A look at capacitance-to-digital converter technology in healthcare applications

Introduction

In recent years, advances in electronic technology have created conditions for many innovations and improvements in the healthcare industry. Challenges facing healthcare equipment include coming up with new diagnostic and treatment methods, enabling remote monitoring, developing home care devices, improving quality and reliability, and enhancing flexibility and ease of use.

For more than 40 years, ADI's rich and comprehensive linear, mixed-signal, MEMS, and digital signal processing technologies have brought significant changes to the design of medical equipment in the fields of instrumentation, imaging, and patient monitoring. This article will focus on capacitance-to-digital converter (CDC) technology, which makes it possible to use high-performance capacitance sensing in healthcare applications.

Capacitive Touch Sensor Controller--A New User Input Method

Capacitive touch sensors provide a user interface in the form of buttons, sliders, wheels, or other methods similar to those shown in FIG. 1 .

Figure 1. Touch sensor layout example

Each blue geometric area represents a sensor electrode on the printed circuit board (PCB), forming one plate of a virtual capacitor. The other plate is formed by the user's finger, which is actually grounded with respect to the sensor input.

The AD7147/AD7148 CapTouch controller family is designed to stimulate and interface with capacitive touch sensors, capable of measuring changes in capacitance from single-electrode sensors. The device first outputs an excitation signal to charge the capacitor plates. When an object (such as a user's finger) approaches the sensor, the user acts as the other plate of the capacitor, forming a virtual capacitor (Figure 2). This capacitance can be measured using a capacitance-to-digital converter (CDC).

Figure 2. Capacitive sensing schematic and typical response

The CDC senses changes in capacitance from external sensors and uses this information to log sensor activation events. The AD7147 and AD7148 have 13 and 8 capacitance inputs, respectively, and are equipped with on-chip calibration logic to compensate for measurement changes caused by environmental changes, ensuring that no false trigger events are generated on external sensors due to temperature changes or humidity changes.

The AD7147 and AD7148 offer multiple operating modes, user programmable conversion sequences, and extremely flexible control features. These features make them ideal for high-resolution touch sensor functions, such as sliders or wheels, with minimal software requirements. In addition, button sensor applications can be fully implemented using the on-chip digital logic without the need for any software.
Basic principles of capacitance sensing and measurement

Capacitance is the ability of a capacitor to store energy in an electric field. In its nominal form - a parallel plate capacitor - capacitance C is a measure of the charge Q stored in the capacitor at a given voltage V, calculated as C = Q/V.

For a parallel plate capacitor, the essence of the capacitance detection and measurement technique is shown in Figure 3.

Figure 3. Measuring the capacitance of a parallel plate capacitor

A parallel plate capacitor consists of two conductors (metal plates) and has the following properties:

●Conductor area, a × b

●The distance d between the two conductor plates

●The dielectric between two conductors is represented by the dielectric constant er

●Based on this geometry, the capacitance calculation formula is as follows

Where ε0 is the dielectric constant of free space

Where ε0 is the dielectric constant of free space.

Figure 4. Sensor electrical configuration

Since the sensor capacitance is determined by a, b, d, and er, by varying the values ​​of these parameters, or observing the change in their values, the CDC technique can be used to directly measure capacitance and for a variety of other applications, depending on the sensor type. For example, if a, b, and εr are constant, the CDC input is inversely proportional to the distance between the two conductors.
Applications

The AD714x, AD715x, and AD774x families of CDC products are suitable for a wide range of applications involving various sampling rates, resolutions, input ranges, and input sensor types. The potential applications of capacitive sensing technology are limited only by the creativity of the user, and we will introduce some possible applications in the field of healthcare below.

Liquid level monitoring

In many applications, such as infusion, the amount of liquid used must be measured, or the infusion must be stopped before the infusion bottle becomes empty. To save medical staff time, automatic liquid level detection technology can be used to eliminate the need for manual checks.

