High PSRR nano-watt MOS-only threshold voltage monitor circuit

Abstract

This work presents a high PSRR nano-watt resistorless threshold voltage (VT0) monitor circuit that can be used in temperature sensors, voltage and current references, radiation dosimeters and other applications such as fabrication process monitoring and verification. In this circuit design the MOS transistors operate in subthreshold and near-threshold regimes, the circuit analysis is based on a current-voltage relationship derived from a continuous physical MOSFET model, valid from weak to strong inversion. The bias condition is established from the equilibrium between two self-cascode cells operating at different inversion levels, and the high PSRR results from a high gain feedback path. The circuit is MOSFET-only, and can be implemented in any standard digital CMOS process. Post-layout simulations show that it operates with less than 1 V of power supply, consuming only tens of nW, and resulting in a VT0 error lower than 1%, when compared to its modeling value, for a -40 to +125°C temperature range. A very high rejection to VDD variation is achieved in this design, with PSRR lower than -63.9 dB at 100 Hz, and a line sensitivity lower than 252 ppm/V was found for a supply range from 1 V to 3 V. Monte-Carlo simulations are presented to evaluate the fabrication variability sensitivity, presenting a maximum error of 4% for a 3σ spread range. The circuit area is very small, around 0.0047 mm2 including the start-up stage.