A Mixed-Signal Offset-Compensation System for Multi-Gbit/s Optical Receiver Frontends

Abstract

Offset compensation (OC) systems are indispensable parts of multi-Gbit/s optical receiver (RX) frontends. Effects of offset are addressed in this paper. 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 (IDACs) to compensate the static offset in the limiting amplifier (LA) and transimpedance amplifier (TIA), as well as continuously track and compensate the TIA offset. 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$\mu$A residual input offset-current and it is compensating the RX via two feedback loops. These are achieved using an area of only 1345 $\mu \mathrm{m}^{2}$, nearly half of RC-filter based architecture. Although the SC implementation needs less area, its residual offset is 8 times higher. Both conventional implementations have a higher high-pass characteristic of about 20 kHz and can compensate only the offset of the TIA. It is concluded, that 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.