Dead Band

Dead band refers to the inactive area on both ends of the slider where no change in positions are reported on touch. If you have a touch contact only on an end channel and no capacitance change on adjacent channel, there is no second data to use for position interpolation calculation. The reported position will remain same.

For slider with a position resolution of 8 bits (position 0 – 255), touch in the dead band area shows position 0 on one end and position 255 on the other end of slider. Sliders may have dead band area of around 10% of the length of the slider on both sides. For example, a 100mm slider may have a dead band of around 10mm each on both sides.

For longer slider with standard sensor design techniques, the dead band area will be slightly larger.

If a 300mm slider is designed, ~30 mm (10% of the total length of the slider) may be inactive (dead band) on both ends. Thus for a 300mm slider, approximately 60mm would be unusable. Dead bands of such considerable length would be undesirable in case of applications with stringent space constraints.

Optimizing the length of end channel electrodes helps to reduce the dead bands. For a desired length of slider, length of the end channels and middle channels can be calculated as follows.

End Channel Middle Channel Middle Channel …. Middle Channel End Channel
middle_channel_length = (slider_length*1.25)/(no_of_channels-1)
end_channel_length = [slider_length-(middle_channel_length*(no_of_channels-3))]/2
Note:
  1. 1.At least one extra segment should be present between End Channel and Middle Channel.
  2. 2.Rtotal (total resistance between channels) should be same for every channels (same Rtotal value should be used for both end channels and middle channels).

Sliders designed based on the above calculation can have size of dead bands reduced to 3% instead of 10% at both ends.

Figure 1. Channel Measurements for a 240mm Slider with Seven Channels