[How to] Touch sensor validation and circuit analysis

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The process of designing and validating a touch sensor depends on the level of experience and expertise of the designer. However, no matter the case, a design begins from the high level (system-level) product specifications of the touch sensor. Those specifications describe the desired outcome and what is expected from the touch sensor when it is ready for production. For example, a product specification table of mutual and self-capacitive touch sensors is shown in Figure 1.

 

Figure 1. Typical product specifications of a touch sensor.

 

In this figure we can see that the capacitance touch sensor is expected to have a response time of 30ms, to be able to support 2 concurrent touches with a minimum diameter of 7mm and many more. Other high level specs can be the size of the screen, restrictions on the controller and material choices, the thickness of the protective cover lens and many others.

 

Once the specifications are clearly defined, the designer can depict them in an equivalent circuit, or a Target Schematic, which ideally transforms those specifications into the electrical circuit of the product.

 

Figure 2. Capacitive touch sensor schematic.

 

Finally, when the target schematic is ready, the designer is ready to produce the sensor, that is the sensor layout. The physical layout should incorporate all the product specifications and remain consistent with the target schematic in terms of the typical requirements that the product definition describes.

 

Read and download our free guide HERE about different representations for capacitive touch sensors, that include schematic diagrams, equivalent circuit diagrams, netlist and layouts.