Image sensing apparatus includes an image pickup plate disposed generally
orthogonally with respect to an expected direction of movement of an
object, such as a finger, multiple image drive plates in spaced relation
to the image pickup plate to define sensor gaps between respective image
drive plates and the image pickup plate, and a reference plate disposed
substantially parallel to the image pickup plate. The reference plate is
spaced from the image pickup plate to permit common mode noise and
coupling to be cancelled and is spaced from the image drive plates to
permit a differential image signal to develop between the image pickup
plate and the reference plate. A differential amplifier coupled to the
image pickup plate and the reference plate provides noise cancellation.
The apparatus may further include a comb plate spaced from the reference
plate and coupled to a reference potential, such as ground.