Improvement of Gray ROM-Based Encoder for Flash ADCs

Abstract

In this paper, gray ROM-based encoder is proposed for the implementation of flash ADCs encoder block based on converting the conventional 1-of-[Formula: see text] thermometer codes to 2-of-[Formula: see text] codes ([Formula: see text]). The proposed gray ROM-based encoder is composed of three stages. In the first stage, the thermometer codes are converted to 2-of-[Formula: see text] codes by the use of two-input AND and four-input merged AND–OR gates. In the second stage, 2-of-[Formula: see text] codes are turned to [Formula: see text] gray codes and a binary code by a quasi-gray ROM encoder and a binary ROM encoder, respectively. Finally, in the third stage, [Formula: see text] MSB bits and LSB bit are determined by a quasi-gray-to-binary converter and a CMOS inverter, respectively. The advantages of the proposed encoder over the conventional encoder are higher speed of second stage, low power, low area and low latency with the same bubble and meta-stability errors removing capability. To demonstrate the mentioned specifications, two 5-bit flash ADCs with the conventional and proposed encoders in their encoder blocks are analyzed and simulated at 2-GS/s and 3.2-GS/s sampling rates in 0.18-[Formula: see text]m CMOS process. Simulation results show that the ENOBs of flash ADCs with the conventional and proposed encoders are equal. In this case, the proposed encoder outputs are determined to be approximately 30[Formula: see text]ps faster than the conventional encoder at 2 GS/s. The power dissipations of the conventional and proposed encoders were 19.50[Formula: see text]mW and 13.90[Formula: see text]mW at 3.2-GS/s sampling rate from a 1.8-V supply and also the latencies of the encoders were 4 ADC clocks and 3 ADC clocks, respectively. In this case, the number of D-FFs and logic gates of the proposed encoder is decreased approximately by 37% when compared to the conventional encoder.