CE2.1 I was student of the Biomedical Department of Sir Syed University of Engineering & Technology, Karachi having roll number 057-BM-2005. During my study of third year, I hada subject named “Application of Filter Design in Biomedical Engineering”. I worked on a class project during which I was required to use different type of filters. I decided to design Analog ECG using instrumentation amplifier. The project startedin June 2007 and was completed in December 2007.
CE2.2 The purpose of the project is to understand how these filter designing works in Medical devices plus how analog signal are converted into digital signals in the industry.Electrocardiogram (ECG or EKG) is recording of electrical activity of heart over some time period and it produces electrocardiograph. It is a noninvasive recording via skin electrodes. Electrical impulses in heart originate in senatorial node and travel through heart muscle where they impart electrical initiation of systole or contraction of heart. Electrocardiograph indicates overall rhythm of heart and weaknesses in different parts of heart muscle. It is the best way to measure and analyze abnormal rhythms of heart, particularly abnormal rhythms caused by damage to conductive tissue that carries electrical signs, or abnormal rhythms caused by levels of dissolved electrolytes, such as potassium, that are too high or low. In myocardial infarction (MI), ECG can detect broken heart muscle. But it can only identify damage to muscle in certain parts, so it can’t rule out loss in other areas.
CE2.3 The electro cardiographer uses two types of leads, unipolar and bipolar. The former have an different electrode at the center of the Einthoven’s triangle (which can be linked to the “neutral” of a wall socket) at zero potential. The direction of these leads was from the “center” of the heart radically outward. These include the precordial (chest) leads and augmented limb leads—VR, VL, & VF. The bipolar type, in contrast, has both electrodes at some potential, with the direction of the corresponding lead being from the electrode at lower potential to the one at higher potential, e.g., in limb lead I, the direction is from left to right. These include the limb leads—I, II, and III.
– Lead I is a dipole with the negative (white) electrode on the right arm and the positive (black) electrode on the left arm.
– Lead II is a dipole with the negative (white) electrode on the right arm and the positive (red) electrode on the left leg.
– Lead III is a dipole with the negative (black) electrode on the left arm and the positive (red) electrode on the left leg.
CE2.4 Leads aVR, aVL, and aVF are augmented limb leads. They were derived from the same three electrodes as leads I, II, and III. However, they view the heart from different angles (or vectors) because the negative electrode for these leads is a modification of Wilson’s central terminal, which was derived by adding leads I, II, and III together and plugging them into the negative terminal of the EKG machine. This zeroes out the negative electrode and allowed the positive electrode to become the “exploring electrode” or a unipolar lead. This was possible because Einthoven’s Law states that I + (-II) + III = 0. The equation can also be written I + III = II.
An additional electrode (usually green) is present in modern four-lead and twelve-lead ECGs. This is the ground lead and is placed on the right leg by convention, although in theory it can be placed anywhere on the body. With a three-lead ECG, when one dipole is viewed, the remaining lead becomes the ground lead by default.
CE2.5 A typical ECG traced of a normal heartbeat (or cardiac cycle) consists of a P wave, a QRS complex and a T wave. A small U wave is normally visible in 50 to 75% of ECGs. The baseline voltage of the electrocardiogram is known as the isoelectric line. Typically the isoelectric line is measured as the portion of the traced following the T wave and preceding the next P wave.
Personal engineering Activity:
CE2.6 To understand the working principle of amplifier, I did a research work on studying different amplifier, which have high precision, low noise, instrumentation amplifier designed primarily for use with low level transducers, including load cells, strain gauges and pressure transducers & then I selected AD624. It has an outstanding combination of low noise, high gain accuracy, low gain temperature coefficient and high linearity make the AD624 ideal for use in high resolution data acquisition systems. The AD624 does not need any external mechanisms for prêt rimmed gains of 1, 100, 200, 500 and 1000. Additional gains such as 250 and 333 can be programmed within one percent accuracy with external jumpers. A single external resistor can also be used to set the 624?s gain to any value in the range of 1 to 10,000. In this way we can amplify input coming from ECG leads or stimulator having selecting a gain of 1000.
CE2.7 The Butterworth low-pass filter offers maximum pass band flatness.It plots the gain reply of different orders of Butterworth low-pass filters versus the normalized frequency axis, ? (? = f / fC); the higher the filter order, the longer the pass band flatness.
CE2.8 A band-rejection filter is used to suppress a certain frequency rather than a range of frequencies. Two of the most popular band-rejection filters are the active twin-T and the active Wien- Robinson circuit, both of which are second-order filters. To cause the transfer function of a second-order band-rejection filter, replace the S term of a first-order low-pass response with the transformation.
The twin-T circuit has the advantage that the quality factor (Q) can be varied via the inner gain (G) without modifying the mid frequency (fm). However, Q and Am cannot be adjusted independently.
The transfer function of the active twin-T filter is
Comparing the variables of with provides the equations that determine the filter parameters
To set the mid frequency of the band-pass, specify fm and C, and then solve for R
The above mentioned formulas are used by using Resistance and capacitor values. Different resistance used such as 100 ohms or 4.7k Ohms plus using of Capacitor of values 0.5% pF or maximum 2% PF to set the frequency range plus the set the inner gain and to control the gain.
CE2.9 After obtaining the Analog signal my major challenge is to convert the ECG signal into Digital signal. I designed the clamper circuit to convert the signal. The clamper circuit was used to set the dc level of the ECG to provide compatibility for the digital circuit. Analog to digital converter ADC0804 which can accept the input of 2 volts to 5 volts, the ECG analog signal must have dc level of 2 volts to 5 volts for sampling so that’s why we need clamper circuit to increase the dc level of ECG analog signal. Inverting summing amplifier of unity gain is used it is design according to the filter output.
CE2.10 After increasing the DC level of the ECG analog Signal I used CMOS IC ADC 0804 8-bit successive approximation A/D Converter. This ADC appear like memory locations or I/O ports to the microprocessor and no interfacing logic is needed. Differential analog voltage inputs allow increasing the common-mode rejection and offsetting the analog zero input voltage value. In addition, the voltage reference input can be adjusted to allow encoding any smaller analog voltage span to the full 8 bits of resolution.
CE2.11 As I was using CMOS IC I need to handle those IC carefully as they are sensitive and as per instruction I need to I should not carry a static charges. When I was using power dual power supply I need to make sure that finger guards are used to avoid current shock. When I was about to use the oscilloscope in lab I was instructed to make sure that I use insulated work surface; keep one hand in your pocket when using a scope probe; remember that ac power may be present in the equipment even when it is turned off.
CE2.12 I have long discussion with my teachers while understanding of the filter plus the how to obtain analog ECG. He guide me do research work using different research article and free data base of article to understand the working aspects.
CE2.13 In the end of the project I was asked to submit the complete report of the project for which I have to explain them everything in detail such designing of the filters plus what problem I faced and how I overcome them.
CE2.14 It was a really challenging of me as it was not easy to design an analog ECG system. Then the another interesting aspect is interoperability how analog signal is changes into digital signal, During the initial testing oscilloscope showed inverted ECG display, then after doing some troubleshooting of Filters, that the ground circuit was not performing its activity properly, then again I have to go through dozens of research paper to understand, why the volts are going into ground. After finding some errors in my clamper circuit I fixed and finally it gives the correct result on oscilloscope.