June 27, 2019
| By: Michael Jackson
Principal Writer, Maxim Integrated
Calorie counter? Check.
Step counter? Check.
Sleep tracker? Check
Heart-rate monitor? Check.
ECG monitor? DOH!!!
Just as you were about to bring your latest chest-worn offering to the health and fitness wearables market, you realize that customers are increasingly seeking electrocardiogram (ECG) functionality on top of everything else. This is going to set you back months, while you go back and try to design in this feature, right? Wrong! Really? How can this be possible, you ask yourself? Well, let’s look at what you really need to make this happen and you might just be surprised that it can all be developed in a much more straightforward manner than you thought.
Step 1: the Analog Front-End
The analog front-end (AFE) required for detecting an ECG signal requires several different building blocks. These include (amongst others) an input amplifier with lowpass filter, a programmable gain amplifier (PGA), and a high-accuracy analog-to-digital converter (ADC) with digital filtering options. Clearly in the confined space of a wearable device, a discrete implementation of the AFE is not feasible. Therefore, an integrated approach is required. When selecting an integrated biopotential ECG AFE for a chest-worn wearable, there are some important specifications and features to look for. It should ideally use a single input channel with very high series resistance (> 500MΩ) and high CMRR (> 100dB) . Along with electrostatic discharge (ESD) compliance (IEC61000-4-2) and electromagnetic interference (EMI) filtering, the IC should be able to detect if leads are connected (even in sleep mode) or if they have become detached from the wearer in normal operation, while also having the ability to quickly recover from overvoltage conditions (e.g. defibrillation). This functionality must be provided with the lowest power consumption possible.
Figure 1 shows the functional block diagram for the MAX30003, a fully integrated biopotential ECG AFE for use in wearable designs that meet these requirements. An advantage of this device is that it provides ECG waveforms using only ONE pair of electrodes (none of this multi-lead, wires everywhere ECG nonsense you see in the hospitals!). Another great feature of this IC is that it also performs heart-rate detection in the same package. Other ECG AFE ICs do not perform heart-rate detection; instead, they rely on the microcontroller to perform the calculation which typically consumes an extra 40µW of power. With a typical current consumption of only 150µW (almost 70% lower than similar parts), this AFE can be powered using a single coin-cell battery. It meets the IEC60601-2-47 ECG specification, making it suitable for clinical as well as fitness applications.
Figure 1. MAX30003 Biopotential AFE
Step 2: Design a Motion Artifact Bandpass Filter
Motion artifacts are spurious measurements caused by the movement of the wearer. You need a bandpass filter to remove them (or reduce their effects). This is best done in the analog domain, prior to conversion of the electrode signals into the digital domain. The primary way to do this is by reducing the bandwidth using highpass and lowpass filters. Using the MAX30003, the single pole highpass corner frequency can be set by connecting an external capacitor, CHPF, to the CAPP and CAPN pins, as shown in Figure 2. The value used should set the highpass corner at 5Hz, especially for high-motion usage such as most sports and fitness applications. For clinical applications, this can be a lot lower, typically down to 0.5Hz or even 0.05Hz. When there is little to no movement, this provides better quality ECG information for diagnosis.
Figure 2. Input Analog Bandpass Filter Network
Figure 3 shows the analog bandpass bode plot for a chest-strap application.
Figure 3. Analog Bandpass Filter Bode Plot for Chest Strap
Step 3: Think About Your Power Options
Depending on the battery type being used, there are several options for powering a complete wearable. The simplest option is to use a linear regulator (Figure 4) to create a common 1.8V DC rail from a coin cell that typically varies from 3.4V down to 2.2V. However, this approach is not particularly power efficient.
Figure 4. Simple Linear Dropout Regulator Power Scheme
While using a buck regulator instead of a low-dropout (LDO) regulator would improve on efficiency, the best solution is to use a power-management IC (PMIC) as shown in Figure 5.
Figure 5. PMIC and a 3VDC Coin-Cell Battery
The advantage of using this type of solution is that a PMIC can deliver separate power outputs for the microcontroller, AFE, and the digital interface.
Step 4: ?????
The great news is that there is no Step 4. You can put your feet up and relax as you realize that adding ECG functionality to a wearable is almost as easy as your well-earned piece of cake (not so good for your heart, by the way!).