The basic principle of liquid level detection is shown in Figure 5. A parallel plate capacitor is constructed with its plates tightly attached to the outer wall of the infusion bottle and extending to near the bottom of the infusion bottle. As the infusion level changes, the amount of dielectric between the plates changes, resulting in a change in capacitance. In order to be able to use a variety of infusion substances with different dielectric constants, a second capacitive sensor is placed near the bottom of the infusion bottle to act as a reference channel to form a ratiometric measurement. The 24-bit AD7746, equipped with two capacitance measurement channels, can be used for this type of application.

Figure 5. Liquid level detection

Connection detection between electrodes and human body

For devices used near human skin, such as those shown in Figure 6, it is often beneficial to first understand the quality of contact between the device surface and the patient’s skin before activating the device or taking a measurement. End uses may range from medical probes that need to contact the skin, biopotential electrode sensors, or housings that hold catheters in place. To obtain this additional information, multiple capacitive sensor electrodes (shown in blue) may be embedded directly into the plastic housing of the device during the injection molding stage of the production process. With the electrode information, a simple algorithm running on the host controller can determine whether all sensor electrodes are making good contact with the skin.

Figure 6. Device using capacitive sensor electrodes

The example shown in Figure 6 uses capacitive sensors in an unconventional way: the user fixes a device containing capacitive sensing electrodes to the human body, whereas in traditional capacitive sensing human-machine interface applications, people generally initiate contact events with sensor electrodes by touching them with their fingers. Using the AD7147/AD7148, it is very simple to develop an application like the one shown in Figure 6.

Sweat testing

In some medical and healthcare testing equipment, it is necessary to measure the sweat exuded by the human body. This is generally achieved by measuring the conductivity of the skin. However, if it is necessary to measure without electrical contact, this function can be achieved by using a capacitive sensor to detect humidity near the human body.

When a person sweats, the humidity (dielectric constant) near the skin increases; the change in capacitance caused by sweating can be measured using a non-contact electrode near the skin where the sweat is occurring.

A second capacitive sensor can be added to measure the ambient humidity and used for common-mode compensation.

Respiration rate measurement

Respiration rate measurement is an important module in patient monitoring systems.

In one implementation (shown in Figure 7), an excitation plate is placed on the patient's back, while a sensor electrode strip is attached to the right side of the patient's chest. The chest movement caused by lung breathing changes the distance between the two plates. The dielectric constant also changes due to the complex physiological activities during breathing. These changes in capacitance can be measured by a CDC device.

Figure 7. Respiration rate measurement

The reason for placing the sensor electrode on the right side of the patient's chest is that this location is least affected by other physiological activities. However, by placing multiple sensor electrodes at different locations on the patient's chest, more information about human function can be obtained. This topic is very interesting and needs further research.

Blood pressure measurement

In blood pressure measurement applications using an inflatable cuff, an important task is to measure the pressure at the air valve. Capacitive sensors are very simple to use in such pressure sensing applications.

As shown in Figure 8, the membrane of a pressure sensor is basically made up of two capacitor plates. When pressure is applied to the sensor, the distance between the capacitor plates decreases. The decrease in distance between the plates results in an increase in capacitance.

A temperature sensor can be used to detect temperature changes in the sensor to compensate for characteristic changes caused by temperature changes. The AD774x series has a built-in temperature sensor for measuring the on-chip temperature - and an ADC voltage channel is also equipped to measure the temperature at the location of the capacitive sensor.

Figure 8. Detecting pressure with a capacitive sensor

Conclusion

This article briefly introduces some of the achievements of Analog Devices in the field of CDC technology, hinting at the great potential of CDC technology in healthcare applications. However, the sensor design - including style, size, and location - the related detailed electronic circuit design, as well as the need for in-depth research, comprehensive experiments, and effective testing depends largely on the nature of the various applications, so we can only scratch the surface in this article.