| With the development of integrated circuit technology and the rise of internet of things, the application of biometrics in personal authentication is becoming more and more important, more and more widely. The fingerprint recognition is the most mature technology in many biometric technologies and it is also the most widely used now. Ultrasonic fingerprint is still in the experimental stage. And only few manufacturers have the technology of RF fingerprint. Capacitive fingerprint is the main sensor in the semiconductor fingerprint sensor market. Capacitive fingerprint sensor has small size, high sensitivity, imitation fake finger etc.Compared with common capacitive fingerprint, feedback capacitive fingerprint has higher sensitivity. This thesis is mainly focus on the analog design of feedback capacitive fingerprint. The digital output of fingerprint has 8 bits. It quantizes the distance of chip and finger to 256 gray value.Firstly, different kinds of fingerprint and the history of fingerprint development are introduced briefly. Then it introduces the principle of fingerprint sensor based on the feedback capacitive. Before we design the circuit modules, we have a problem to solve. Because in circuit model, there is a fringing capacitance, being formed the edge electric field between chip and finger. And fringing capacitance is very difficult to be calculated by hand. So we simulate the relationship of the capacitance and the distance between chip and finger using ANSYS Maxwell. The analog circuits of this sensor is composed of collection unit, VGA, 8 bits pipeline ADC, ring oscillator, other auxiliary circuit. Taking into account the dimension, collection unit does not adopt an operational amplifier. It uses a cascade structure. And the gain is up to 70 dB. VGA with resistance feedback changes its gain through changing the feedback resistance value. The range of VGA gain is 1 to 2.4. Step size is 0.2. 8-bits ADC adopts pipeline structure. There are two-stage. The first stage is 4-bits. The second stage is 5-bits. The extra bit is used as digital error correction. Its sampling frequency is 1 MHz. The ring oscillator mainly provides the system clock. There are three kinds of frequency to be selected, respectively 12 MHz, 16 MHz, 22 MHz. Other auxiliary circuits include follower, register and selection circuit etc…Based on 0.5um process, the circuits are simulated with Spector. Through simulation the whole system, we can get the relationship of the distance of finger and chip and gray value. Finally, every module can work normally by the tape-out result. The fingerprint image is clear. |
