Those Things about capacitive Touch (Part 2)

 —— Microchip Technology Inc.

         Senior embedded application engineer

         James Li

It will be easier to understand the principle of capacitive touch by first reviewing the concept of equipotential or communicator learned in middle school.

As shown in the figure below, there is a pressure difference between the two containers, and liquidity will be formed. You can view the blue part of the container as a liquid or an electric charge, and as long as the bottom is effectively connected, given enough time, you'll get the stable value every time.

We mentioned in "The Things about Capacitive Touch (I)" that the equivalent capacitance of each sensor channel in the actual capacitive touch project is on the order of 10 pF. In the previous low-cost schemes, it is difficult to directly measure the variation of the value of this small capacitance, so indirect measurement is usually adapted, such as through oscillating circuit, charging time, and external amplification. These methods are no longer mainstream and will not be introduced in depth here.

With the continuous improvement of semiconductor integration and accuracy, capacitor signal sampling can now be read directly by ADC or enhanced ADC. Simple operation and fast speed can release MCU's work load, so that MCU can have more time to do other work or sleep to reduce power consumption. It is the mainstream method of capacitive touch measurement at present.

CHOLD with green bottom in the figure above is the sampling and holding capacitance of ADC, and CX is the equivalent capacitance of the external touch sensor. The two capacitors can be regarded as the containers at both ends of the communicator. There are some switches in the middle, which can cooperate to complete the charging, discharging, connecting and other actions of the capacitor on both sides. It used to be controlled by software, but now it is basically completed by hardware, and the efficiency is further. The way this works is that the charge is transferred to an equipotential and then sampled.

The latest chip will also integrate hardware filtering, magnification adjustment, signal to noise ratio is also greatly improved; CHOLD can usually be adjusted to match the size of the external CX, as much as possible to get a large variation.

Touch signals are usually input with more or less noise. First through the hardware low-pass filtering, if the hardware still does not filter the noise clean, there are software filtering, frequency hopping and other mechanisms, the effective signal recovery. As long as the structure of the sensor is properly designed and the parameters are properly configured, the accuracy of touch operation can still reach 100%.