The capacitance between the device and the ground is referred to as C1, between the person and the
ground C2 and between the person’s finger and the electrode C3.
The B6T series does not directly measure C3, but rather, Cx, the capacitance between the GND of the
detection circuit and the detection electrode.

The following section explains how measurement of the variation of Cx relates to the variation of C3:

Here, C1 is a few hundred pF and C2 is between about 100 and 300 pF, while C3 represents a very small change of a few pF.
None of the capacitance values is stable and may vary greatly depending on changes of the operating environment including humidity and location. 
However, C3 is significantly smaller than C1 and C2 and a change in C3 is dominant in relation to Cx even if the capacitance values change at the same rate.
According to this theory, measuring Cx can detect the approach of a human finger.
The following section explains how the change of Cx is measured specifically.

Generally, measurement of capacitance value, C, requires conversion of the value into an electrical signal such as a frequency, voltage, etc.
For B6T, the value Cx is converted into voltage.
Although various methods are available for converting capacitance into voltage, B6T series uses the “series capacitance division comparison system” (patent pending).
When the voltage on pin A of B6T is represented with Va and the voltage on pin B with Vb, measuring Vb allows detection of change in an unknown capacitance value, Cx.

Specifically, the voltage ratio of the fixed capacitance value Cr to the varying and unknown capacitance value Cx is measured.
Under this method, measuring change in the voltage ratio detects the capacitance change generated by the approach of a person to the electrode, which allows sensing of whether a person has touched the electrode.
For more detailed technical data, click the link below: