The capacitance of a shielded capacitive load cell is determined so as to
minimize the effect of stray or parasitic capacitance between the load
cell and other objects including the shield. The load cell conductors are
coupled across input and output terminals of an operational amplifier
that is tied to a reference voltage. A constant current is applied to the
load cell, and the resulting rate of change in voltage at the amplifier
output is measured as a representation of the load cell capacitance. In a
vehicle seat sensor application including an electromagnetic interference
shield between the load cell and the seating surface, the amplifier
output is coupled to the load cell electrode furthest from the shield,
the amplifier maintains the other load cell electrode at a virtual
reference voltage, and the shield is tied to the reference voltage.