區(qū)分噪聲與實際觸摸——魯棒電容傳感的關(guān)鍵

　　電容感應正在成為一種流行的接口替代開關(guān)和旋鈕在消費電子，前面板顯示應用，以及許多工業(yè)和汽車傳感器。本文描述了影響各種電容傳感技術(shù)的各種噪聲，以及在不同環(huán)境條件下可以克服各種噪聲的方法。

　　電容式傳感可以降低成本、健壯性、靈活性和直觀的用戶界面，而不會破壞與現(xiàn)有形式因素和特征集的兼容性。電容感應還使其他創(chuàng)新的用戶界面（UI）功能，如鄰近效應和LED（發(fā)光二極管）調(diào)光和褪色效果。所有這一切都依賴于具有強大的電容傳感性能。一個強大的電容傳感性能的關(guān)鍵方面是區(qū)分噪聲和一個真正的信號（即來自手指觸摸）。電容傳感技術(shù)的魯棒性取決于如何做到這一點，以避免在惡劣、嘈雜的環(huán)境中錯誤地按下按鈕。

　　Capacitive sensing basics

　　At the heart of any capacitive-sensing system is a set of conductors on a PCB which interact with electric fields. Placing a finger near fringing electric fields adds capacitance to the system. As shown in Figure 1 below， the additional charge storage capacity added by the finger is known as CF （finger capacitance）。 The capacitance of the sensor without a finger present is denoted as CP （parasitic capacitance）。 The measurement system determines the variation in CX （total capacitance） due to the addition of CF for the detection of the finger. This variation in capacitance due to a finger is in the range of less than 1 pF. External noise affecting the sensors either has roots in charge or voltage， and this produces noise at the digital output that looks like capacitance change on the input. This induces a challenge in terms of avoiding any noise， which can also cause the same amount of capacitance change.

　　Sensor Capacitance = CX

　　CX = CP + CF