Study On Digital Programmable Gain Amplifier(DPGA) Based On CMOS Image Sensor

This paper describes a digitally programmable gain amplifier controlled byseven-bit inputs, which is an important part of the high performance big dynamicrange CMOS image sensor. It is made up of several modules, such as clock generatingcircuit, reference and bias circuit, fully differential amplifier, switched capacitoramplifier and digital logic units and etc. This paper introduces the main performanceparameters of DPGA , describes the design idea and system architecture of DPGA,illustrates the operation principle of each stage, analyzes the characteristics and designmethod of the fully differential amplifier and how to use the switched capacitor circuitto realize the function to sample and hold. At last, it analyzes each module of theDPGA system and displays the simulation results.There is a digitally programmable gain amplifier which can properly amplify thesignals with different luminance in the CMOS image sensor, for short is DPGA. It canexpand the dynamic range of the whole system. In CMOS image sensor, rowamplifier transfers the signals from pixel units to DPGA. Generally, we use theswitched capacitor amplifier to realize signals sampling and holding, use digital logicunits to control switches in the capacitor segment of switched capacitor amplifier, anddifferent switches logic combination can realize the different gains. The rowamplifiers' inputs are the signals from the pixel units, and they transfer the signals tothe first stage amplifier, which is instrumentation amplifier;the second amplifier isfully differential amplifier;and the DPGA amplifies the input signals with differentamplitudes controlled by seven bits digital inputs, and produces the signals which canbe dealt with by 10b pipelined A/D converter.DPGA has three stages which takes on the pipeline structure. The first stage isan invariable gain amplifier, which samples the outputs from the second stage rowamplifier and amplifies the signals two times as much (6dB expressed by dB unit).The second and the third stage of the DPGA are controlled by low 3-bits and high4-bits of the 7-bits digital inputs and the gain step are 1/8dB and 1dB, respectively.