This paper presents a scaling friendly mostly digital voltage-controlled-oscillator (VCO)-based 0-1 multistage noise shaping (MASH) analog-to-digital converter. A novel background calibration technique corrects conversion errors due to VCO linear gain drift, residue generating digital-to-analog converter mismatches, and nonlinearity of the VCO voltage-to-frequency conversion. The proposed architecture minimally modifies the basic 0-1 MASH architecture and directly calibrates the main VCOs without relying on replica matching. A redundant first-stage coarse quantizer enables fast error estimation in the digital domain. A 12-b prototype implemented in 180-nm CMOS achieves 12-b ENOB over 2.5 MHz and consumes 4.8 mW from a 1.8 V supply.