Designing an ECG

We will be going through the process for designing and constructing a simple ECG circuit. This signal acquisition circuit will read and display a human ECG onto an oscilloscope.

This circuit will require 3 main components:

1. Instrumentation Amplifier (INA)
2. 60 Hz notch filter
3. 150 Hz low pass filter

An instrumentation amplifier is needed to amplify the ECG signal from ~1mV to 1V so that all peaks can be observed. The 60 Hz notch filter is needed to remove noise produced by surrounding electrical equipment. The 150 Hz low pass filter is needed to remove physiological noise, as the most important signals we will be observing occur at lower frequencies.

• circuit simulation software (we used LTSpice)
• breadboard
• wires
• 5 uA741 op amps
• resistors
• capacitors
• two 9V voltage sources
• electrodes
• alligator clamps
• oscilloscope
• resistance substitution box

The first figure above shows a standard INA circuit. Circuit values were calculated based on the equation shown below. The expected value of Vin2 - Vin1 = 1 mV, the amplitude of an ECG signal. Vout should be equal to 1 V, so we need a magnification factor of 1000. The resistor values should be within the kOhm range to prevent any large currents to be drawn from the body and through the circuit. R1 and R2 values were selected arbitrarily as 2.2 and 56 kOhms, respectively, as resistor values we knew would be available. We then utilized Excel to calculate the ratio of R4/R3. R3 was selected arbitrarily as 1.5 kOhms, a resistor value we knew would be available. R4 was then found to be 28.9 kOhms from the R4/R3 ratio, via Excel.

VoutVin2 - Vin1 = (1 + 2*R2R1)(R4/R3)

The above theoretical values were used to develop the LTSpice circuit design shown in the second figure above. The circuit was simulated to ensure that a gain of ~1000 could be attained with the selected values. A value of 27kOhms was used for R4, as this was the closest resistor value available for creating the real circuit. These values can be adjusted based on the available resistors, as long as the values still satisfy the equation shown.

The circuit was constructed with the values shown in the schematic above, and tested to ensure a gain of ~1000 was achieved. The breadboard circuit constructed is shown in the final image above. It is important to note the locations of Vin and Vout for each circuit component, as this is where the next circuit component will be connected.

The first figure above shows the schematic of the VCVS band-reject filter circuit that we used to design our 60Hz notch filter. This circuit design allows for high Q values; we selected a Q value of 8, as it allowed us enough leniency with the available resistors to still acquire an accurate reject band.

The equations above, which are defined by the VCVS filter design, were input into an excel spreadsheet to allow for easier manipulation of values. We began by converting the cutoff frequency f0 = 60Hz to ω0 = 376.8 radians. We defined C = 100nF and Q = 8, and used these values to solve for the resistor values of R1 = 1.6kOhms, R2 = 424 kOhms, R3 = 1.6 kOhms. A value of 220nF was used for 2C, as this was the closest capacitor value available. The LTSpice circuit shown above was simulated to ensure these values would produce the expected 60Hz notch.

To construct the circuit shown above, the resistor values shown in the table were connected in series to attain the most accurate values. R1 and R2 were connected into the breadboard circuit, and a resistance substitution box was connected in place of R3. The value of R3 was tested near the theoretical value to find the value needed for an accurate 60Hz notch. We found that an R3 value of 1.4 kOhms was needed. The circuit constructed is shown in the final image above.

Our low-pass filter was based on the second order VCVS low pass filter shown below with a gain of 1. The equations shown above are for finding the values of the circuit components, which are defined by the VCVS filter design with a gain of 1. For a circuit with a gain of 1, R3 is an open circuit and R4 is a short circuit (R4 = 0). The filter coefficient a = 1.414214.

The equations above were input into an excel spreadsheet to allow for easier manipulation of values. We began by converting the cutoff frequency fc = 150 Hz to ωc = 942 radians. C2 was calculated as 0.0667µF. C1 was calculated as 0.033µF, R1 as 20 kohms, and R2 as 25 kOhms. 24 and 1.1 kOhms resistors were placed in series for R2. These values were then put into an LTSpice simulation to check for the expected output. This schematic can be seen above. These values were then used to construct the circuit shown.

Next we need to connect each of the three circuit components we have constructed. The Vout of the INA will be connected to the Vin of the 60 Hz notch filter. The Vout of the notch filter will be connected to the Vin of the low pass filter. The Vout for the low pass filter will be connected to an oscilloscope so the the signal can be viewed. In the image above, the large green wire connects the INA to the notch filter, and the large yellow wire connects the notch filter to the low pass filter. In the next step we will configure the Vin of the INA to receive human signals.

3 electrodes will be placed on the human subject. The ground wire will be located on the left ankle. The two inputs indicated in the schematics will receive input from electrodes placed on the right ankle and left wrist. Ensure that electrodes are placed on the inside of the wrist and ankles. While reading the ECG signal, the subject should sit completely still. The ECG wave will be visible on the oscilloscope monitor. Adjust oscilloscope settings to view a steady waveform with a clearly indicated QRS complex. Our achieved waveform is shown above.

Troubleshooting:

• make sure all electrodes are positioned properly and securely (avoid bones)
• apply and hold gentle pressure to electrodes to ensure a good connection
• switch signal inputs (right ankle and left wrist)

Image from: https://www.google.com/url?sa=i&url=https%3A%2F%2Fwww.primemedicaltraining.com%2F12-lead-ecg-placement%2F&psig=AOvVaw38SEfOqBUOsBPheXoyopq5&ust=1651305570532000&source=images&cd=vfe&ved=0CAwQjRxqFwoTCOimzunmuPcCFQAAAAAdAAAAABAJ

This circuit is the most simplified version of a functional ECG. There are many improvements, changes, and additions that can be made to this design. Some further developments include:

• Integrating an Arduino system and code to find and display the BPM
• Additional filtering to remove noise
• Further amplification to view smaller peaks present