Design Of 8 Bit 1.5 GSPS CMOS Track-and-hold Circuit

Analog to digital converter converters(A/D converters) are the bridge which connect the analog world and digital world, and are widely applied in communication and information system, automatic control, radar, military, aviation and spaceflight, medical, electric appliances and many aspects. With the drive of high frequency telecommunication and software radio communication technologies, A/D converters are pushed to develop at high frequency and high speed trend. High speed A/D converters track the fast changing signal, sample the amplitude of signal at every moment. The sampling speed of thousands of Megahertz guarantees great dynamic characterization of process high frequency signal, therefore high speed A/D converters have been the essential parts of the high performance signal acquisition and processing systems. The development of future wireless digital signal processing system will force A/D converters to close up to the radio frequency terminal of system, consequently the research about high speed A/D converters have been attached importance by academic and industrial circles.The track-and-hold, or THC, is a critical part of high speed data acquisition systems. It is often used at the front-end of A/D converters to take samples of its analog input signal and hold these samples for quite enough time during a short period of time. And during such times the A/D converters accomplish quantification and coding to achieve higher target, such as speed and accuracy. THC can be used in any place where transient signals need to be held. Most data acquisition systems have to apply THC, especially high speed data converters. THC determining the whole A/D converters'characterization, including speed and accuracy, thus it is of great importance to research T/H circuit to enhance A/D converters'function.This paper orientates to the research tendency of high speed A/D converters, and does certain study about high speed THC. First of all, background of study is described including international semiconductor and data converters development and the development and present situation of high speed A/D converters in academic and industrial circles. In the second place, basic working theorem of A/D converters is introduced and basic architectures and Performance Metrics are compared. Moreover, basic theorem of THC is presented, including the comparison of traditional open loop and closed loop architectures, four mode specification of THC and the choice of manufacturing technologies. Last, a time-interleaved two-stage THC is proposed.The designed circuit is implemented in a standard 1 Poly 5 Metal 0.18μm CMOS process. Cadence simulation results show that with coherent sampling method and coupled with 1.5GSPS sampling rate and 1Vpp sinusoidal input, the proposed THC achieves 67.69dB Spurious Free Dynamic Range (SFDR), -67.44dB Total Harmonic Distortion (THD)and 6.64 bit accuracy. The test results of typing out show that under Nyquist-rate sampling, 45.79dB SFDR , -42.05dB THD and 6.64 bit accuracy are achieved with sampling frequency 1.5GHz. The performance of the THC is comparable to most bipolar designs, both in speed and accuracy. The proposed THC can be used as a front-end THC in Flash and interpolating & folding A/D converter architectures.