Ec stingy

PCB Design

Directory overview

This directory includes several files with project metadata, but the key files are:

pcb.kicad_pro: KiCad 6 project file. Use this to properly load and open the schematic and layout files.

pcb.kicad_sch: The schematic file.

pcb.kicad_pcb: The layout file.

libraries/: Contains symbols, footprints, and 3D models for components used in the project that are included in the KiCad library.

manufacturing/: Contains a bill of materials, a component placement file, and gerber files required to get the board produced.

V for Via: Establishing a workflow

We heavily based the overall design and microcontroller layout on the KiCad 6 STM32 tutorial, with several notable additions:

• The AD8232 and all of its associated circuitry (a near replica of the SPICE model in this project)
• The CP2102N USB-to-UART bridge controller, which acts as both a voltage regulator and PC interface in this design

We also added several peripherals to facilitate use and debugging:

• A micro-USB port, to power the board
• A 3.5 mm jack, to plug the electrode cable into
• A boot switch, to reset the microcontroller
• A serial wire debug pin header, to modify the firmware
• An ECG signal pin header, to inspect the analog signal
• LEDs to indicate a power connection and heartbeat readings, respectively

The schematic ends up falling roughly into 5 sections:

Before approaching the layout, we first consult PCB layout guidelines provided by the various ICs used. From the AD8232 datasheet:

Keep all of the connections between high impedance nodes as short as possible to avoid introducing additional noise and errors from corrupting the signal. To maintain high CMRR over frequency, keep the input traces symmetrical and length matched. Place safety and input bias resistors in the same position relative to each input. In addition, the use of a ground plane significantly improves the noise rejection of the system.

And from the STM32 datasheet:

Each power-supply pair (VDD/VSS, VDDA/VSSA etc..) must be decoupled with filtering ceramic capacitors as shown above. These capacitors must be placed as close as possible to or below the appropriate pins on the underside of the PCB, to ensure the good functionality of the device. The 10 nF capacitor must be ceramic (good quality) and it must be placed as close as possible to the chip.

With these principles in mind, a heavy draw of inspiration from the layout of the SparkFun AD8232 breakout, and a modest amount of trial-and-error, the resultant two-layer board layout came out like so:

How cost-effective is it, really?

Time and effort were put into ensuring that the components chosen were economical and readily available to reduce the manufacturing cost as much as possible. Below is a breakdown of the per-board manufacturing costs at the time of writing this, ordering from JLCPCB (minimum quantity of 5):

Although it's difficult to directly compare this device to commercially-available products, the closest would be a Holter monitor like the Prince 180B1 from Heal Force which currently retails at $140 CAD. The total cost of our board comes to $11.56 CAD, which is roughly 9% of the market equivalent. Not bad!

The cutting room floor

Commission a board review: Despite passing KiCad's electrical and design rule checks, a third-party formal review could validate the schematic and layout. This review would also ensure adherence to best practices on differential trace length matching, passive component sizing, and decoupling capacitor layout, helping to prevent potential costly or time-consuming errors before manufacturing.

Add more safeguards: This device is far from being fully IEC 60601-compliant, but replacing the PC power supply with an external battery, adding TVS diodes to sensitive traces for ESD protection, and using an optocoupler to provide electrical isolation between the patient and the ECG would all increase the device's overall safety.

Compactify the layout: The board size comes in at roughly 61 mm by 33 mm, but there is still quite a bit of unused space which could be used for more peripherals or done away with altogether by rearranging components.

Design an enclosure: A hard casing with cutouts for peripheral access and LED viewing could enhance this project. The board already has mounting holes for fastening. KiCad's StepUp tool allows the 3D model above to be easily imported into FreeCAD and would come in handy here.