Acquisition and Analysis of an ECG (electrocardiography) Signal

1. Acquiring an ECG Signal

1. Adjust the voltage of the sources to +5 V and -5 V and connect them in series.
2. Build the circuit shown in Figure 4. Calculate the values of the resistors and capacitors. For the high pass filter, the cut-off frequency should be 0.5 Hz. The capacitor value should be chosen from the table below (according to availability).

Available Capacitor Values (µF )
0.001	1	100
0.022	2.2	220
0.047	4.7	470
0.01	10	1000
0.1	47	2200

3. Place electrodes on the right arm, left arm and right leg (this is reference) of the patient, and connect them to the circuit.
4. Use the oscilloscope to view the ECG signal (V_o). Press Auto Set and adjust the horizontal and vertical scales as needed. You should be able to see the R peaks despite of the noise in the signal.

2. Displaying the ECG signal using Instrumention Software

1. In this demonstration we used LabVIEW. Write a program that displays the ECG signal using a graphical interface for configuring measurements and a waveform graph. Once an analog input has been selected, configure the program with the following settings:

• Signal input range >> Max = 0.5; Min = -0.5
• Terminal Configuration >> RSE
• Acquisition mode >> continuous
• Samples to read = 2000
• Sampling rate = 1000

2. Acquire the ECG signal and observe the waveform. You will see a signal similar to Figure 1.
3. Adjust the scale of the x-axis to show time in seconds.
4. It is often necessary in instrumentation to amplify the signal of interest to a specific amplitude. Create a gain control and set it so that the amplitude of the ECG is 2 Vp.

3. Analyzing the ECG signal

In this section, an ECG signal will be filtered and analyzed to determine the heart rate. The following block diagram shows the components of the program.

1. Use a waveform graph to display the signal.
2. Evaluate the spectrum of the signal using the Amplitude and phase spectrum subvi (in Signal processing → Spectral) and display its magnitude using a waveform graph. The horizontal axis corresponds to frequency. It is discrete because the computer uses a Fast Fourier Transform (FFT) algorithm to calculate the spectrum of the signal. The frequency goes from k = 0 to k = (N-1)/2, where N is the length of the sequence, in this case 4000. To calculate the corresponding analog frequency, use the following formula:
   
   where fs is the sampling frequency. Note that most of the energy of the signal is in the low frequency range and also that there is a peak of high intensity in the medium frequency range. Calculate the frequency of that peak using the formula provided above.
3. Implement a low-pass filter using Butterworth of Chebyshev functions. Choose a cut-off frequency equal to 100 Hz. Make sure that the filter provides an attenuation of at least -60 dB/decade in the stopband.
4. Connect the output signal of the read from spreadsheet subvi to the input of the low-pass filter.
5. Implement a stop-band filter using Butterworth or Chebyshev functions. The objective is to reduce the 60 Hz interference without modifying the other frequencies. Try border frequencies close to 60 Hz.
6. Connect the output of the lowpass filter to the input of the stopband filter.
7. Find the peaks using the peak detector subvi (it is located in Signal processing → Sig Operation). For the threshold, look at the signal's amplitude and choose the most appropriate value.
8. Extract the locations of the peaks using the index array subvi (in Programming → Array).
9. Subtract the lower position from higher position, then, multiply by the sampling period T = 1/fs to obtain the RR interval.
10. Calculate the reciprocal and adjust units and place an indicator to display the BPM.