Offset-Compensation Systems for Multi-Gbit/s Optical Receivers

Abstract

Offset compensation (OC) systems are indispensable parts of multi-Gbit/s optical receiver (RX) frontends. Effects of offset are addressed in this chapter. The analytical expression for the highest lower-cut-off frequency of the OC with minimum impact on the sensitivity is found. Existing OC solutions are discussed. Then, a novel mixed-signal (MS) architecture is introduced which uses digital filtering of the signal, and current-digital-to-analog converters to compensate the static offset in the limiting amplifier. In the transimpedance amplifier both static and dynamic offset are compensated. By using two feedback loops and a continuous tracking the presented solution offers more functionality than other existing MS architectures. Three RX implementations, with RC, switched-capacitor (S-C) and with the MS-OC architectures, in the same 28 nm bulk-CMOS are compared quantitatively with measurements. The presented MS design reaches a lower-cut-off frequency of under 9 kHz, a dynamic range of over 1 mA, 3.2 \(\upmu \)A residual input offset-current and it is compensating the RX via two feedback loops. The presented system offers a higher flexibility and functionality in implementation, as well as a very good compromise between area, precision and performance over the commonly used RC-filter and S-C filter based solutions.