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Details, datasheet, quote on part number:QT113H-D
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Datasheet text preview:
CHARGE-TRANSFER TOUCH SENSOR
! ! ! ! ! ! ! ! ! ! ! ! Projects a proximity field through air Less expensive than many mechanical switches Sensitivity easily adjusted via capacitor value Turns small objects into intrinsic touch sensors 100% autocal for life - no adjustments required 2.5 to 5V, 600µA single supply operation µ Toggle mode for on/off control (strap option) 10s, 60s, infinite auto-recal timeout (strap options) Gain settings in 2 discrete levels HeartBeatTM health indicator on output Active-low (QT113) or active-high outputs (QT113H) Only one external part required - a 1¢ capacitor
QProxTM QT113 / QT113H
Vdd O ut O pt1 O pt2
1
8
Vs s S n s2 S n s1 Gain
Q T113
2 3 4
7 6 5
APPLICATIONS ! !
Light switches Prox sensors
! !
Appliance control Security systems
! !
Access systems Pointing devices
! !
Elevator buttons Toys & games
The QT113 charge-transfer ("QT'") touch sensor is a self-contained digital IC capable of detecting near-proximity or touch. It will project a proximity sense field through air, via almost any dielectric, like glass, plastic, stone, ceramic, and most kinds of wood. It can also turn small metal-bearing objects into intrinsic sensors, making them responsive to proximity or touch. This capability coupled with its ability to self calibrate continuously can lead to entirely new product concepts. It is designed specifically for human interfaces, like control panels, appliances, toys, lighting controls, or anywhere a mechanical switch or button may be found; it may also be used for some material sensing and control applications provided that the presence duration of objects does not exceed the recalibration timeout interval. The QT113 requires only a common inexpensive capacitor in order to function. Power consumption is only 600µA in most applications. In most cases the power supply need only be minimally regulated, for example by Zener diodes or an inexpensive 3-terminal regulator. The QT113's RISC core employs signal processing techniques pioneered by Quantum; these are specifically designed to make the device survive real-world challenges, such as `stuck sensor' conditions and signal drift. Even sensitivity is digitally determined and rem ains constant in the face of large variations in sample capacitor CS and electrode CX. No external switches, opamps, or other analog components aside from CS are usually required. The option-selectable toggle mode permits on/off touch control, for example for light switch replacement. The Quantum-pioneered HeartBeatTM signal is also included, allowing a host microcontroller to monitor the health of the QT113 continuously if desired. By using the charge transfer principle, the IC delivers a level of performance clearly superior to older technologies in a highly cost-effective package.
TA 00C to +700C 00C to +700C -400C to +850C -400C to +850C
AVAILABLE OPTIONS SOIC QT113-S QT113H-S QT113-IS QT113H-IS
8-PIN DIP QT113-D QT113H-D
Copyright Quantum Research Group Ltd R1.10/0104
Quantum Research Group Ltd
1 - OVERVIEW
The QT113 is a digital burst mode charge-transfer (QT) sensor designed specifically for touch controls; it includes all hardware and signal processing functions necessary to provide stable sensing under a wide variety of changing conditions. Only a single low cost, non-critical capacitor is required for operation. Figure 1-1 shows the basic QT113 circuit using the device, with a conventional output drive and power supply connections.
