ECG Sensor

The measurement of the ECG involves the connection of between twelve and fifteen leads to a patient’s chest, arms and right leg via adhesive foam pads. It records a short sampling of the heart’s electrical activity between different pairs of electrodes. This study develops a sensor board that provided continuous ECG monitoring by measuring the differential across a single pair of electrodes.

2.2.1.1. ECG

The ECG measures the electrical activity of heart. The beating heart generates an electric signal that can be used as a diagnostic tool for examining the functions of the heart. This electric activity of the heart can be approximately represented as a vector quantity. Cardiologists have developed a simple model to represent the electric activity of the heart. In this model, the heart consists of an electric dipole located in the partially conducting medium of the thorax. This dipole moment, knew as the cardiac vector, is shown in Fig. 2-2

Fig. 2-2 Cardiac vector diagram [11]

The cardiac vector is defined as including 12 leads to form the exact ECG. However, in our case this is a portable device that cannot employ the whole 12 leads in our sensor, so we choose only lead II (shown in Fig. 2-3), which is considered to be a typical example of ECG monitoring. The lead II ECG waveform is also considered to provide typical clinical data for diagnosing heart disease. In fact, hospitals widely use the portable lead II ECG machine in the emergency ward.

Fig. 2-3 Lead II ECG diagram [11]

2.2.1.2. ECG Waveform

There are two premises in the ECG waveform. One is that the cardiac muscle is formed by excitable nerves that express electrical signals (voltage). The electrical signals are termed the ‘action potential’ of the cardiac nerve cells. The other premise is that the cardiac muscle (the atrium and the ventricle) cells systole and diastole together, which produces every beat of the heart. If they do not systole and diastole together, the heart beat is irregular -- so called cardiac fibrillation -- and the person die.

The explanation of the ECG waveform is that electric stimulation is activated by the SA node; the SA node expresses an action potential, as shown in the first curve of Fig. 2-4. Then the AV node receives the stimulation and expresses its action potential, which is shown on the third curve of Fig. 2-4. Then the signal (electric stimulation by the SA node) keeps conducting to the atrial muscle and then the ventricular muscle.

Cardiologists have determined that the P wave of the ECG waveform is mainly contributed by the atrial muscle, as shown on the second curve of Fig. 2-4. The QRS complex and the T wave are mainly contributed by the ventricular muscle, seen in the last curve of Fig. 2-4. Because the cardiac muscle is formed by excitable nerves, the electrical stimulus from the SA node is propagated to the other part of the heart. According to the difference between propagation time and action potential of every part of the heart, the ECG wave form is decomposed to the P wave, QRS complex, and T wave. Every component of the ECG waveform is shown in Fig. 2-4. Note that the action potential of bundle branches lags behind the action potential of the AV node by 100 ms., so it takes 100 ms. to pump blood from the ventricle to the atrium.

Fig. 2-4 Components of ECG waveform diagrams [11]

2.2.1.3. Measurement for ECG Signal

Because the ECG has very small signals, at the range of a few mV (usually less than 10) it is often interfered with by the 60 Hz noise created by the power line or the human body. In this case, we knew that signal conditioning is very important for bio-potential measuring. The best-fitting conditioning for the bio-potential signal can make it much simpler to do further signal processing. Therefore, it is necessary to employ an instrumentation amplifier to reduce the 60 Hz noise and to amplify the ECG signals we are interested in. Then, we filter the low frequency DC noise by a low pass filter and amplify the signal by a gain and filter stage. The fault often occurs that the output waveform is very sensitive to

motions like breathing and even slight movement of the human body, so it is necessary to add an anti-motion artifact stage to isolate the signal from motion artifacts. To create clearer signals with less 60 Hz noise interference on the baseline of the ECG waveform, we apply a DRL (Driven Right Leg) circuit to reduce the 60 Hz noise. The block diagram of the ECG sensor is shown in Fig. 2-5.

Fig. 2-5 Block diagram of ECG sensor

2.2.1.4. Circuit Design for the ECG Sensor a. The Different Input Stage

The differential input stage [12] is shown in Fig. 2-6. Here we choose a micro-power consumption instrumentation amplifier AD627 (Analog Device). Its max supply current is only 85 μA, and it has a wide power supply range from +2.2 V to ±18 V. It also provides gain for the signal.

The gain is adjusted by the resister R^G, in the term:

5 200

−

= Ω Gain R^G K

1 R G1

Fig. 2-6 semantic diagram of the differential input stage for the ECG sensor We use

b. The Gain and Filter Stage

In the gain and filter stage, shown in Fig. 2-7, we choose OP296 for the operation amplifier. Its max supply current is only 85 μA and consists of 2 OP amplifiers in one chip. For the sake of minimizing the scale, ICs with SMD packages are used. In this stage, the first OP amplifier serves as a gain stage where G² = 20, and the second OP serves as a low pass second-order Butterworth filter where the cut-off frequency Fc = 75 Hz.

1K

Fig. 2-7 Semantic diagram of the gain and filter stage for the ECG sensor

c. The Anti-Motion Artifact Stage

As for the anti-motion artifact stage, a second-order band-pass filter [12] is used, as in Fig. 2-8, and

the resonance frequency Fr =

Fig. 2-8 Semantic diagram of the anti motion artifact stag for the ECG sensor

d. Driven-Right-Leg (DRL) Circuit

In the last stage, the Driven-Right-Leg (DRL) circuit [8] (Fig. 2-9), we use a small capacitor with a

value of 10 nF to block the 60 Hz noise, and an auxiliary OP amp to feed the noise back to the human

Fig. 2-9 Semantic diagram of the Driven-Right-Leg (DRL) circuit for the ECG sensor