37.5.1.1 CVD Self Sensing
A CVD operation begins with the internal Sample-and-Hold capacitor (S&H Cap) being disconnected from the path that connects it to the external capacitive sensor node. While disconnected, S&H Cap is precharged to VDD or discharged to VSS. The sensor node is either discharged or charged to VSS or VDD, respectively, to the opposite level of S&H Cap. When the precharge phase is complete, the VDD/VSS bias paths for the two nodes are disconnected and the paths between S&H Cap and the external sensor node is reconnected, at which time the acquisition phase of the CVD operation begins. During acquisition, a capacitive voltage divider is formed between the precharged S&H Cap and sensor nodes, which results in a final voltage level setting on S&H Cap, which is determined by the capacitances and precharge levels of the two nodes. After acquisition, the ADC converts the voltage level on S&H Cap. This process is, then, repeated with the selected precharge levels inverted for both the S&H Cap and the sensor nodes. The waveform for two CVD measurements, which is known as the differential CVD measurement, is illustrated in the following figure.
