A 250 mV 7.5 μW 61 dB SNDR SC ΔΣ Modulator Using Near-Threshold-Voltage-Biased Inverter Amplifiers in 130 nm CMOS

An ultra-low voltage switched-capacitor (SC) ΔΣ converter running at a record low supply voltage of only 250 mV is introduced. System level aspects are discussed and special circuit techniques described, that enable robust operation at such a low supply voltage. Using a SC biasing approach, inverter-based integrators are realized with overdrives close to the transistor threshold voltageVthwhile compensating for process, voltage and temperature (PVT) variation. Biasing voltages are generated on-chip using a novel level shifting circuit, that overcomes headroom limitations due to saturation voltageVsat. With an oversampling ratio (OSR) of 70 and a sampling frequency (fS) of 1.4 MHz at 250 mV power supply the converter achieves 61 dB SNDR in 10 kHz bandwidth while consuming a total power of 7.5 μW.