A novel bubble tolerant thermometer-to-binary encoder for flash A/D converter

A thermometer-to-binary encoder, which is suitable for high-speed and low-resolution flash-ADC, is proposed. The encoding scheme can effectively reduce the bubble induced error, by the use of separated one-out-of-N circuit for each bit of binary codes. In comparison with the most commonly adopted Gray-ROM encoder, the proposed method can outperform it by a factor of 2 in the bubble error reduction. The long latency gray-to-binary encoder which limits the total conversion speed can also be removed from the proposed circuit. Two types of implementation circuit including a ROM-based encoder and a fully digital encoder are developed.