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QT118H-G Datasheet(PDF) 3 Page - Quantum Research Group |
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QT118H-G Datasheet(HTML) 3 Page - Quantum Research Group |
3 / 14 page 1.3 ELECTRODE DESIGN 1.3.1 ELECTRODE GEOMETRY AND SIZE There is no restriction on the shape of the electrode; in most cases common sense and a little experimentation can result in a good electrode design. The QT118H will operate equally well with long, thin electrodes as with round or square ones; even random shapes are acceptable. The electrode can also be a 3-dimensional surface or object. Sensitivity is related to electrode surface area, orientation with respect to the object being sensed, object composition, and the ground coupling quality of both the sensor circuit and the sensed object. 1.3.2 KIRCHOFF’S CURRENT LAW Like all capacitance sensors, the QT118H relies 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. 1.3.3 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 1-4), which is more than two orders of magnitude greater than that required to create a return path to the QT118H via earth. The QT118H's PCB however can be physically quite small, so there may be little ‘free space’ coupling (Cx1 in Figure 1-4) between it and the environment to complete the return path. If the QT118H 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 QT118H’s own circuit ground to: - A nearby piece of metal or metallized housing; - A floating conductive ground plane; - Another electronic device (to which its might be connected already). 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. ‘Ground’ as applied to capacitive fields can also mean power wiring or signal lines. The capacitive sensor, being an AC device, needs only an AC ground return. 1.3.5 SENSITIVITY ADJUSTMENT 1.3.5.1 Gain Pin The QT118H can be set for one of 3 gain levels using option pin 5 (Table 1-1). This sensitivity change is made by altering the internal numerical threshold level required for a detection. Note that sensitivity is also a function of other things: like the values of Cs and Cx, electrode size, shape, and orientation, the composition and aspect of the object to be sensed, the thickness and composition of any overlaying panel material, and the degree of ground coupling of both sensor and object. The Gain input should never be connected to a pullup or pulldown resistor or tied to anything other than SNS1 or SNS2, or left unconnected (for high gain setting). lq 3 QT118H R1.08 / 0405 Figure 1-3 Internal Switching & Timing Cs Cx SNS2 SNS1 ELECTRODE C ha rge Amp R esult Do ne Start Figure 1-4 Kirchoff's Current Law S e nse E lectrode CX2 S u rrou n d in g en viro n m e n t CX3 SENSO R CX1 |
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