Fig.2 depicts a schematic of the proposed batteryless gastric
acid sensor circuit. The base station and the sensor module are
inductively coupled. First, the base station supplies power for the
sensor module operation by 13.56-MHz sinusoidal waves, which
is Industrial Scientific Medical (ISM) frequency band [7]. Then,
the sensor circuit converts the output voltage of a pH sensor into
a pulse interval modulation (PIM) [8] code and transmits the
code to the base station.