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AN4038 数据表(PDF) 5 Page - STMicroelectronics |
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AN4038 数据表(HTML) 5 Page - STMicroelectronics |
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5 / 39 page  AN4038 Main factors influencing proximity sensitivity Doc ID 022686 Rev 1 5/39 1 Main factors influencing proximity sensitivity To design a proximity sensor system using an STM8TL5xxx device, the following parameters must be considered: STM8TL5xxx setting ● Whatever the sensor design of an application, the detection distance can be fine tuned thanks to the STM8TL5xxx sensitivity. Sensitivity is determined by three parameters: the sampling capacitance (CS), the target reference, and the detection threshold. The CS is a capacitor used to accumulate charge from the parasitic capacitor created by the sensor electrodes. The target reference is the gain and the detection threshold defines the detection trigger level. The higher the sensitivity is, the better the detection distance is, but the noise on the acquisition signal increases. When the noise increases, the threshold should be increased to filter the noise. However, this effectively decreases the sensitivity. A compromise must be defined between the detection range and the noise immunity. ● The approach speed is one of the constraints of a proximity application. STM8TL5xxx devices are able to compensate environmental change (temperature, VDD variation) thanks to the environment control system (ECS) feature. The response time of the ECS should be in line with the desired approach speed. The lower the approach speed is, the slower the compensation is. Electrode design ● The detection distance and detection area of a system are determined by the sensor design which is influenced by both the sensor size and shape. Generally, increasing the sensor size, increases the proximity detection distance. The electrode is built with two electrodes. The length, the width, and the spacing between the two electrodes are the three parameters that define the sensor and influence the detection distance. ● Coupling with ground strongly influences the sensitivity of the proximity sensor system. Ground plane or ground surrounding are not recommended. The nearer the ground is from the proximity sensor, the lower the sensitivity is. But, the surrounding ground can have a beneficial effect on the directivity. The directivity can be an important parameter and can be improved by the electrode shape and the surrounding ground. In addition, the mechanical environment around the sensor can significantly modify the detection area and distance. Front panel ● Front panel materials and thickness modify the detection distance and the projected capacitance. The better the dielectric value is, the better the sensitivity of the sensors is. |
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