When Analog Devices introduced the AD8232 single-lead heart-rate monitor analog front end (AFE) in August, it doubled down on the design effort it put into the ADAS1000 AFE for diagnostic-quality electrocardiograph (ECG) instruments that it announced in 2011.
The ADAS1000 chips cost something more than $18 each in thousand-unit quantities. Yet their potential market in hospitals, clinics, and doctor’s offices, although reasonably large, is relatively small compared to the potential market in exercise equipment, remote patient monitoring, and home healthcare, which is the end-use market the AD8232 (priced at introduction at $1.36 each) plays in.
The larger market would not exist, though, had ADI not established a reputation among medical-products OEMs with the ADAS1000 devices and the company’s microelectromechanical systems (MEMS) sensors.
The AD8232 provides signal conditioning for just one task, sensing the minute electrical pulses that accompany the contractions of a subject’s heart, allowing the rest of the instrument to extract heart rate and, in some cases, a single ECG lead (see the figure). In all these cases, instead of monitoring differential signals from an array of electrodes, the AFE monitors a single pair that can be attached in many different ways, including in the hand grips of sports equipment.\
Establishing a Reputation
Creating the market required establishing a track record at the high end, which is what the ADAS1000 does. In its clinical environment, it deals with a host of electrodes. This is a tough environment. Essentially, there are six chest electrodes in a full ECG, plus electrodes on the patient’s right arm, left arm, and left leg. Combinations of differential signals between combinations of electrodes are processed to create the “leads” on the ECG display. Other combinations monitor breathing rate and verify the proper connection of all the electrodes.
In a clinical environment, an ECG also must deal with the possible presence of a signal from the subject’s pacemaker and, most challenging for a chip designer, with the possibility that an emergency may call for the application of a defibrillator.
On the Body and in the Gym
Obviously, all that is challenging for a clinical AFE, and it justifies the 20-times price differential between the ADAS1000 and the single-channel AD8232. Yet the less expensive device with the far greater potential market is far from simple and offers no easy path to knockoff copies for other chip makers.
On the other hand, OEMs should find product development comparatively easy. For wearable exercise devices, the AD8232 can be placed in a pod near the heart, with just two sense electrodes placed underneath the subject’s pectoral muscles. There is no need for a driven electrode. The datasheet for this configuration shows just five resistors and four capacitors.
It’s only a little more complicated to configure the end-user product for heart rate measured at the hands. In this case, the bill of materials goes up to nine resistors and seven capacitors. This is more challenging than the chest-worn system, since the heart rate signal is measured at the hands with stainless steel electrodes.
One problem is that the user’s arm and upper body movement create large motion artifacts, and the long lead length makes the system susceptible to common-mode interference. Consequently, a very narrow band-pass characteristic is needed to separate the heart signal from the interference.
ADI’s reference design employs a two-pole high pass filter set at 7 Hz, followed by a two-pole low pass filter at 24 Hz. Because the ECG signal is measured at the hands, it is weaker than when it’s measured closer to the heart, so the reference low-pass filter stage also provides a gain of 11 to bring the total system gain close to 1100. To deal with hands-off interruptions, a drive signal drives to the third electrode, which can also be located at the hands, to cancel common-mode interference.
It’s even possible to design an end-user product that provides cardiac monitoring, along with simple rate information, as long as the subject remains relatively still. ADI provides a reference design in the datasheet in which the AD8232 is configured with a 0.5-Hz two-pole high pass filter followed by a two-pole, 40-Hz, low pass filter, and a third electrode is driven for common-mode rejection.