This paper presents a physical unclonable function (PUF) embedded in a 10-bit successive-approximation register (SAR) analog-to-digital converter (ADC). There are two operation modes in this design: PUF mode and ADC mode. In the PUF mode, this design realizes the physically unclonable function by manipulating the capacitor array in the SAR ADC. Only a little area overhead is required for implementing the PUF. An offset cancellation circuit is employed to improve the uniformity of the PUF design. The proposed PUF is authenticated using Challenge Response Pairs (CRPs). Reliability and Uniqueness of the CRPs are 97.69% and 48.01%, respectively. Uniformity of the CRPs is 50.59% that is very close to the ideal value of 50%. In the ADC mode, this design achieves an ENOB of 9.66 under 20 MS/s.