3-Part ECG Circuit

An electrocardiogram (ECG) is considered one of simplest and fastest ways to evaluate heart rate. It is able to show the pace, rhythms, strength, and timing of the heartbeats using the natural electrical impulses created by the contractions of the heart. In this project we created a working model of an ECG using three parts in series: an instrumental amplifier, a notch filter, and a low pass filter. By designing each component from our desired values we successfully modeled each component and the combined components using simulated and physical models.

For all 3 parts:

• Breadboard
• Oscilloscope
• Function Generator
• 3rd machine
• circuit connection wires
• Cables / cable adapters for reading input and output as well as the oscilloscope and function generator

For Instrumental Amplifier:

• (2) 820 Ω resistors
• (2) 39 kΩ resistors
• (2) 8.2 kΩ resistors
• (3) LM741 operational amplifiers

For Notch Filter:

• (2) 2.2 kΩ resistors
• (1) 1 kΩ resistors
• (2) 1.3 uF capacitors
• (1) 2.6 uF capacitors
• (1) LM741 operational amplifier

Low-Pass Filter:

• (1) 6800 Ω resistor
• (1) 0.1 uF capacitor

The signal taken in by an ECG typically is not large enough to be seen on a display. Therefore, some type of amplification is needed in order to see the ECG peaks. In this design an instrumentation operational amplifier uses a 1000dB gain to provide a high amount of gain for the low signal in the presence of high levels of noise.

Methods:

Using the instrumental op amp equation we did the following calculations to figure out the values for each resistor below. Since the exact value were not possible without creating large series of resistors on the breadboard we used the following for each one:

• R1 = 820 Ω
• R2 = 39k Ω
• R3 = 820 Ω
• R4 = 8.2k Ω

Using the closest values from above, we created a circuit in LTSpice to test its output. The circuit produced the desired output so we built it on a circuit board for physical testing. The output for physical circuit was also the desired output, with a gain of about100 dB.

The inputted ECG signal also has a large amount of noise associated with it. Typically most buildings have a powerline noise signal occurring around 60Hz. This noise can be removed by using a notch filter which is used to pass all frequencies except a single specified signal. In this design, a notch filter is used to remove this 60Hz powerline noise from the desired ECG signal.

Methods:

Using the notch filter equation we calculated the desired values for the components. The variables in the notch filter equation stand for the following:

Q: quality factor (wanted value of 8)

ω0 = 2*pi*f0 : center frequency in rad/sec

f0: center frequency in Hz (wanted value of 250Hz)

β: bandwidth in rad/sec

ωc1 ωc2: cutoff frequencies (rad/sec)

Since the exact value were not possible without creating large series of resistors on the breadboard we used the following for each one:

• R1 = 2.2k Ω
• R2 = 2.2k Ω
• R3 = 1k Ω
• C = 1.3 μF
• 2C = 2.2 μF

Using the closest values from above we created a circuit in LTSpice to test its output. The circuit produced the desired output so we built it on a circuit board for physical testing. It gave the desired output of having no signal at 60 Hz.

In addition to the instrumentation amplifier and notch filter, a low pass filter is needed in our circuit. This low pass filter allows low frequencies to pass through the circuit to allow easier detection of the ECG signal spike .  In this design we used a cutoff frequency of 250Hz

Methods:

We used the following equation for the low pass filter:

fcutoff = 1 / 2*pi*R*C

Using the above equation and the desired cutoff frequency of 250Hz,  we completed calculations to figure out the values for each resistor. These can be seen in part two of the appendix. Since the exact value were not possible without creating large series of resistors on the breadboard we used the following for each one:

• R = 6800 Ω
• C = 0.1μF

Using the closest values from above we created a circuit in LTSpice to test its output. The circuit produced the desired output so we built it on a circuit board for physical testing. It gave the desired output of a cutoff frequency of 250 Hz.

Finally, we connected all three constructed components. We imputed the signal directly into the instrumentation operational amplifier. For the positive and negative voltage supplies on the LM 741 operational amplifiers, we used a 9V battery in the actual simulation and a value of +/- 15V in the LTSpice simulation. The amplifier then led directly into our notch filter containing a buffer on the end. We used the same voltage supplies for the operational amplifier in the buffer as the first few. After the buffer, we connected our low pass filter which then directly led into where we measured the output ECG signal.