device that takes the signal from a human heart beat and displays the signal numerically with waves. These waves correspond to different parts of eh cycle of a heart beat and each wave is important as it tells a story of how the human's heart is working. An ECG needs to be able to take the small signal from a human heart and make it readable, by making it bigger and getting rid of all of the weird distortion that can happen on waves from things like motion when getting the ECG. Building an ECG is not as hard as it seems, and in fact can be broken down into subparts to help.

This is the most important step! As is said before the ECG needs to be readable for the doctor, and of he can't read it lives may be at risk. Some common things you will find out about an ECG is that there are three main components; an instrumentation amplifier (INA), a notch filter, and a low pass filter. Each other component has a specific role.

The gain, or amplification factor, of the INA needs to be theoretically at 2000. Anywhere from 1000 to 2000 is good in reality, but 2000 is the best value. The cutoff frequency for the notch filter needs to be 60 Hz to limit the noise, or distortion of the signal. Lastly the cutoff frequency of the low pass filter needs to be 100 Hz to keep any high signals out.

In this step you need to take your knowledge about an ECG and the restraints to calculate the values for the components of the circuits. For input voltages it is typical to use 9V, so do calculations based off of a 9V battery or power supply.

Here are examples of how to do the math for the components of the ECG.

This step is the most important, because it tells you if your individual components are working, and if one does not work it will mess up the entire ECG circuit. To do this you need to use a circuit simulation software to construct the circuits with your calculated value and run them, which for me is LTSpice. First, you need to build all three components, separately. Use the values you calculates in step 2. Next, run the individual components to see if they work. To do this use an input of a 9V sin wave to simulate a battery and a wave like a heart beat. Make sure the desired outputs are happening by looking at the input and output graphs.

Use the schematics you built on LTSpice and build the components individually. Put them close to each other, because for the combined circuit later they will be in series.

Once again test each individual component and make sure they are all working before combing them together. This can be done with a function generator, power supply, and an oscilliscope. Here are what the outputs should look like displayed on the screen

To combine the components use the output of the INA to be the input of the notch filter and the output of the notch filter to be the input of the low pass filter. The input will go into the INA and the output will come from the low pass filter.

This can be done with ECG leads, and the leads will go on your ankles and right wrist in Configuration 2. If you do not have access to these leads, a simulation of a cardiac wave from a function generator will also work.

Use an Arduino one board and code to display the heart rate in beats per minute and ECG on your computer screen, and you're done!