An electrocardiogram (ECG/EKG) is the most common medical heart recording that must be accurate in numerous disruptive environments. At each source the potential for noise interference is prevalent.
In the last decade, the introduction of portable ECG technology has made it possible to measure the heart’s activity in a variety of settings. As these wearable devices make their way into mainstream hospital-at-home environments, manufacturers face continued pressure to reduce costs, development time, and maintain performance levels.
There are some unique hardware challenges and requirements associated with the design and development of an ECG dry electrode device.
Apart from the need to be a small, low power solution, the analog front end (AFE) choice is vital as it will help manage signal quality issues which arise during ECG signal acquisition.
In this article we explore two of the main challenges associated with the development of a dry electrode ECG device; the AFE choice and hardware design.
The importance of ECG specific AFE choice
Today’s design engineer is faced with a choice between developing a sensor conditioning circuit based on discrete components or making use of a more integrated sensor interface analogue front end (AFE).
AFEs are used for amplifying, filtering, and digitising in vivo physiological signals. As all physiological signals differ in amplitude and bandwidth, an ECG capable AFE should be used.
When comparing discrete components to an sensor interface AFE, the AFE has many advantages such as ease of use, performance and functionality.
ECG AFEs must meet a certain set of requirements to ensure the input ECG signal is appropriately handled, whilst preserving the integrity of the signal, and ensuring the data is fit for analysis. This requires well-defined input signal characteristics, dynamic range, and a data sample rate, all while maintaining a low power solution and minimising power dissipation.
ECGs are highly variable amongst the general population. The AFE choice is crucial to the device hardware design as an unsuitable AFE, external configuration, or register configuration will impact the end performance of the device. If set up incorrectly, the AFE may compromise the collected raw data and thus inhibit accurate and reliable algorithm performance.
Hardware schematic and layout design
Following AFE selection, there are additional hardware considerations which may be affected by your end application.
Good hardware layout design is crucial to a high performing ECG device as it will minimise the internal noise, which could interfere with the ECG signal.
A lead I wrist wearable device will deal with different challenges than a chest-based device. It is vital that the use of the device is considered before designing the hardware. The overall goal of the hardware design of an ECG device is to minimise all forms of noise, both internal and external.
Despite its importance, the electrode design and performance will not matter if the internal hardware is poorly designed as this will infer significant noise e.g. mains noise, onto the raw signal. The performance of the ECG device will also be affected by PCB track routing, track length, mechanical constraints of the ECG device, and whether the device is wired or wireless.
Implementing health monitoring technology like ECG in wearable devices presents unique challenges. The AFE plays a vital role in overcoming these challenges and allowing acquisition of clinical-standard ECG signals on a personal device such as a smartwatch.
This technology has immense potential to dramatically shift the healthcare system and change health monitoring forever.