Summary of Design and Implementation of a 12 Lead Portable ECG
This 2016-2017 McMaster University project, led by Christina Riczu and colleagues under Dr. DeBruin, created a compact, battery-powered 12-lead ECG that won first place in the biomedical category. The device focuses on signal conditioning, analog-to-digital conversion, and data transfer to a host for graphical display. Key design priorities included mechanical robustness, rechargeable battery life, safe electrical isolation via a DCDC converter, and a clean user interface.
Parts used in the Compact 12-Lead Portable ECG:
- Battery
- Isolated DCDC converter
- 3.3V linear regulator
- Contec ECG cable with DB15 connector
- Button snap connectors
- 3M Red Dot silver-silver chloride electrodes
- Continuity meter (for pinout determination)
- Circuit boards designed for reusability
During the academic year of 2016-2017 at McMaster University, in conjunction with Dr. DeBruin, Christina Riczu, Thomas Phan and Emilie Corcoran, we developed a compact, battery powered, 12-lead electro-cardiogram. The project won 1st place in the biomedical category at the ECE Capstone Poster Day.
The final report we handed in for the course is attached at the end of this post and includes background information, a design overview, schematics and bill of materials for the hardware we developed. This post will introduce the project and serve as a personal account of the considerations and problems associated with the portion of the project that I focused on.
Before we begin I should note that if you decide to replicate this design or develop a derived design that you are doing so at your own risk. Attaching a device with a low impedance connection to a person can be dangerous. We developed this project under a supervisor with experience developing and maintaining such devices and protocols were put into place to ensure safety.
System Design
My goal during the project was to develop the most compact device possible while providing robust mechanical design, usable battery life, convenient connectivity, a clean user interface, and good analog performance. For this to occur components must be selected carefully across electrical, mechanical and software domains. In addition to this I wanted the design to be reusable and extendable in the future, choices were made to allow the circuit boards to be used for different applications beyond this project.
Overview
The purpose of the hardware is to perform signal conditioning, analog to digital conversion and transfer the data to a host.
The software processes the data and displays it in a graphical interface.
Patient Interface
Low cost cables for connecting skin electrodes to a data acquisition system is available on EBay. If you search for Contec ECG cable on EBay you will find them for about $20. They interface with a DB15 connector and come terminated with button snap connectors for connecting with commonly available ECG electrodes such as the 3M Red Dot silver-silver chloride electrodes.
We could not find a pin out for the cable so we used a continuity meter to figure it out, I include it here in case anyone needs it.
Electrical
We begin with electrical component selection as this dictates the direction we must go for mechanical and firmware integration.
Isolated Power
As mentioned before, electrical isolation is required for safety. Batteries can be used to provide a simple isolated supply, however, requiring the replacement of batteries puts an unnecessary burden on the user. We wanted the device to be convenient which meant we wanted the device to be battery powered and rechargeable even during use. This necessitated an isolated DCDC converter between the battery and electronics connected to the patient for safety.
The system was developed to allow the DCDC converter to be sized smaller thus allowing the device to fit into a smaller box. From the top-level schematics, you can see that some components are powered by the battery through a 3.3V linear regulator and only the components with a direct connection to the patient is powered by the isolated DCDC.
Read more: Design and Implementation of a 12 Lead Portable ECG
- What was the primary goal of the system design?
The goal was to develop the most compact device possible while ensuring robust mechanical design, usable battery life, convenient connectivity, a clean user interface, and good analog performance. - How does the device ensure electrical safety?
Safety is ensured through an isolated DCDC converter between the battery and patient-connected electronics, developed under supervision with strict protocols. - Can the device be used while charging?
Yes, the device was designed to be battery powered and rechargeable even during use. - What type of cables were used for connecting skin electrodes?
Low cost Contec ECG cables available on EBay were used, which interface with a DB15 connector and terminate with button snap connectors. - Which specific electrodes are compatible with this setup?
The system uses commonly available 3M Red Dot silver-silver chloride electrodes connected via button snap connectors. - How was the cable pinout determined?
The team could not find a pinout online so they used a continuity meter to figure it out. - What components are powered by the isolated DCDC converter?
Only the components with a direct connection to the patient are powered by the isolated DCDC converter. - What components are powered by the 3.3V linear regulator?
Some components are powered by the battery through a 3.3V linear regulator rather than the isolated converter. - Does the hardware support future applications?
Yes, choices were made to allow the circuit boards to be reused for different applications beyond this specific project. - What software function does the host computer perform?
The software processes the transferred data and displays it in a graphical interface.
