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Capacitive Touch Sensing Solutions Slide 36

This slide illustrates how auto-tune works. In this example there are four channels that may have four different starting conditions. As shown in this case, Channel One has a 2pF starting capacitance. All is well because it is within the 7.5 pF dynamic range. Similar conditions exist on Channel Two, with 6pF, which is still acceptable. However, in the case of Channel Three, there is a 24pF starting capacitance. What happens here is that the device will automatically position the 7.5 pF dynamic range window for each channel as long as the initial capacitance value is within 30pF. In fact, looking at Channel Four, there is a 45pF starting capacitance which is still within the total dynamic range of the device, but outside of the 30pF auto-tuning range. In this case, the device is not able to move the dynamic range window high enough to “capture” the initial capacitance value. Thus, an additional external capacitor is needed. However, the advantage is that this will only have to be added only in extreme cases.

PTM Published on: 2011-12-12