Capacitance Sensor Design using Analog Devices Capacitance-to-Digital Converter (CDC)
Capacitive sensors have become more popular after the success of the scroll wheel used in iPod. It gives the user an interface with greater sensitivity and control than standard mechanical input technologies. Product as diverse as hand phone, digital camera LCD TV and media player has start implementing capacitance sensing into it.
Analog Devices' capacitance sensing solution consists of an excitation source connected to a transmitter, which generates a capacitive field to a receiver. The capacitive field lines measured at the receiver is translated into the digital domain by a Σ-Δ analog-to-digital converter. The total capacitance measured at the receiver decreases when a grounded object, such as a finger, comes close to the induced capacitive field. The excitation source and Σ-Δ CDC are implemented on the AD7150, while the transmitter and receiver are constructed on the sensor PCB.
The sensor PCB is glued to the underside of the case or covering of the finished product. The capacitive field lines extend above the sensor PCB for about 4 mm. The field also extends above any covering material over the sensor PCB. One advantage of this sensor arrangement is that the user is never in contact with the sensor PCB itself, so there is no wear on the sensor.
The case or covering material housing consumer products such as MP3 players, digital still cameras, and handsets, is made from a variety of materials. Materials such as plastic or glass are suitable covering materials for use with capacitance sensing; metal cannot be used.
The response of the capacitance sensor depends on three factors:
• The size and type of the sensor element
• The size of the object touching the sensor
• The thickness and type of the covering material
Each of these factors affects the magnitude of change measured by the CDC when the sensor is touched. If the change in CDC output is very small, then it becomes difficult to differentiate between the sensor-touched and the sensor-not-touched conditions. This application note details how each of these factors affects the sensor response and can be used as a guideline when deciding the size and form of the sensor configuration, as well as the covering plastic specification.
Grounding around sensor board must be carefully design, if the sensor is close to any AC power or high speed switching circuit a ground shielding must be added into it.
Any standard PCB material is suitable for capacitance sensor design, which allows the sensors to be manufactured using industry-standard techniques. Sensor board and trace material examples are shown in Table 1.
Table 1 - Materials Suitable for Sensor Manufacturing
|FR4 (and Similar)||Copper|
|Flex (FPC or Polyamide)||Copper|
|PET (Plastic)||Indium tin oxide (ITO)/silver/carbon|
The button can be any size (the minimum size is 3 mm in diameter). Each button sensor is connected to one CIN input of the AD7150/AD7151. Buttons can be circular, oval, square, or irregularly shaped.
The following guidelines apply for all sensor layouts:
• Recommended trace width is 0.2 mm.
• Minimum clearance between traces is 0.15 mm.
• Maximum recommended distance between AD7150/AD7151 and the sensor is 10 cm.
• Do not route any switching signals directly underneath the sensor electrode or the traces to/from the sensor.
• Floating traces should not be routed next to the sensor traces. Ensure LED traces are not floating by including a 100 nF capacitor to ground on any LED signal.
• The serial interface signals should be routed as far away from the sensors as possible.
Written by Lim Chee Yang