How to Simplify Analog Front-End Design for Heart Rate Monitors

My New Year's resolution to get in shape hasn't been as successful as I had hoped. Like many people, I started out with great enthusiasm but may be slipping back into my old ways.

During one of my experiments, I began to wonder how difficult it would be to design a heart rate monitor to monitor my own health. I was seeing more and more portable devices integrating patient monitoring capabilities, with the goal of detecting cardiac abnormalities in advance and saving lives.

About three years ago, Texas Instruments (TI) released the ADS1298 family of ECG/EEG analog front ends, designed to make analog front end design for biopotential devices a little easier. Now, to put it a little more simply, I’m talking about right leg drive (RLD) design, Wilson center termination (WCT) design, leads-off detection methods, etc. (integrated into the chip, reducing board space by up to 95%). This high level of integration reduces board space and enables smaller designs. Combined with very low power consumption (under 1mW), this family is really well suited for portable applications. Let’s take a closer look at a chest strap heart rate monitor to see how difficult it is to design an analog front end.

When the heart beats, it generates a small electrical pulse with a small bandwidth of 40Hz for the main frequency component; the amplitude is about a few millivolts. This electrical pulse can be acquired and digitized by a high-precision ADC (such as the ADS1291) without any external gain stage setting. So keep in mind, if we disassemble the chest strap design, we will see that it is simplified with two electrodes placed on the chest to mark a single lead (called Lead I). There is no need to use a third electrode to bias the gym enthusiast or patient. Instead, we use a different design method to ensure that the input is within the range of the converter. The input is AC coupled to remove the DC operating point from the patient. Then, we use high impedance resistors to set the operating point.

As I mentioned earlier, the RLD electrode does not need to drive the patient's limb. Instead, the RLD electrode can be used to set the bias point at the input of the converter through a weak resistor. There are other ways to bias the input: a power supply or an external LDO can be used. In fact, any source can be used as long as it ensures that the input is within the operating range of the ADC.

Since most signal components have a small bandwidth of 40H, the ADC data rate can be limited to help tune down the noise. The digital filter of the ADS1298 family limits the data rate to 1/4, which allows us to use 250SPS or even 500SPS and still meet the 1μVrms converter noise requirement.

Really, that’s all you need to design an analog front end—nothing too complicated. Advanced medical equipment or consumer heart rate monitors could even use the same analog front end design. The “what” of an ECG design depends on how you utilize the data in digital processing. That’s what makes this system so cool. I’ll explain a little more about digital processing in a future article, so stay tuned.