A 0.028mm2 19.8fJ/step 2nd-order VCO-based CT ΔΣ modulator using an inherent passive integrator and capacitive feedback in 40nm CMOS

This paper presents an OTA-less 2^nd-order VCO-based CT ΔΣ modulator featuring a passive integrator that makes use of the VCO's inherent parasitic effect. A low-power capacitive feedback technique is also presented for robust loop compensation. Fabricated in 40nm CMOS, the prototype occupies 0.028mm² of active area and consumes 524μW when sampling at 330MHz. The ΔΣM achieved peak Walden FoM of 19.8fJ/step with 68.6dB SNDR over 6MHz BW.