A Pixel-Level TAC Design For Readout Circuits

In the focal plane readout circuit,Trans-Impedance Amplifier converts the photocurrent detected by an infrared detector into a time signal,Time-to-Amplitude Converter then converts the time signal into a voltage signal,the distance information of the detected object can be obtained.TAC is one of the most critical circuit focal plane readout modules,its performance directly determines the performance of the entire focal plane readout circuit,therefore,how to design a high-performance TAC is the most important subject in the focal plane readout circuit.In integrated circuits,integrator can convert time signal to voltage signal,this article is based on this idea to design a pixellevel TAC.This article first introduced the principle of this pixel-level TAC.The principle is to generate a global ramp signal outside the pixel.The Buffer pushes the ramp signal into each pixel as a buffer.Sample and hold circuit in the pixel signal sampling time end signal corresponding to voltage ramp.According to the size of the sampling voltage,the arrival time of the time-ending signal can be obtained.Final the row and column selection module reads out the sampling voltage in each pixel.After introducing the principle of the pixel-level TAC in this paper,this has analyzed the principle of its sub-modules in detail.First of all,this paper introduces the design of the pixel-level TAC auxiliary circuit module,including the operational amplifier,bandgap reference voltage,reference circuit,and ramp.Produce and push the design of the module,then this article has analyzed the design of the pixel,row selection and column selection.This paper simulates the design using Cadence IC616 based on 0.18?m CMOS process.this pixel-level TAC as a whole achieves a 64×64 array size,12-bit effective number of digits,500 ps time resolution,0~2?s time measurement range,and 1kHz frame rate.Due to the limited response speed of the op amp and source follower,the linearity of the TAC is poor when the time interval is relatively short.When the interval is greater than 10 ns,the linearity of the pixel-level TAC becomes very good.Its DNL is only 0.5LSB,and the largest of INL is 1.2LSB.Based on the good performance of this pixel-level TAC,this paper continues to design the layout.The final designed pixel area is 50?m×50?m.Each pixel contains not only TAC but also TIA.The total area of the chip is 4166?m × 4066?m,a total of 4096 pixel units are integrated,so the design of this pixel-level TAC meets the requirements of the focal plane readout circuit.