A Digitally Controlled On-chip Monotonic Reference Current Generator With Low LSB Current For Fast And Accurate Optical Level Monitoring

This paper describes a digitally controlled on-chip monotonic reference current generator with 8-bit resolution and a LSB (least significant bit) current as low as 100 nA. It was designed as a building block of a generic DC-coupled burst mode laser diode driver (BM-LDD) for GPON (gigabit passive optical network) applications and acts as an on-chip reference current generator with a settling time of 18 ns for fast and accurate optical level monitoring with guaranteed monotony. The proposed architecture of the on-chip reference current generator is based on an 8-bit segmented current-steering digital-to-analog converter (IDAC) and a bandgap voltage reference. The bandgap voltage reference deviates only 0.6% of the nominal value over temperature and power supply variations. A cascade current mirror with a super beta helper circuit is used to guarantee monotony and high accuracy. The linearity errors caused by systematic influences and random variations are reduced by the proposed 2D double centroid symmetrical architecture. The design was realized in a 0.35 mum SiGe BiCMOS technology with 3.3 V power supply