Figure 1-1 Standard mode options
+2.5 to 5
SENSING E LE C TR O DE
1 2 3
V dd OU T SNS 2
7 5 Cs
10nF
OP T1
G A IN
1.1 BASIC OPERATION
Cx
4 6 The QT113 employs bursts of charge-transfer cycles to OP T2 SNS 1 acquire its signal. Burst mode permits power consumption in OUTPUT=DC Vs s the microamp range, dramatically reduces RF emissions, TIM EOUT=10 Secs 8 = lowers susceptibility to EMI, and yet permits excellent TOGGLEGOFF GAI N= H I H response time. Internally the signals are digitally processed to reject impulse noise, using a 'consensus' filter which requires three consecutive confirmations of a detection 1.2 ELECTRODE DRIVE The internal ADC treats Cs as a floating transfer capacitor; as before the output is activated. a direct result, the sense electrode can be connected to The QT switches and charge measurement hardware either SNS1 or SNS2 with no performance difference. In both functions are all internal to the QT113 (Figure 1-2). A 14-bit cases the rule Cs >> Cx must be observed for proper single-slope switched capacitor ADC includes both the operation. The polarity of the charge buildup across Cs required QT charge and transfer switches in a configuration during a burst is the same in either case. that provides direct ADC conversion. The ADC is designed to dynam ically optimize the QT burst length according to the It is possible to connect separate Cx and Cx' loads to SNS1 rate of charge buildup on Cs, which in turn depends on the and SNS2 simultaneously, although the result is no different values of Cs, Cx, and Vdd. Vdd is used as the charge than if the loads were connected together at SNS1 (or reference voltage. Larger values of Cx cause the charge SNS2). It is important to limit the amount of stray capacitance transferred into Cs to rise more rapidly, reducing available on both terminals, especially if the load Cx is already large, resolution; as a minimum resolution is required for proper for example by minimizing trace lengths and widths so as not operation, this can result in dramatically reduced apparent to exceed the Cx load specification and to allow for a larger gain. Conversely, larger values of Cs reduce the rise of sensing electrode size if so desired. differential voltage across it, increasing available resolution The PCB traces, wiring, and any components associated with by permitting longer QT bursts. The value of Cs can thus be or in contact with SNS1 and SNS2 will become touch increased to allow larger values of Cx to be tolerated (Figures sensitive and should be treated with caution to limit the touch 4-1, 4-2, 4-3 in Specifications, rear). area to the desired location. Multiple touch electrodes can be The IC is responsive to both Cx and Cs, and changes in Cs used, for example to create a control button on both sides of an object, however it is impossible for the sensor to can result in substantial changes in sensor gain. distinguish between the two touch areas. Option pins allow the selection or alteration of several special features and sensitivity.
1.3 ELECTRODE DESIGN
Figure 1-2 Internal Switching & Timing
E LE C T R O D E
R esul t
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 QT113 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 com position, and the ground coupling quality of both the sensor circuit and the sensed object. If a relatively large electrode surface is desired, and if tests show that the electrode has more capacitance than the QT113 can tolerate, the electrode
S NS 2
S ing le -S lo p e 14-bit S w i tch e d Cap a c ito r AD C
B u r s t Controller
Cs Cx
S NS 1
S ta r t
Do n e
C ha r g e Am p
-2-
can be made into a sparse mesh (Figure 1-3) having lower crum pled into a ball. Virtual ground planes are more effective Cx than a solid plane. Sensitivity may even remain the same, and can be made smaller if they are physically bonded to as the sensor will be operating in a lower region of the gain other surfaces, for example a wall or floor. curves.
1.3.2 KIRCHOFF'S CURRENT LAW
Like all capacitance sensors, the QT113 relies on Kirchoff's Current Law (Figure 1-4) 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 m anner 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 exam ple, 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.
1.3.4 FIELD SHAPING
The electrode can be prevented from sensing in undesired directions with the assistance of metal shielding connected to circuit ground (Figure 1-5). For example, on flat surfaces, the field can spread laterally and create a larger touch area than desired. To stop field spreading, it is only necessary to surround the touch electrode on all sides with a ring of metal connected to circuit ground; the ring can be on the same or opposite side from the electrode. The ring will kill field spreading from that point outwards. If one side of the panel to which the electrode is fixed has m oving traffic near it, these objects can cause inadvertent detections. This is called `walk-by' and is caused by the fact that the fields radiate from either surface of the electrode equally well. Again, shielding in the form of a metal sheet or foil connected to circuit ground will prevent walk-by; putting a sm all air gap between the grounded shield and the electrode will keep the value of Cx lower and is encouraged. In the case of the QT113, sensitivity can be high enough (depending on Cx and Cs) that 'walk-by' signals are a concern; if this is a problem, then some form of rear shielding may be required.
1.3.3 VIRTUAL CAPACITIVE GROUNDS
W hen detecting human contact (e.g. a fingertip), grounding
1.3.5 SENSITIVITY
The QT113 can be set for one of 2 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 value of Cs, electrode size, shape, and orientation, the com position 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. 1.3.5.1 Increasing Sensitivity In some cases it may be desirable to increase sensitivity further, for example when using the sensor with very thick panels having a low dielectric constant.
Figure 1-3 Mesh Electrode Geometry
Sensitivity can often be increased by using a bigger electrode, reducing panel thickness, or altering panel com position. Increasing electrode size can have diminishing of the person is never required. The human body naturally returns, as high values of Cx will reduce sensor gain (Figures 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 Figure 1-4 Kirchoff's Current Law a return path to the QT113 via earth. The QT113'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 environm ent to complete the return path. If the QT113 circuit CX2 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 QT113'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 m axim ize exposed surface area in a flat plane if possible. A square of metal foil will have little effect if it is rolled up or
S e n s e E le c t r o d e
SENSO R
CX 1
S u r r o u n d in g e n v ir o n m e n t
CX3
-3-
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