ECG Design Project - Nick Most

An electrocardiogram, known as an ECG, is used to detect heart problems and monitor the health of the heart through bioelectric signals. As electric signals flow through the heart beginning in the sinoatrial node and ending in the ventricles, the electrocardiogram displays an output with five different phases. As seen in the diagram, these phases are the P, Q, R, S, and T waves. This Design Project utilizes three filters to simulate a simplified ECG that when hooked up to a patient, will display their ECG reading. The three filters used are an INA filter, a notch filter, and a low pass filter. These filters will then be connected in series, the patient will be set up using ECG leads as the input, and the output will be displayed on an oscilloscope.

The supplies needed for this design project include a variety of resistors and capacitors, 5 op-amps, jumper wires, an oscilloscope, and a function generator to supply power to both the circuit and each individual op-amp.

The first step is constructing the INA filter. This filter is set up to have 3 op-amps connected through a series of resistors. These resistor values were chosen in order to receive an output gain of 2 using an output voltage equation. Once the resistor values had been chosen, the circuit was built as seen in the LTSpice schematic. The picture displayed on the left is the real circuit. Each op-amp was supplied with 15 V on the positive terminal and -15 V on the negative terminal. The positive and negative voltages are supplied through the rows at the top and bottom of the bread board. The positive and negative terminals were then connected to each op-amp through jumper wires.

The second step is creating a notch filter. This filter is set up by a resistors in series as well as capacitors in series set up in parallel flowing through an op-amp. The capacitor and resistor values were chosen by trying to find a quality factor of 8 as well as achieving a resonance frequency of 60 Hz. Once the resistor and capacitor values have been chosen, the circuit was built as seen above. The theoretical notch filter will have a high magnitude at an frequency less than or greater than 60 Hz. At 60 Hz, it will drop to almost 0. The positive and negative terminals were connected through jumper wires to the op-amp.

The third step is constructing the Lowpass filter. This filter restricts frequencies from being allowed through that are above a certain threshold. The resistor and capacitor values were calculated by determining a certain cutoff frequency. Once this cutoff frequency is determined, the other values can be calculated as seen above. The lowpass filter uses a series of resistors as well as capacitors connected to ground and the output of the op-amp to restrict the frequencies above the desired cutoff frequency. As done with the other filters, the positive and negative terminals were connected through jumper wires into the op-amp.

The final step in building the circuit is to connect the outputs of each op-amp into the inputs of the next filter. For example, the output of the INA will be connected with a jumper wire to the input of the notch filter. The output of the notch filter will then be connected with a jumper wire to the input of the lowpass filter. The oscilloscope will then be attached to the output of lowpass filter and the function generator will be connected to the input of the INA filter. The last step is to ensure that each op-amp is connected to the positive and negative terminals because each op-amp needs a power supply to function properly. When everything is put together, the oscilloscope will display a simulated ECG.

To ensure the project works, a patient was hooked up as the input into the circuit. If the project was built correctly, the oscilloscope should display the patient's ECG signal. As seen above, when the patient was connected, a proper ECG signal was displayed on the oscilloscope proving our project was built correctly. Once finished, a simplified ECG signal amplifier has been built!