A 40GS/s 6b ADC in 65nm CMOS

Abstract

Progress in 40Gb/s optical dual- polarization (DP) QPSK systems inspired an idea of 100G transmission by optical frequency division multiplexing (FDM) of QPSK-modulated channels [1]. A practical solution suggests two 58Gb/s DP QPSK channels, spaced by 50GHz (Fig. 21.7.1). The challenge is in implementing a 6b ADC operating at sampling rate of 29Gs/s, as compared to 24Gs/s reported before [2]. The other challenge is reduction of ADC sampling jitter. In an interleaved architecture, jitter is limited by the timing mismatch between the clocks of T&H circuits. While initial timing error is compensated during ADC calibration, its spread over the input frequency range and drift may still impact jitter performance. This paper presents, to our knowledge for the first time, a 6b ADC operating up to 40Gs/s with power dissipation ≪ 1.5W. The 30% margin for the sampling rate reduces interleaved timing errors and therefore sampling jitter below 0.25ps-rms. The ADC also includes on-chip test signal synthesizer that generates a gigahertz range sinusoidal signal to simplify production testing.