Design Of Analog-to-digital Converter For Image Sensor On Medical Field

With the development of image sensor on medical field, it is important to use high speed high resolution A/D converter on medical field.This paper systematically presents A/D converter used on medical field. Many types of A/D converters and how to design core circuit blocks are also introduced. In this project, 11bit resolution 50MHz sampling frequency of A/D converter has been completed based on UMC 0.18um process. All core circuit blocks including fully-differential gain-boost amplifier, preamplified-comparator, bootstrapped switch, sampling and holding circuit, precision mutiply-by-two circuit, two-phase non-overlapping clock and digital error correction circuit are completely simulated using Spectre emulator in Cadence environment. Bootstrapped switch is used to reduce charge injection. Fully differential gain-boost amplifier has large gain bandwidth product so that setting up time relatively decreased and A/D conversion speed improved. DC gain of OTA is 87dB and open-loop gain bandwidth is 2.71GHz.Phase margin is 70 degree. Closed-loop gain bandwidth is above 1GHz.Settling up time is 2.2ns and differential output voltage swing is 1V. Preamplified comparator which plays an important part in the improvement of resolution in A/D converter can decrease offset voltage and can also be used in high frequency.In this project, low-power consumption has been realized in two ways: on one hand, capacitor scaling technology has been adopted to reduced back-end OTA requirement so that sampling capacitor and OTA current has been optimized. On the other hand, flip-around S/H circuit has been set in order to minimize noise and power dissipation in S/H stage. Also, to improve accuracy, preamplified comparator is used so that offset voltage can be reduced. Bootstrapped switch is also used in sampling-and-holding circuit to reduce signal distortion. The minimum resolution of comparator is 100uV and output response time is about 1.5ns.After circuit design in schematics, layout design, parasitic extraction and postlayout of operational transconductance amplifier(OTA) and comparator have been completed based on"UMC 0.18um MM process". How to reduce the signal link between digital circuit and analog circuit are also introduced in order to improve the chip performance.