US Patent No. 7,965,877, FINGERPRINT SENSING DEVICE USING PULSE PROCESSING
This technology is about a fingerprint sensing device that measures the capacitance between an array of electrode plates and finger skin using pulse processing, in which pulse width rather than voltage level, is used for capacitance measurement and digital signal conversion. A pulse, the width of which is compared and adjusted with that of a reference pulse, is generated when voltage at sensing electrodes in discharging is compared with a reference voltage. The comparison results are then digitalized in a grade image sensor or output directly in a binary image sensor. The sensor can communicate with a CPU using serial communication, parallel communication, or memory map scheme. Since no A/D is used, there is no extra time and hardware cost for the conversion from analog signals to digital signals. Due to the pulse processing nature, the circuits can be configured insensitive to the change or fluctuation in voltage supply. This feature enables the sensing device work with a variety of voltages, and thus it can be better used in portable, battery powered or passive devices.
Uniqueness and benefits of this technology:
1. Using “pulse width” rather than voltage level for capacitance measurement and signal conversion. No analog signal exists in the sensing device, and no ADC is required;
2. This technology enables a “one-bit” binary detection of fingerprint images, which is fast and energy efficient;
3. The discrete nature makes the fingerprint sensor insensitive to variations in voltage supply.