ECE 480 Design Team 6 Electrocardiography and Design

Transcription

1 ECE 480 Design Team 6 Electrocardiography and Design Alex Volinski November 16 th, 2012

2 Executive Summary Recently there has been a large increase in consumer demand for a new and functional ECG (Electrocardiograph) demonstration board. Texas Instruments currently has an outdated ECG demonstration board that needs to be updated to the new technical standards. This application note is focused towards teaching the reader how an ECG is supposed to work in its current incarnation in the medical field, as well as show the methods and design techniques that go into the creation of an ECG demonstration board. Keywords ECG, INA333, OPA2333, micro-controller, amplification, biopotential, Right Leg Drive

3 Table of Contents Executive Summary...1 Table of Contents...2 Understanding an ECG...3 Designing the Circuit...4 Power...7 References and Appendix...8

4 Understanding the ECG Before delving into designing an Electrocardiograph circuit, it is important to understand how an ECG machine actually works. The first concept to understand is a biopotential. Each cell in the human body acts like a little battery, having stored up potential energy. This can be measured as a voltage, and a biopotential is the difference between voltages measured between one cell and another. This concept is used in an ECG machine through the use of electrodes. Electrodes are attached in multiple locations to the patient, including the left arm, the right arm, the right leg, and multiple leads running to the chest. These electrodes have the ability to convert the ionic signals from the body and the cell biopotential into electrical signals on wires. The ECG machine itself is designed to be the measure of the heart working as an electrical generator, being able to determine the heart rate of the patient hooked up to it.

5 Designing The Circuit The first concept of designing an electrocardiograph demonstration board is understanding the principle of a right leg drive circuit. This circuit takes advantage of multiple concepts, but the main idea behind the circuit is taking inputs from the right arm and the left arm of the patient through leads, and amplifying that with feedback from the right leg of the patient to drive the circuit. A simple example of this can be seen in figure 1. Figure 1. Driven Right Leg Circuit As shown in figure 1, an instrumentation amplifier is used to create the output needed for an ECG board. In this case and the case for any board produced by Texas Instruments, the INA333 amplifier is used. This 3-op amp designed chip is great to use because of its very low offset voltage and its high common mode voltage rejection. Having good common-mode rejection is essential in the design of an ECG circuit, because the human body works as an antenna. This means that it is producing common-mode interference that needs to be canceled out to produce an accurate reading.

6 Once the concept of the right leg drive circuit is understood, the enhancing of the circuit is the next step. This step involves making the design better to get the outputs desired, as well as making the ECG easier to use and safer for its users. One such addition is to add series resistance to the input leads of the circuit. This will allow for limiting the input current to the circuit. Another technical addition will be to filter the inputs and the outputs of the circuit to get better results. This can be achieved by putting a resistor and capacitor to ground. A last addition to the circuit is input buffers and circuit amplifiers. This is shown specifically for the right leg in the above example, by using another Texas Instruments product in the OPA2333 operational amplifier. The input of the circuit uses these op-amps to buffer the input signal, while other parts of the circuit use these op-amps to enhance the signals in the circuit. Figure 2 shows a finalized prototype of what an ECG circuit could look like after all the design considerations.

7 Power Figure 2. Electrocardiograph Example Circuit When dealing with power in this type of circuit, there are multiple components that need different voltage levels to operate at their peak performance. Most demonstration boards, like the one above and the one being created for the design project, a 9 volt battery is being used for power. However, the INA333 and OPA2333 amplifiers both need around 5 volts as their voltage to operate correctly and at their best efficiency. To solve this problem, a buck converter is used drop the 9 voltage input down to 5 volts. This can then be spread throughout the circuit where it is needed.

8 References A precision low-level DAS/ECG Cardiotachometer Demo Board, a Texas Instruments technical presentation created by John Brown. Analog Fundamentals of the ECG Signal Chain, a Texas Instruments technical presentation prepared by Matthew Hann. Appendix Data Sheet for INA333 n_precision_amps%252520&%252520linear&247sem Data Sheet for OPA2333