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QT310 参数 Datasheet PDF下载

QT310图片预览
型号: QT310
PDF下载: 下载PDF文件 查看货源
内容描述: 可编程电容传感器IC [PROGRAMMABLE CAPACITANCE SENSOR IC]
分类和应用: 传感器
文件页数/大小: 20 页 / 830 K
品牌: QUANTUM [ QUANTUM RESEARCH GROUP ]
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of the QT310, 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.  
1.4 SENSITIVITY ADJUSTMENTS  
There are three variables which influence sensitivity:  
1. Cs (sampling capacitor)  
2. Cx (unknown capacitance)  
3. Signal threshold value  
There is also a sensitivity dependence of the whole device on  
Vdd. Cs and Cx effects are covered in Section 1.2.1.  
The threshold setting can be adjusted independently from 1 to  
255 counts of signal swing (Section 2.3).  
Note that sensitivity is also a function of other things like  
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 mutual coupling of the sensor circuit and the object (usually  
via the local environment, or an actual galvanic connection).  
Threshold levels of less than 5 counts in BG mode are not  
advised; if this is the case, raise Cs so that the threshold can  
also be increased.  
Figure 1-5 Shielding Against Fringe Fields  
1.4.1 INCREASING  
S
ENSITIVITY  
In some cases it may be desirable to greatly increase  
sensitivity, for example when using the sensor with very thick  
panels having a low dielectric constant, or when sensing low  
capacitance objects.  
A ‘virtual capacitive ground’ can be created by connecting the  
QT310’s own circuit ground to:  
(1) A nearby piece of metal or metallized housing;  
(2) A floating conductive ground plane;  
(3) A fastener to a supporting structure;  
(4) A larger electronic device (to which its output might be  
connected anyway).  
Sensitivity can be increased by using a bigger electrode,  
reducing panel thickness, or altering panel composition.  
Increasing electrode size can have diminishing returns, as  
high values of Cx load will also reduce sensor gain (Figures  
5-1 and 5-2). The value of Cs also has a dramatic effect on  
sensitivity, and this can be increased in value up to a limit.  
Because the QT310 operates at a relatively low frequency,  
about 500kHz, even long inductive wiring back to ground will  
usually work fine.  
Increasing electrode surface area will not substantially  
increase sensitivity if its area is already larger than the object  
to be detected. The panel or other intervening material can be  
made thinner, but again there are diminishing rewards for  
Free-floating ground planes such as metal foils should  
maximise 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.  
1.3.4 FIELD  
S
HAPING  
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  
moving 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  
small air gap between the grounded shield and the electrode  
will keep the value of Cx lower and is encouraged. In the case  
Figure 1-6 Burst Detail  
LQ  
4
QT310/R1.03 21.09.03  
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