As capacitive sensors integrate more with emerging technology, replacing more traditional buttons, design engineers are able to get rather creative with how they can design them into devices. This article reviews how capacitive sensing can be applied to determine the quality of the contact between a medical device’s surface and the wearer’s skin.

Figure 1: Smart devices using capacitive sensor electrodesWhile capacitive sensors continue to be used to replace mechanical buttons in human interface applications, a little imagination, combined with the basic principles used in human-interface designs, will allow many other applications to take advantage of this technology. For example, capacitive sensing can be applied to monitor the integrity of skin contact of a body worn sensor. Figure 1 shows a couple of example application concepts that can be enhanced to include human body contact sensing.

For the devices shown in Figure 1, it is often beneficial to have information about the quality of contact between the device’s surface area and the skin before the device is activated or a measurement is taken. The range of devices could include a medical probe that needs to rest flush on the skin, a bio-potential electrode sensor, or the housing holding a catheter tube in place. To achieve this additional performance, several capacitive sensor electrodes, shown in yellow, could be embedded directly into the device’s plastic housing at the injection molding stage during manufacturing. Once the electrode information is available, a simple algorithm running on the host controller can be applied to determine if all sensor electrodes are making proper contact with the skin.

Figure 2: Analog Devices AD7147 CapTouch programmable controller functional block diagramThe examples shown in Figure 1 use capacitive sensors in a non-traditional way, where a user positions a device containing the capacitive sensing electrodes on the human body, as compared to traditional capacitive sensing human interface applications where a person initiates contact with the sensor electrodes, typically by finger touch. Developing the type of applications shown in Figure 1 is rather straightforward, but a few key guidelines should be followed to ensure a robust, reliable, high-performance solution.

Select a High-Performance Capacitance-to-Digital Controller
Developing a high performance contact sensing application starts with the capacitance-to-digital controller. For applications shown in Figure 1, the device’s surface to skin contact is measured directly from the small change in energy distributed across the array of capacitive sensor electrodes when contact is made with the skin. The accuracy of this type of measurement greatly depends on the sensitivity of the capacitance-to-digital controller’s analog front-end and the number of sensor electrodes. When selecting a capacitance-to-digital controller, start by identifying some key features, such as a high-resolution analog front-end, programmable sensitivity settings, programmable offset control, on-chip environmental calibration, a product family that has enough capacitive input channels to support the number of sensor electrodes used in the application, and an integrated design that does not require external RC components for sensor calibration.

Optimize the Number and Size of the Capacitive Sensor Electrodes
The goal of the measurement is to determine exactly how flush the device is on the skin. The final accuracy of this measurement will be determined by the number (resolution) and size of electrode sensors distributed across the device’s surface area. For the applications described in Figure 1, the surface area is typically small, therefore, requiring the designer to develop the application using small sensor electrodes. Reliably measuring the small energy associated with small sensor electrodes requires a highly sensitive analog front-end controller. Keep in mind that the type of overlay material and thickness further attenuates the small signal emitting from the sensor. The controller’s analog front-end measurement must be sensitive enough to measure this small signal while maintaining good signal margin between the measured signal and the threshold level detection setting under all operating conditions (e.g., supply, temperature, humidity, thickness, and type of overlay materials). A low signal margin increases the risk of false detection and erratic sensor behavior.

Analog Devices’ AD7147 (Figure 2) and AD7148 CapTouch programmable controllers for single-electrode capacitance sensors, with 16-bit femto farad measurement resolution range, are capable of supporting sensor electrodes as small as 3.0 × 3.0 mm beneath a 1.0-mm plastic overlay material with a dielectric constant of 3.0 while maintaining a full scale signal margin of 1,000 ADC output codes. Together with 16 programmable threshold detection level settings across the full scale, the AD714x family provides a very robust and reliable measurement solution ideally suited for small capacitive electrode sensing applications.

Figure 3: AD7147/AD7148 on-chip environmental calibrationMaintain Reliable Performance Under Changing Environmental Conditions
Capacitive sensor electrodes are developed using standard copper material on the printed circuit board or flex material. The characteristics of this material will vary as the temperature and humidity changes. This variation will create a shift in the baseline level to which all of the sensor threshold levels are referenced. Large shifts increase the risk that the contact threshold levels will be too low or too high depending on the direction of the offset. This situation can lead to false contact errors or threshold levels that are too sensitive or under sensitive, thus leading to erratic contact behavior. In order to maintain the original sensor signal to threshold detection level margins (sensitivity), the capacitance-to-digital controller needs to automatically track the magnitude of the baseline offset error and rescale the threshold settings accordingly. Figure 3 provides an example of how the AD7147 and AD7148 threshold levels automatically track and adjust for any baseline offset changes due to changing environmental conditions.

Figure 4: Paths for stray capacitanceEliminate Measurement Errors Due to Stray Capacitance
Retrofitting a device to include an array of capacitive sensor electrodes may impose space limitations, thus forcing the designer to locate the capacitance-to-digital controller far from the capacitive sensors, resulting in long, closely routed parallel sensor traces. Long parallel sensor traces are a disadvantage in a capacitive sensing application because the adjacent sensor traces, which will be at different DC potentials when in use, establish a stray coupling path between each trace. A flooded ground plane does not prevent this because stray capacitance will still be formed between the electrodes/traces and the ground plane since they are also at different DC potentials. One way to eliminate stray capacitance errors is to flood the plane separating the adjacent traces and then drive the plane at exactly the same DC level as the capacitive sensor electrodes and traces. The AD7147 and AD7148 devices eliminate stray capacitance problems by providing a dedicated ACSHIELD output that is capable of driving a flooded plane at the same dc potential as the capacitive sensor traces. The examples shown in Figure 4 describe this behavior.

If one were looking to add contact feedback between a body worn sensor and a person’s skin, Analog Devices offers two high precision devices suitable for this application. The AD7147 and the AD7148 support 13 and 8 independent sensor patterns respectively. Analog Devices offers complete sensor libraries and software support to get a design prototyped and to the market as quickly as possible.