Coplanar (One-layer) Design

In coplanar design, both X and Y electrodes are fabricated on the same layer of the PCB. The length of a coplanar slider can be extended marginally by increasing the gap between each key, but the response will become increasingly non-linear with signal dropouts. The field strength decreases on a sensor electrode before it starts to rise on the next.

Figure 1. Capacitive Effect for Two Widely Seperated Sensor Keys

However, it is possible to extend the interpolation between each channel by introducing a series of sub-electrodes, each separated by a resistor. Instead of a single segment per channel, each channel in the slider is formed by one or more segments. The extra segments are created using resistive dividers on the X lines as shown in the following figure.

Figure 2. Use of Extra Segments to provide Interpolation on a Long Slider

This arrangement provides smooth transition between channels as the users move their finger along the sensor electrodes. Up to seven extra segments can be used between channels to achieve the required sensor length. It is possible to achieve 350mm slider length with this resistive interpolation method.

The following figure depicts the dimension of Resistively Interpolated sensor design.

Figure 3. One-layer (Coplanar) Long Slider (Resistively Interpolated)