A simple and reliable controlled sampling interval (CSI) method is proposed for improving the sampling resolution of periodic signals of analog-to-digital converters (ADC). The proposed CSI method uses K periods to obtain P sampling points. By combining these P sampling points into one period interval, the period is virtually sampled with a high number of samples. This new technique includes two main stages: (i) determining the values of P and K, and (ii) performing the sampling and signal recombination process. Experimental results show that the proposed CSI method, which requires no additional hardware, improves the sampling resolution of periodic signals.