Medical device capacitance measurement technology analysis and application (1)

Capacitance measurement not only on your smart phone; in the must in contact with human skin medical equipment and other products, it is also helpful.

This article describes how to use a capacitor testing to determine the device surface in contact with the user of the skin.

Capacitive measuring technology in a traditional HMI applications continue to be popular, such as laptop touchpad, MP3 player, touch screen display and short range detector, etc.

In addition to using capacitive sensors replace mechanical buttons, with a little imagination, coupled with the basic principles of human-computer interface design, will make a lot of other applications can take advantage of this technology. Figure 1 shows an example for some application concepts, these concepts can be used to detect human contact.

Figure 1. Use capacitive sensor electrode device

As shown in Figure 1, the device could be starting devices or the measurement devices and before you know the contact between the skin quality, often more advantageous.

These devices include the need for close to the skin of medical probe, biological potential electrode sensor or for fixed catheter tube shell. To determine the contact conditions, can in the injection moulding process will figure in the green of capacitive sensor electrodes directly embedded devices in plastic casing. Host-controller IC capacitive sensor controller read some status register, the register indicates that capacitive sensor how close off the skin. Subsequently, the master runs on a micro-controller of basic detection algorithm processing status register information to determine the sensor electrodes in contact with the skin.

In the tradition of capacitive measuring machine interface application, usually by finger touch start contact sensor electrodes.

The example in Figure 1 is a non-traditional manner using capacitive sensor, users in the human body to place a detection with capacitive electrode device. Developing this type of application is very simple, but in order to build a stable and reliable system, or should follow some key guidelines

Capacitance to digital controller.

Development of high performance contact detection applications, you first select an appropriate Capacitance to digital controller (CDC). As shown in Figure 1, the device surface contact with the skin either directly through the energy of small variations in the measurement of such energy distribution in capacitive sensor electrode array, when the device is in contact with the skin may occur. This kind of measurement accuracy depends on the sensitivity of the CDC analog front end and the number of sensor electrodes. Using the traditional PCB process of capacitive sensor accuracy is typically 50 fF to 20 pF range, so using 16-bit CDC's high-precision measurement technology is ideal.

In the select the CDC, the first to clear some of the key features, such as with 16-bit ADC in high resolution analog front ends, programmable sensor sensitivity settings, programmable sensor offset control, on-chip environmental calibration, support the ideal quantity sensor electrode adequate capacitive input channels, and need no external RC devices for integrated design of sensor calibration.

These features are support for reliable and flexible applications, brings the best user experience. For example, programmable sensitivity enables interface designers for specific application presets the best sensor sensitivity, and non-use can lead to poor sensitivity of fixed solutions. Programmable offset control for interface designers are another important feature, because every production batch of the sensor plate offset values may differ slightly. Rapid ahead characterization allow new sensor plate into production before change host firmware settings. For ambient temperature or humidity is expected to change in the application, chip environmental calibration can achieve more reliable solution. Please note that the electrode sensor is a standard PCB copper trace construction; substrate properties vary with temperature and humidity changes, so will change the sensor output of the baseline level. If the CDC supports on-chip calibration, this baseline wander during product use in dynamic compensation.

Small electrodes require high sensitivity.

Measurement of objectives is to identify the device and the skin immediately; skin contact with the device, the better the quality of the equipment, the more accurate read. Measurement accuracy depends on the distribution of the devices contact surface area electrode sensor quantity (electrode, the higher the resolution) and size. As shown in Figure 1, the device surface areas generally very small, need designers in developing applications with small sensor electrodes.

For reliable measurements and small sensor electrode capacitance changes related to small (generally less than 50 pF), you need to use high sensitivity analog front-end controller.

Remember, the plastic cover material type and thickness will further influence the sensor through emitting a signal of small plastics. Controller's analog front end measurement must have sufficient sensitivity to measure this kind of small signal, at the same time in all operating conditions (e.g. different power supply voltage, temperature and humidity, as well as coverage of material thickness and types), in the measured signal and the threshold level detection settings between a better signal. A low signal levels will increase the false detections and sensor instability risk. In order to minimize the risk, when used with 16-bit ADC with CDC, the baseline level in the sensor (sensor has no contact with the skin) and contact threshold level should be maintained between the margin of at least 1000 LSB.

AD7147 and AD7148 CapTouch programmable controller for single electrode capacitance sensors, with 16-bit resolution, dollar slightly Farah-level measurement in the full range of set 16 programmable threshold detection level values.

Both controllers support 1 mm plastic covering materials (dielectric constant 3.0) of 3 mm × 3 mm small sensor electrodes, while retaining the full 1000 ADC LSB signal. Full signal margin means that there will be no contact with skin and skin contact by the difference between the sensor output.