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In this paper, two magnetic-to-digital converters (MDCs) using single-amplifier-based second-order delta-sigma modulators (DSMs) are presented to detect the dc magnetic field. The proposed second-order DSM required only a single-operational amplifier to achieve the second-order noise shaping. Both the proposed circuits have been fabricated in a 0.5-μm CMOS DPDM process, and the resolution of 11 bits can be achieved. The measured sensitivities are 1.486 and 0.459 mV/mT, and the minimum detectable magnetic fields are 0.6 mT and 0.4 mT for the MDC with and without the pre-amplifier, respectively. Both the measured nonlinearities are smaller than 1.3% within the range of ± 100 mT.