In self-contained mixed-mode systems, a voltage reference (VR) is an essential building block that provides bias voltages for modules such as linear regulators, A/D converters, and RF communication blocks. For systems with tight power budget, such as a miniaturized wireless sensor nodes, VRs should meet certain major criteria. 1) Since VRs serve as a peripheral to some main blocks, it is vital that VRs consume very little power, especially when the total system power budget limited to tens of nW. 2) Minimizing the power should not compromise the performance of VR, such as temperature coefficient and line sensitivity. 3) VRs should be able to operate across a wide range of supply voltage, in particular below 1V, since some power sources, such as energy scavenging units, provide low output voltages.
We propose a voltage reference using a depletion-mode device that addresses and exceeds all the aforementioned criteria by achieving single digit pW power consumption and sub-1V operation without sacrificing temperature and supply voltage immunity. Measurements show a temperature coefficient of 19.4ppm/C (3.4 uV/C), line sensitivity of 0.033%/V, power supply rejection ratio of -67dB, and power consumption of 2.2pW. It requires only two devices and functions from Vdd=3V to 0.5V with a small footprint of 1350um2. The 2-T voltage reference can be stacked to generate different bias voltages. A digitally trimmable voltage reference is also proposed that can compensate for chip-to-chip process variation.