As wearables become more widespread, designers will be increasingly interested in ICs that support the typical improvements that we expect from electronic products—more features, longer battery life, smaller form factor. The term “wearables” is rather vague and could theoretically refer to a wide variety of electronic devices that are attached to the human body in one way or another. However, I think that we tend to associate the word “wearable” with medical technology, and it just so happens that this article focuses on an IC intended for medical applications.
The MAX30004 is described as a “biopotential (R-to-R detection) analog front end.” Let’s unpack this name.
Biopotential refers to electrical signals associated with the functioning of the human body. As you probably know, the human body was using electricity long before the intellectual beings affiliated with those bodies started designing circuits. There’s something enigmatic about the idea that we needed to discover and harness electricity in order to build the devices that allow us to detect and monitor and analyze the electrical signals used by the body to perform the physiological and intellectual processes that enabled us to discover and harness electricity. . . .
Anyways, so this or that biopotential can tell us something about the state of the human body. Next we have “R-to-R” detection. Unfortunately for those of us who know much more about amplifiers than about the details of the human heart, this doesn’t have anything to do with “rail-to-rail.” The R-to-R interval refers to a specific measurement associated with a person’s heart rate. I’m not going to say more than that because I don’t want to be wrong; if you’d like to try your luck with a few medical definitions, you can click here. You can also find more information at the following page provided by National Instruments: Using LabVIEW for Heart Rate Variability Analysis.
The R-to-R interval as part of an ECG (electrocardiogram). Image courtesy of National Instruments.
OK, now we’re back in familiar territory: “analog front end,” if I recall correctly, is defined as “exceedingly complicated IC—typically designed by mythological beings employed by TI, ADI, Maxim, et al.—that provides comprehensive analog signal processing and analog-to-digital conversion.”
The Block Diagram
You don’t need to spend much time with the MAX30004’s block diagram before you realize that there’s a lot going on here. Just think how much board space would be required if you had to implement most of these blocks as discrete components.
In keeping with the general analog-front-end architecture, the MAX30004 conditions the input signal, digitizes it, applies some digital signal processing according to the needs of the application, and provides the final data to an external processor via a standard serial interface (in this case SPI).
The following diagram gives you more details regarding the (rather nontrivial) circuitry involved in the first signal processing block. Note that the input signal is differential.
You might be wondering what else is needed in addition to the MAX30004. As usual, few external components are required. If you look at the typical application circuit on page 37, you see some capacitors, and a microcontroller, and the circuit that produces the differential input signal. The fundamental components are electrodes, which are things that almost all of us have heard of but very few of us have actually used in a design.
In this context an electrode is something that attaches to the body and introduces physiological electrical signals into the environment of a standard electronic circuit. I don’t know exactly what sort of amplitudes you would expect in a standard heart-rate monitor application, but they’re small—generally less than 1 mV, I’m guessing. Hence the need for carefully designed analog signal-conditioning circuitry.
The following diagram shows you an equivalent circuit that can be used to model the electrode interface. It also specifies the EMI filter that constitutes a large portion of the external components required in a typical MAX30004 application.
Diagram taken from the MAX30004 datasheet.
Before we finish up I’ll mention one other feature that I found particularly interesting. The MAX30004 can actually be configured to enter a low-power sleep mode between heart beats. From a human perspective it seems rather stressful to be falling asleep and waking up so frequently, but apparently it’s not an issue for the MAX30004, and it certainly seems like a good way to extend battery life.
If you have any knowledge or experience with regard to heart-rate measurements, feel free to share your thoughts in a comment.