A side-channel attack on a reference-charge flow in SAR ADCs discloses analog information acquired at a sensor frontend. A random interrupt dithering masks the correlation between analog input and reference charge flow by injecting extremely-large dither without input range reduction. An unpredictable randomness based on comparator metastable operation by thermal noise enhances the security level. The additional silicon area for the proposed technique is only 7% of an unprotected ADC core area. Fabricated in 0.18 um CMOS, a 10-bit 1MS/s secure SAR ADC demonstrate the suppression of information leakage from 4.6-bit to 0.8-bit against reference-charge side-channel attack.