Fully Digital Rail-to-Rail OTA With Sub-1000-μm² Area, 250-mV Minimum Supply, and nW Power at 150-pF Load in 180 nm

A fully digital operational transconductance amplifier (DIGOTA) architecture for tightly energy-constrained low-cost systems is presented. A 180-nm DIGOTA testchip exhibits an area below the 1000-μm
²
wall, and 2.4-nW power under 150-pF load, and a minimum supply voltage Vmin of 0.25 V. In the 0.3-0.5-V supply range, DIGOTA improves the area-normalized small (large) signal energy FoM by at least 836× (267×) over prior sub-500-mV OTAs, while reducing area by 27×-85×. The low-V
_min
and nW-power features are shown to enable direct harvesting at the mm-scale.