Capacitance Detection Technology in Medical Equipment

Capacitive detection technology continues to be favored in traditional man-machine interface applications, such as laptop touch panel, MP3 player, touch screen display and short-range detector. In addition to using capacitive sensors to replace mechanical buttons, a little imagination and the basic principle of man-machine interface design will enable many other applications to use this technology. Figure 1 shows examples of application concepts that can be improved by using human contact detection.Figure 1. Device using capacitive sensor electrodes

For the device shown in Fig. 1, it is often advantageous to know the contact quality between the device and the skin before starting the device or measuring. These devices include medical probes that need to be close to the skin, biopotential electrode sensors, or housings for fixing catheters. In order to determine the contact conditions, several green capacitive sensor electrodes in the figure can be directly embedded into the plastic shell of the device during injection molding production. The host microcontroller reads some status registers on the capacitive sensor controller IC, which indicate how close the capacitive sensor is to the skin. Then, the basic detection algorithm running on the host microcontroller processes the status register information to determine whether the contact between each sensor electrode and the skin is appropriate.

In the traditional man-machine interface application of capacitive detection, people usually touch the sensor electrode by finger touch. The example in Fig. 1 uses a capacitive sensor in an unconventional way, and the user places a device containing a capacitive detection electrode on the human body. Developing such applications is simple, but in order to build a stable and reliable system, some key guidelines should be followed

Capacitance digital controller. To develop high-performance contact detection applications, we must first select a suitable capacitor digital controller (CDC). For the application shown in Fig. 1, the contact between the device surface and the skin is directly measured by a slight change in energy, which is distributed in the capacitive sensor electrode array and occurs when the device contacts the skin. The accuracy of this measurement depends on the sensitivity of the CDC analog front end and the number of sensor electrodes. The accuracy of capacitive sensor manufactured by traditional PCB process is usually in the range of 50 FF to 20 PF, so the high-precision measurement technology using 16 bit CDC is ideal.

When selecting CDC, we should first clarify some key characteristics, such as high-resolution analog front-end with 16 bit ADC, programmable sensor sensitivity setting, programmable sensor offset control, on-chip environment calibration, sufficient capacitive input channels supporting an ideal number of sensor electrodes, and integrated design without using external RC devices for sensor calibration. These features support reliable and flexible applications and bring the best user experience. For example, programmable sensitivity allows the interface designer to preset the best sensor sensitivity for a specific application rather than adopting a fixed solution that may lead to poor sensitivity. Programmable offset control is another important feature for interface designers, because the offset value of sensor boards in each production batch may be slightly different. The quick preview feature allows you to change the host firmware settings before putting a new sensor board into mass production. For applications where ambient temperature or humidity is expected to change, on-chip environmental calibration can achieve a more reliable solution. Note that the electrode sensor is constructed using standard PCB copper traces; The properties of the substrate will change with the change of temperature and humidity, so the baseline level of the sensor output will be changed. If CDC supports on-chip calibration, this baseline drift can be dynamically compensated during product use.

Small electrodes require high sensitivity. The objective of measurement is to determine the closeness of the equipment to the skin; The better the contact quality between the skin and the equipment, the more accurate the reading of the equipment. The accuracy of measurement depends on the number (the more electrodes, the higher resolution) and size of electrode sensors distributed in the contact area of the device. For the application shown in Figure 1, the surface area of the device is generally very small, and designers need to use small sensor electrodes when developing applications.

In order to reliably measure the small capacitance change related to the small sensor electrode (generally less than 50 PF), a high-sensitivity analog front-end controller is required. Remember that the type and thickness of plastic covering material will further affect the small signal emitted by the sensor through the plastic. The analog front-end measurement of the controller must have sufficient sensitivity to measure this small signal, and maintain a good signal margin between the measured signal and the threshold level detection setting under all operating conditions (such as different power supply voltage, temperature and humidity, and the thickness and type of coverage material). Low signal margin will increase the risk of false detection and sensor instability. In order to minimize risk, when using CDC with 16 bit ADC, a margin of at least 1000 LSB shall be maintained between the sensor baseline level (the sensor is not in contact with the skin) and the contact threshold level.

Ad7147 and ad7148 captouch programmable controllers are used for single electrode capacitance sensors. They have 16 bit resolution, can carry out nano farad level measurement, and can set 16 programmable threshold detection level values in the full-scale range. Both controllers support 3 mm & Tides under 1 mm plastic covering material with a dielectric constant of 3.0; 3 mm small sensor electrode while still maintaining the full-scale signal margin of 1000 ADC LSB. The full-scale signal margin is the difference between the sensor output without skin contact and with skin contact.

Maintain reliable performance. The capacitive sensor electrode is made of standard copper material or flexible material on PCB. The properties of this material change with temperature and humidity. This change will offset the baseline level (the electrical average of all sensor thresholds is based on the baseline level). A large baseline offset increases the risk that the contact threshold level is too low or too high (too low or too high depends on the direction of baseline offset), which will cause false contact error, or make the threshold level either too sensitive or not sensitive enough, resulting in the instability of contact state. In order to maintain the original signal contact threshold detection level margin (sensitivity) of the sensor, CDC needs to automatically track the amplitude of baseline offset error and readjust the threshold setting accordingly. The example in Figure 2 shows how the threshold levels of ad7147 and ad7148 are automatically tracked and adjusted for baseline offset changes caused by changes in environmental conditions.

Figure 2. Ad7147 / ad7148 on chip environment calibrationEliminate measurement errors. The modification of capacitive sensor electrode array in the device may cause space constraints and force the designer to place the CDC away from the capacitive sensor. This will increase the length of parallel sensor routing and make the wiring dense, which is not conducive to capacitive detection applications, because routing at different DC potentials will form the stray coupling path shown in Fig. 3a. The grounding layer of PCB cannot prevent this situation because the wiring and grounding layer are at different DC potentials, which will still form stray capacitance (Fig. 3b).Figure 3. The path of stray capacitance shows the results of the following parallel routing: parallel routing (a) without copper pouring layer, parallel routing (b) on grounding copper pouring layer, and parallel routing (c) on copper pouring layer with the same DC potential as the routing

To eliminate the stray capacitance error, one method is to surround the adjacent routing with a layer driven by the DC level (the DC level is the same as the DC level of the capacitive sensor electrode and routing). The ad7147 and ad7148 devices eliminate stray capacitance by providing a dedicated acschield output with this function, as shown in Figure 3C.

Consumer health care equipment such as spa and skin care products are entering ordinary families from professional institutions, and users are no longer technicians who are specially trained and familiar with products and their applications. Therefore, many of these products need a more intelligent user interface to enable untrained users to master the correct product use methods. Capacitive detection provides new choices for user interface designers, enabling them to explore various innovative methods to meet new user interface requirements. Capacitive digital technology provides contact information between capacitive sensor electrode and skin, which can be used to maintain the best product performance and safety.