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Cap-Sensing Solution Using ADI AD7xxx Series

 2009-05-15

Cap-sensing is a dilute version to touch screen solution. Similar to the capacitive touch screen solution, The Cap-sense solution is made easy because the design of the human interface portion can be done using the Printed Circuit Board (PCB). For instance, should the user need to create a 10-digit touch pad, it can be done through square pad designs onto the PCB assembly of his project.


Fig 1 Basic working of Cap-sense solution using AD7147 (picture from ADI specification for AD7147)

Figure 1 demonstrates the basic working of a Cap-sense function. To create the capacitor effect, the human finger acts as the grounding plate of the capacitor. The Cap-sense device measures the changes in capacitance value of the pad and if this exceeds the threshold, the device interprets this as sensor activation. (for detail information refer to ADI web-site on Cap-Touch Programmable Controller)

There are many applications to using Cap-Sensing. Depending on the complexity of their application, different parameters are seen as important. The most notable requirements are the resolution, number of channels and sampling frequency as these determine how sensitive the system can be.

A simpler application that Cap-sense can be used is switch buttons or keypad design. Depending on user requirement, the number of sensor pads that can be put in ranges from single unit up to a maximum of 14 units. For convenience, we consider AD7147 with 13 channels with 111 samples per second. The advantage of using AD7147 is its diversification in interface facility. It can operate using either SPI or I 2C communication. As the device is a supporting component, it is necessary to at least work with a micro-controller.


Fig 2 Block Diagram of Cap-Sense connectivity

The design is made easy as it does not require the measurement accuracy of AD7147 and merely setting the reading threshold. One notable effect of the insulation layer at the sensor is retaining the charge difference generated during the “touch” process. As long as the finger continues to “touch” on the pad, the charge continues to exist and AD7147 continues to sense the activation of the particular pad. This would be different if the insulation is removed.

We can find various applications from the set up in Fig 2. It can function as simple on/off switch, key pads (mostly requires at least 12 sensors), glide gauge with resolution of 14 levels. This is of course only when Cap-sense is used as on-off function.

For more complex system, it is possible to use a single channel device such as AD7151. In this application, we would require a more accurate reading of the capacitance level. Since capacitance is determined by the surface area which 2 electrodes interact with each other as demonstrated in Fig 3, varies this parameter therefore changes the capacitance which will be detected by AD7151. The system is therefore able to detect movements of the plate through this change.


Fig 3 Single channel gliding gauge

Cap-sense is relatively new, while we apply this technology to our designs, we continue to find more ways of using this component. Already we have seen more complex applications as seen in Apple iPOD and Creative Zen, its application is only limited to the imagination of the user.