This paper presents a second-order Sigma-Delta modulator for electroencephalogram applications with 10 bits of resolution, 1.2 V of supply voltage, and only 140 nW of power consumption over a bandwidth of 25 Hz. Low-voltage operation has been achieved using quasi-floating-gate-based circuits. The use of a new class-AB operational amplifier in weak inversion allows very low power consumption. Experimental results show an energy efficiency of 1.6 pJ per quantization level, making it the most energy-efficient converter reported to date in the very low signal bandwidth range.

Matching accuracy of capacitors at the first stage of the residue amplifier limits the overall resolution in pipelined analog-to-digital converters. A new residue amplifier is presented in this paper which is inherently insensitive not only to capacitor ratio but also to amplifier offset voltage and gain errors. Experimental and simulation results are provided to show how the proposed technique senses and compensates these errors.