Capacitive touch sensing using an internal capacitor of an analog-to-digital converter (ADC) and a voltage reference

摘要

An internal sampling capacitor of an analog-to-digital converter (ADC) in a digital device is charged to a reference voltage, then some of the voltage charge on the internal sampling capacitor is transferred to an external unknown capacitor through a low resistance switch internal to the digital device. After the charge transfer has stabilized, the voltage charge remaining on the internal sampling capacitor is measured. The difference between the known reference voltage and the voltage remaining on the internal sampling capacitor is used to determine the capacitance value of the external capacitor. Alternatively, the external capacitor may be charged to a reference voltage then the external capacitor is coupled to the internal sampling capacitor, e.g., having no charge or a known charge on it, and the resulting voltage charge on the internal sampling capacitor is measured and used for determining the capacitance value of the external capacitor.

主权项

1. A method for determining a capacitance of an external capacitor with an integrated circuit device, said method comprising the steps of:
using only a sample and hold capacitor of an analog-to-digital converter (ADC) within the integrated circuit device to measure a sensor capacitor having a first electrode connected with an external pin of the integrated circuit device, wherein a measurement is performed by the following steps: charging from a voltage reference within the integrated circuit device the sample and hold capacitor to a first voltage by connecting the voltage reference with the single internal capacitor, wherein the sample and hold capacitor has a known capacitance; while keeping the voltage reference connected to the sample and hold capacitor, measuring a first voltage on the sample and hold capacitor with the ADC; discharging the external capacitor with a first switch in the integrated circuit device; disconnecting the voltage reference and coupling the sample and hold capacitor to the external capacitor with a second switch in the integrated circuit device so that some of the charge on the sample and hold capacitor is transferred to the external capacitor; de-coupling the sample and hold capacitor from the external capacitor and measuring a second voltage across the internal capacitor with the ADC in the integrated circuit device; and calculating a capacitance of the external capacitor from the measured first voltage, the measured second voltage, and the known capacitance of the sample and hold capacitor with a digital processor in the integrated circuit device.