This paper proposes a high-accuracy readout system with a high-efficient DC-DC converter. Thanks to a switching DC-DC converter with high conversion efficiency, the power loss in a regulator can be reduced. In the proposed system, sampling is performed using a synchronization signal generated from the switching regulator to avoid deterioration of readout accuracy caused by the ripple noise of the switching converter. Since the proposed technique does not require a large capacitor for ripple reduction, the wake-up time can be reduced, which is suitable for IoT devices operating intermittently. A prototype system developed in this work has achieved a reduction in sensor resistance readout variation to 0.006% while maintaining a high-speed wake-up.