In this paper, we give a concrete example of a 10-bit video rate ADC and introduce the effect of top-down design methodology with analog-HDL from the viewpoint of utilization techniques. First, we explain that analog top-down design methodology can improve chip performance by optimizing the architecture. Next, we concretely discuss the importance of modeling and verification. Verification of the full system does not require extracting all the information for each block at the transistor level in detail. The flexible verification method that we propose can provide good and fast full chip verification. We think analog top-down disign methodology will become increasingly more important from now on because "system-on-chip" requires one chip mixed-signal system LSIs.

This paper describes a study to determine if a current-mode circuit is useful as an analog circuit technique for realizing submicron mixed analog-and-digital MOS LSIs. To examine this, we designed and circuit simulated a new current-mode ADC bit-block for a 3 V, 10-bit level, 20 MHz ADC with a pipeline architecture and with full current-mode approach. A new precision current-mode sample-and-hold circuit which enables operation of a bit block at a clock speed of 20 MHz was developed. Current mismatches caused by the poor output impedance of a device were also decreased by adopting a cascode configuration throughout the design. Operation with a 3 V power supply and a 20 MHz clock speed in a 3-bit A/D configuration was verified through circuit simulation using standard CMOS 0.6 µm device parameters. Gain error, mismatch of current, and linearity of the bit block with changing threshold voltage of a device were carefully examined. The bit block has a gain error of 0.2% (10-bit level), a linearity error of less than 0.1% (more than 10-bit level), and a current mismatch of DAC current sources in a bit cell of 0.2 to 0.4% (more than 8-bit level) with a 3 V power supply and 20 MHz clock speed. An 8-to 9-bit video-speed pipeline ADC can be realized without calibration. This confirms that the current-mode approach is effective.