November 29, 2016
|By: Andrew Baker
Executive Director for Industrial and Healthcare Products, Maxim Integrated
Comprehensive hardware/software reference design includes sensors for diverse health parameters
Starting a new project is both an exciting time and a challenging one for a design team. it’s a chance to work with new components and tools on (hopefully) interesting projects to create something special. But it’s also a challenge since the team must deal with time-to-market pressure, marketing changes, and customer feedback, in addition to design-related issues. That's why a good reference design can make a major difference, by compressing the up-front efforts and minimizing the unexpected.
Certainly, some preliminary software development can be started right away using a PC, development tool, a processor emulator, or even a rough breadboard of a part of the circuit. Soon, however, actual circuitry – "hardware" – is needed to do serious development, debugging, hardware/software integration, and even some pre-production pilot units.
For wearables, the team's problems are magnified by an unavoidable fact: the device needs to interface with different analog sensors, each with unique signal interface level, type, and noise issues. For example, the EKG chest strap delivers a microvolt signal corrupted by a large 50/60-Hz noise component and body noise, while the blood-oxygen sensing photodiodes are milliamp-current sources. This means that the design of the analog front end is much more difficult than when there are multiple channels, but at least of the same type. In addition, this application demands a high degree of functionality including wireless interface in a tiny package, along with extreme low-power demands.
Getting to that reality point can take significant engineering resources and time, leaving little room for the inevitable last-minute problems that require an iteration of the design. If the team is experienced and lucky, this run-up to having a reliable, operating project prototype with needed development tools and interfaces will take at least three months, typically six months, and perhaps more.
A Long Road to Proof of Concept
There's also pressure to have something which looks "reasonably" close to the final product to show marketing, management, key customers, or even focus groups to validate the approach. This provides a "proof of concept" to test out the design appearance and functionally before the design has progressed too far. Obviously, a patched-together prototyping set-up won't be sufficient for this role.
At a minimum, the project team must:
- Study the vendor offerings and their data sheets; select the needed active and passive components based on tradeoffs in performance, power, price, and availability. It's an added challenge when there are different types of sensors at the front ends, rather than multiple channels of the same sensor type.
- Order the needed parts; some companies have a special purchasing group just for these low-volume orders, but in many cases, the team may have to do it themselves. They may even have to put the cost on their company credit card, since cutting a purchase order (P.O.) is a time-consuming process.
- Get all the needed parts, build up a rough prototype, and get it running. If there are problems, determine if it is a fundamental design mistake or just that the rough prototype has problems with timing, signal integrity, and other issues that hopefully won’t exist in the final PC board.
- When the prototype hardware seems OK, get some near-final PC board prototypes fabricated, loaded, and checked out so final system integration, debug, algorithm development, and other tasks can be completed
For these reasons and other reasons, many vendors now offer reference designs, which range from basic, barely tested schematic diagrams all the way to complete, fully vetted evaluation/development boards.
Minimizing Frustration Developing Wearable Health/Fitness Designs
An example of a reference design which saves time and minimizes frustration is Maxim's hSensor Platform for wearable health and fitness applications, which usually involve a variety of very different sensors. The platform (formally designated MAXREFDES100#) simplifies acquiring and processing inputs such as those from chest straps, ECG/EKG patches, wrist-worn devices, thermometers, disposable temperature patches, blood oxygen measurement meters, smart weigh scales, and bio authentication systems. It even has inertial sensors (3-axis accelerometer, 6-axis accelerometer/gyroscope), as shown in Figure 1. The complete package includes the hSensor board, firmware with drivers, a debugger board, and a GUI. (Note that this reference design is also a nominee for the "Best IoT Boards of 2016" category in the Mouser Electronics/Hackster.io "Maker Madness" contest.)
The small board (Figure 2) – just 25.4mm x 30.5mm – runs on a single coin cell, and allows data to be monitored over a Bluetooth low-energy link, stored in flash, or streamed via USB. It offers additional functionality via the ARM® mbed™ hardware development kit. Using the free source code at Maxim’s website, designers can also download algorithms for applications and modify them as needed.
Given the many simultaneous (and often contradictory) pressures facing a design team developing products for the fast-moving health/wellness market, using a reference design such as Maxim's hSensor platform is not only a smart move, but may even be the only one that makes sense.