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QT160 Datasheet(PDF) 3 Page - Quantum Research Group |
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QT160 Datasheet(HTML) 3 Page - Quantum Research Group |
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3 / 12 page  do not propagate fields laterally very well, and will have poor sensitivity in the middle. Experimentation is required. Since the channels acquire their signals in time-sequence, any of the 6 electrodes can be placed in direct proximity to each other if desired without cross-interference. A good example of backlighting can be found in the E160 eval board for the QT160. 1.3.3 KIRCHOFF’S CURRENT LAW Like all capacitance sensors, these parts rely on Kirchoff’s Current Law (Figure 1-5) to detect the change in capacitance of the electrode. This law as applied to capacitive sensing requires that the sensor’s field current must complete a loop, returning back to its source in order for capacitance to be sensed. Although most designers relate to Kirchoff’s law with regard to hardwired circuits, it applies equally to capacitive field flows. By implication it requires that the signal ground and the target object must both be coupled together in some manner for a capacitive sensor to operate properly. Note that there is no need to provide actual hardwired ground connections; capacitive coupling to ground (Cx1) is always sufficient, even if the coupling might seem very tenuous. For example, powering the sensor via an isolated transformer will provide ample ground coupling, since there is capacitance between the windings and/or the transformer core, and from the power wiring itself directly to 'local earth'. Even when battery powered, just the physical size of the PCB and the object into which the electronics is embedded will generally be enough to couple a few picofarads back to local earth. Electrodes connected to the IC themselves act as coupling plates back to local ground, since when one channel is sensing the other channels are clamped to circuit ground. 1.3.4 VIRTUAL CAPACITIVE GROUNDS When detecting human contact (e.g. a fingertip), grounding of the person is never required. The human body naturally has several hundred picofarads of ‘free space’ capacitance to the local environment (Cx3 in Figure 1-5), which is more than two orders of magnitude greater than that required to create a return path to the IC via earth. The PCB however can be physically quite small, so there may be little ‘free space’ coupling (Cx1 in Figure 1-5) between it and the environment to complete the return path. If the circuit ground cannot be earth grounded by wire, for example via the supply connections, then a ‘virtual capacitive ground’ may be required to increase return coupling. A ‘virtual capacitive ground’ can be created by connecting the IC's own circuit ground to: (1) A nearby piece of metal or metallized housing; (2) A floating conductive ground plane; (3) A nail driven into a wall; (4) A larger electronic device (to which its output might be connected anyway). Free-floating ground planes such as metal foils should maximize exposed surface area in a flat plane if possible. A square of metal foil will have little effect if it is rolled up or crumpled into a ball. Virtual ground planes are more effective and can be made smaller if they are physically bonded to other surfaces, for example a wall or floor. 1.3.5 FIELD SHAPING The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected lQ 3 QT160/161 1.07/0904 Figure 1-2 Internal Switching & Timing Cs Cx SNS2 SNS1 ELECTRODE Charge Am p Result Done Start Figure 1-3 Mesh Key Geometry Figure 1-4 Open Electrode for Back-Illumination |
Similar Part No. - QT160 |
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Similar Description - QT160 |
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