» Quick Access Smart.Safe.
AuthenTec fingerprint authentication solutions utilize a silicon-based sensor to capture fingerprint images using active-capacitance sensing technology.
Active capacitive pixel-sensing technology
Each sensor cell (pixel) contains an active capacitive feedback circuit whose effective feedback capacitance is modulated by the presence of live skin close to the surface of the sensor. This active sensing approach provides much higher immunity to parasitic effects. This ensures a higher signal-to-noise ratio and greater capacity to capture a wider range of fingerprints than other silicon-based technologies such as passive capacitive sensing.
How it works
The surface of each pixel is composed of two adjacent metal plates, which are separated from the skin and the environment by an ultra-hard protective coating. These sensor plates create a fringing capacitance between them whose field lines extend beyond the surface of the silicon. When live skin is brought in close proximity to the sensor plates, the skin interferes with field lines between the two plates and reduces the effective capacitance between them. When the skin is on the sensor surface (fingerprint ridge) the feedback capacitance is minimized, while when the skin is far from the sensor surface (fingerprint valley) the feedback capacitance is maximized. The sensor cell works in two phases: in the reset phase, the input and output of the inverter are shorted together through a reset switch, causing the charge integrator output to settle to the logical threshold of the inverter. During the sensing phase the reset switch is opened and a calibrated charge is input into the input plate of the sensor cell, causing the charge integrator output to change by an amount proportional to the feedback capacitance between the two sensor cells. Because the feedback capacitance of a fingerprint ridge is smaller than that of a fingerprint valley, the output swing of a sensor cell under a ridge will be greater than the output swing of a sensor cell underneath a fingerprint valley. A two dimensional array of sensor cells is used to capture the entire fingerprint image. The array is addressed in a random access mode through row and column decoders, enabling advanced functions such as windowing and sub sampling. The output of the sensor array is passed through an analog signal-conditioning block providing the capability to adjust sensor gain and offset before the signal is converted through an on-chip A/D converter into an 8-bit digital signal for output off chip.