Introduction: BME 207 EKG Circuit Design Instructions

The electrocardiogram (ECG) is a vital tool for monitoring cardiovascular function in patients.  In this report, detection, amplification, and filtering circuits for processing of the ECG signal were designed to record, extract and display the PQRST waveform representing the electrical activity of the heart.  To create a working ECG, we set up the circuit in three stages consisting of an instrumentation amplifier (INA), notch filter, and low pass filter.   The INA is the first stage of the circuit responsible for magnifying the recorded signal, allowing for better readability of the heart's electrical signals on the oscilloscope. The second stage of the circuit contains a notch filter design that filters out a frequency of 60 Hz, avoiding powerline interference with the heart’s signals.  The final stage of the circuit is a low pass filter that attenuates signals with frequencies above 150 Hz to reduce noise from interfering with the desired signal.  LTSpice was used to confirm the expected function of the amplifier and filters prior to the construction of the circuit.  Once constructed, a human signal was passed through the circuit and transmitted through the Arduino program for further processing. This resulted in an EKG display and the determination of the human subject’s heart rate.  The signal could also be transmitted to an oscilloscope where the signal can be displayed and evaluated. 

Supplies

  • 1 Breadboard
  • A lot of jumper wires
  • 2, 9V Batteries
  • An oscilloscope
  • 3 sticky EKG pads
  • 3 EKG electrodes
  • BNC to double alligator clip cable
  • extra alligator clip to alligator clip wires

INA:

Resistors

  • 1 kohm
  • 2, 25 kohm
  • 2, 3 kohm
  • 2, 60 kohm

3, UA741 with +-15 V,OpAmps

Notch filter (60Hz)

Resistors

  • 2, 1.6 kohm
  • 33 kohm
  • 390 kohm

Capacitors

  • 2, 0.1 uF
  • 0.22 uF

UA741 with +-15 V, OpAmp

Lowpass Filter

Resistors

  • 18 kohm
  • 27 kohm

Capacitors

  • 0.068 uF
  • 0.033 uF

UA741 with +-15 V,OpAmp

Step 1: Build Instrumentation Amplifier (INA)

Build the first part of the circuit as shown in the pictures above. View the grey and blue circuit schematic for equipment values. It is important to note that this section of the circuit was created to have a gain of about 1000. This amplifies the human input signal so that the EKG can be more easily displayed and read to help determine the BPM later on.

This gain was created with stage 1 having a gain of 50 and stage two having a gain of 20. It is essential to try and have the gains of each stage be close together and not unequal. The equations in Step 2.

Step 2: INA Calculations

Step 3: Build Notch Filter (60Hz)

Design the second part of the circuit as shown in the images above. Have a wire going from pin 6 of the last opAmp in step 1 to the start of this circuit design. The Notch Filter section of the circuit was created to filter out a frequency of 60Hz, to have a quality factor of around 8 and a capacitor value of 0.1uF, and did the calculations shown in Step 4.

Step 4: Notch Equaitons

Step 5: Build Lowpass Filer (Fc=150Hz)

Build the last part of the EKG circuit as shown in the images above. Connect a wire from pin 6 in the opAmp in step 3 to the start of this circuit design. This section is the lowpass filter section that filters out frequencies over 150 Hz. 150Hz was chosen to filter out the noise and high frequencies do not contribute to the QRS complex which is read to determine the BPM. The values for this part were chosen using the math in Step 6.

Step 6: Lowpass Equations

Step 7: Test It Out

Now that the circuit is complete, it can be tested using an oscilloscope to read the EKG. To accomplish this first hook up the 2, 9-volt batteries as shown in the images above. Then use a BNC to double alligator clip cable to hook the output wire (shown in the long yellow wire) from pin 6 in the last opAmp to the red alligator clip and ground the black alligator clip. Then attach the other end of the BNC cable to the oscilloscope.

Next, attach the 3 EKG electrode leads to the sticky pads and place them on the wrists and ankles as shown in the diagram. Then attach the other end of the COM electrode to the ground of the circuit using an alligator clip to alligator clip wire. Using another alligator clip to alligator clip wire attach the G1 electrode to pin 3 on the first opAmp from Step 1: INA, do the same thing with the G2 electrode connecting it to pin 3 on the second opAmp from Step 1: INA. Once everything is connected an EKG signal should show on the oscilloscope (make sure you are sitting still).