Low-pass Filter For High-performance Cmos Analog Circuit Design

The paper is to design analogue lowpass filtering circuits with high performances. The circuits are used directly as anti-alias filters in an analogue front-end of video decoder IC(integrated circuit). The specification is that the cutoff frequency must be more than 15MHz, the maximum passband ripple must be 0.2dB and the minimum stopband attenuation must be 40dB. According to the specification and the scheme three architectures including a GmC(transconductor capacitor) filter, a SC(switched capacitor) filter and a SI(switched current) filter are researched and implemented, simulated in 0.18m CMOS process. The results are as expected. Low sensitivity is desired in the design because of the characteristic of IC. So the passive LC network is used as a prototype for building a corresponding active network by leap-frog method. GmC network has better high-frequency performances and is easier to control on chip than RC network. In the paper a GmC filter with a cutoff frequency of 29.8MHz is implemented. An effective improvement is made in CMFB (common mode feedback) through using a differential amplifier with four inputs. Transconductors are tuned through comparing two voltages, which helps to stabilize the cutoff frequency. There are many applications of SC technology in IC design because of the characteristic of SC technology. In the paper four usual transformations from s-domain to z-domain are introduced to show the superiority of the bilinear transformation in high-frequency applications. Then a novel SC architecture is produced to finish a filter with a cutoff frequency of 17.8MHz. The accurate bilinear result is reached by dual-rate sampling. The researches have proved the current mode superior to the voltage mode in high-frequency applications. In the paper a general integrator is completed to synthesize a bilinear SI filter with a cutoff frequency of 17.2MHz. The results in the paper will help the development of filters in manyapplications such as video. As integrated technology and circuit design are developed, continuous time filters will be designed mainly by GmC and SI technology is likely to replace SC technology as a design method of discrete time filters...