Patent ID: 7239729

Claim:
A circuit for operating a fingerprint sensor, comprising: a circuit portion for detecting sensor signals of individual pixels and for controlling a sequence in which the sensor signals are detected; a first comparing element for determining whether a supplied sensor signal is greater than a stored first value; a second comparing element for determining whether the supplied sensor signal is smaller than a stored second value; a first memory register for storing the first value; a second memory register for storing the second value; a first gate circuit for causing, if the supplied sensor signal has a larger value than a value stored in the first memory register, the larger value to be stored in the first memory register; a second gate circuit for causing, if the supplied sensor signal has a smaller value than a value stored in the second memory register, the smaller value to be stored in the second memory register; a subtracting element in which a difference between the values stored in the first and second memory registers is formed; and a third comparing element for comparing the difference with a prescribed value and for initiating production of a fingerprint image when the difference is greater than the prescribed value.