An on-chip capacitor mismatches measurement technique is proposed. The use of a betamultiplier-biased ring oscillator improves the measurement sensitivity by over 6 times with respect to the state-of-the art. Experimental results using a 90 nm CMOS and thick-oxide transistors are presented. The method enables the measurement of capacitors with mismatches being as small as σ=0.04% only, and the minimum measurable capacitance can be as small 4.3fF. The results also demonstrated that better matching can be achieved with low-density capacitors. ©2009 IEEE.