A second-order ΔΣ current-to-digital converter (CDC) for IoT sensing applications is presented in this paper. The proposed CDC uses pseudo-differential current-starved ring oscillators as phase domain integrators. A negative feedback loop relaxes input ring oscillator nonlinearity. The proposed architecture does not require excess loop delay compensation or nonlinearity calibration. Digital differentiation using XOR implements an intrinsic first-order high-pass shaping of static element mismatch in the current steering digital-to-analog converter. A prototype CDC in 65nm CMOS process achieves 62dB dynamic range at 0.48pJ/conversion-step and has 20X better energy-efficiency than state-of-the-art.