In this paper, we propose a new ratio-metric interface for amplitude-modulated MEMS sensors based on mutually injection-locked oscillators (MILO). The two outputs are down-converted by a peak demodulator to improve the immunity to phase variation and noise. The peak values are then fed into a second-order discrete-time delta-sigma modulator that directly produces a digital ratio-metric output with a worstcase resolution greater than 16 bits and with input full scale of 0.6. The interface is clocked by the resonators output signal and is adapted for a sampling frequency up to 1MHz.