Why do ecg machines contain amplifiers

The FDH low-leakage diodes are used to protect the inputs of the instrumentation amplifier. Whenever the voltage in the circuit exceeds 0.

Figure 5. Protection circuit. The second stage is the instrumentation amplifier, IA, which uses three operational amplifiers op-amp. There is one op-amp attached to each input to increase the input resistance. The third op-amp is a differential amplifier. This configuration has the ability to reject ground-referred interference and only amplify the difference between the input signals.

Figure 6. Instrumentation amplifier. The third stage is the high pass filter, which is used to amplify a small AC voltage that rides on top of a large DC voltage. The ECG is affected by low frequency signals that come from patient movement and respiration. A high pass filter reduces this noise. High pass filters can be realized with first-order RC circuits. Figure 7 shows an example of a first order high-pass filter and its transfer function.

The cut-off frequency is given by the following formula:. 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. Electrocardiographs record cardiac activity of the heart and are used to diagnose disease, detect abnormalities, and learn about overall heart function.

Electrical signals are produced by contractions in the heart walls which drive electrical currents and create different potentials throughout the body.

By placing electrodes on the skin, one can detect and record this electrical activity in an ECG. ECGs are non-invasive, making them a useful tool to assess how well a patients heart is performing, such as by measuring how well blood flows to the organ.

This video will illustrate the principals of ECGs and demonstrate how to acquire, process, and analyze a typical ECG signal using a biopotential amplifier. Other biomedical applications that utilize electrical signal processing to diagnose disease will also be discussed.

To understand the principles of an ECG, let's first understand how the heart produces electrical signals. For a normal, healthy heart, at rest, an ECG displays a series of waves that reflect the different phases of a heartbeat.

The ECG starts in the sinoatrial node, also known as the SA node, which is located in the right atrium and acts as a pacemaker in the heart. The electrical signals cause atrial contraction forcing blood into the ventricles. This sequence is recorded as the P wave on the ECG. This signal then passes from the atria across the ventricles, causing them to contract and pump blood to the rest of the body.

This is recorded as the QRS complex. Finally, the ventricles relax and this is recorded as the T wave. The process then begins again and is repeated for every heartbeat. Notice that the QRS wave is much larger than the P wave, this is because the ventricles are larger than the atria. Meaning they mask the relaxation of the atria or the T wave.

Other processes in the body, like respiration or muscle contractions, can interfere with the ECG measurement. As can currents from the circuitry used to obtain them. Often, the electrical signals that the ECG is attempting to record are quite weak. Therefor, a biopotential amplifier is used to increase their amplitude which allows them to be further processed and recorded.

There are three main components to the biopotential amplifier, the patient protection stage, the instrumentation amplifier, and the high pass filter. As the main suggests, the patient protection circuit uses a combination of resistors and diodes to protect, both, the patient and the machine and equipment.

The resistors limit the current that flows through the patient, where as the diodes keep the current flowing in the correct direction. The next stage is the instrumentation amplifier, which amplifies the difference between the inputs from each electrode.

It is composed of three operational amplifiers. Two to increase the resistance from each input, and the third to amplify the difference between the input signals. The last stage is the high pass filter, which reduces the noise and filters out low frequency signals arising from patient movement or respiration.

Now that you know how an ECG is measured, let's see how to construct a biopotential amplifier and process the data to get a clean ECG signal. Having reviewed the main principals of electrocardiography, let's see how to build a biopotential amplifier and acquire an ECG signal. To begin, first gather a proto-board, an AD instrumentation amplifier, and all necessary circuit components. Then, calculate the values of all of the resistors and capacitors in the circuit using the following equation.

Then, plug in the capacitor value to determine the resistance. Next, build a biopotential amplifier according to the provided diagram. Here is what the final circuit should look like.

Attach three wires with alligator clips to the binding posts of a DC power supply, then turn on the power source. Adjust the voltage to plus five volts and minus five volts, and connect the the wires, in series, to the circuit.

Now, use an alcohol prep pad to wipe the patients right wrist, left wrist, and right ankle. Add conductive adhesive gel to the electrodes before placing them on the patient. Then, connect the electrodes to the circuit using wires with alligator clips. Turn on the oscilloscope and acquire the ECG signal. Adjust the horizontal and vertical scales as needed. With these adjustments, you should be able to see the R peak of the wave form.

Connect the circuit to the PXI chassis, then open the instrumentation software and, either, use or write a program that will display the ECG signal and a wave form graph. Configure the data acquisition interface with the following settings. Label the scale of the x-axis to display time and seconds, then display the ECG signal as a waveform. If the signal needs to be amplified, create a gain control and set it so that the amplitude of the ECG is two VP.

Now that we have demonstrated how to acquire an ECG signal, let's see how to analyze the results. Here is a representative ECG signal. This signal needs to be filtered. To transform this signal, first, select Signal Processing then Spectral on the menu.

A Fast Fourier Transform algorithm calculates and plots the spectrum of the signal displaying the frequency as discreet values on the horizontal axis. Most of the energy in the signal is at low frequencies. But, there is a high intensity peak in the medium frequency range, which is assumed to be noise.

Frequency is plotted as k on the horizontal axis and goes from zero to N minus one over two, where N is the length of the sequence. For this experiment, N equals 2, Calculate the analog frequency for each k value using the following equation, where f s is the sampling frequency and determine the frequency of the high intensity peak based on the FFT graph.

Then, create a low pass filter with a cutoff frequency of hertz. Use, either, the Butterworth or Chebyshev function to filter the signal, which should attenuate at least 60 decibels per decade in the stop band. Connect the output signal of the data sub VI to the input of the low pass filter.

This filter removes the extraneous high frequency waves of the ECG. Now, create a Bandstop filter and set the cutoff frequencies at around 55 and 70 hertz. The ECG generated by each cardiac cycle is summarized on Table 1. Figure 2 Basic structure of the heart. Duration at 75 bpm 0. Ventricular diastole. Semilunar valves close. Ventricular filling. Semilunar valves closed. Ventricular systole. Semilunar valves open. Blood pumped into aorta and pulmonary artery.

Table 1 Duration and characteristics of each major event in the cardiac cycle. The ECG is converted into electrical voltage by electrodes. Figure 3 A disposable surface electrode.

The cardiac mechanism of ECG is shown on Figure 4. In the top figure, the electrocardiogram ECG initiates the cardiac cycle. The cardiac sounds are also shown. The bottom figure shows that ejection occurs when the pressure in the left ventricle exceeds that in the arteries.

Once the electrodes convert the ECG into electrical voltage, these voltage can be fed into an instrumentation amplifier, and then be processed. Figure 4 The ECG cardiac cycle. We measure the ECG by connecting two electrodes on the right and left chest respectively, as shown on Figure 5. The body should be connected to ground of the circuits, so that we connect the leg to the ground.

If the body is not grounded, no signal would be obtained. Figure 5 Simplified ECG recording system. Circuits of the ECG system. Most ECG amplifier types are separated from the main power circuits, because any crossover could cause a surge leading to an electrical shock. An optical isolator is often used to prevent such a problem from occurring. Typically built into the power circuit, the primary amplifier generates a current that can be used for an output.

Some ECG machines use a paper-strip recorder to display the readings. Other types transfer data to a computer, magnetic tape, or an oscilloscope that can display the activity of electrical signals. The data are usually converted to a digital format before being transferred to an output device. An ECG, therefore, generally includes an analog-to-digital converter in addition to an amplifier.

Three types of signals from the heart can be processed through an ECG amplifier.