Oscilloscope Electrocardiogram Board Design

Introduction

The electrocardiogram is a surface measurement that captures the electrical potential created by the electrical activity in the cardiac tissue. This flow of electrical current, carried by ions, is the signal responsible for initiating the contraction of cardiac muscle fibers, ultimately driving the heart's pumping action. The ECG output trace is generated by calculating the potential difference between readings from L1 and L2. Meanwhile, the L3 connection establishes a common reference point between the body and the recording device, usually an oscilloscope. Establishing a link between the ECG trace and the electrical events occurring within the heart is termed the "inverse problem of electrocardiology." It involves solving for the electric sources responsible for generating the electrical potential detected on the body's surface.

Circuit Description of the ECG Electronic

Power is supplied to the ECG circuit, the amplifier by two 9V batteries, and the various terminals are soldered on a switch available on the chassis for up and down powering.

The Input Circuitry

During ECG monitoring, the amplifier receives signals from self-adhesive electrodes attached to the subject's body. The connecting cables (represented as blue, red, and green circles in the diagram) must be short and well-shielded to ensure signal quality. RG-174 50 Ohm coaxial cables with Lemo connectors should be considered due to their durability, slimness, and ease of use with the aluminum box housing the circuit.

A diode protection is introduced for the amplifier inputs to address safety concerns. While the circuit runs on two 9-volt batteries, potential power surges from the connected oscilloscope or computer could pose risks. The diodes direct any excess voltage to the ground if it exceeds 0.6 volts, which is well above the heart's electrical signals and should not affect circuit performance. Each input requires two diodes, one for positive and one for negative voltage swings. Alternatively, the circuit could be redesigned with optoisolator components for enhanced safety, providing complete galvanic isolation (up to several thousand volts) between the electrodes and the power supply.

The Output Circuitry

A bulkhead BNC connector is installed on the circuit box to enable the output signal to interface with different devices. This allows a direct connection to an oscilloscope using a properly terminated 50-ohm coaxial cable. Necessary modifications are also done to the output section of the circuit to accommodate connection to the DrDAQ ADC. The DrDAQ ADC accepts voltages in the 0 to 5 volts range, which isn't a perfect match for the ECG signal that contains both positive and negative pulses.

An AC coupling is introduced by adding a 1µF capacitor to address this issue. A pull-up resistor is also included and connected to the +9volt supply, which raises the DC level to approximately 2 volts. A 680k ohm resistor is also used for that purpose, considering that the resistance across the ADC (which forms the other part of the voltage divider) is measured to be around 180k ohms.

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