During the academic year of 2016-2017 at McMaster University, in conjunction with Dr. De Bruin, 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.
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.
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.
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.
We begin with electrical component selection as this dictates the direction we must go for mechanical and firmware integration.
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.
The use of a switching converter to power an analog front end meant that we need some noise filtering. The ROE-3.305S isolated DCDC converter we are using comes from RECOM’s Econoline series of switching converters that are designed to be small but it outputs a lot of ripple because the designers did not include a lot of output filtering. A common mode choke will cut down on any common mode noise generated by the isolated DCDC and the LC output filter will filter out much of the switching transients generated by the 80 kHz switching frequency of the ROE-3.305S. The LC back feed filter is the filter recommended by RECOM to pass some EMC compliance standard test, no calculations were performed by us for back feed filter.
Read More: Design and Implementation of a 12 Lead Portable ECG