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A low power 12bit 50KS/s R-C SAR (successive approximation) ADC is presented in this paper. A R-C DAC structure and modified sample-hold circuit are presented. For the digits distribution of R-C DAC, the mismatch analysis of capacitors and resistances is emphasized because precision of SAR ADC primarily depends on its DAC. Reasonable layout design can decrease the R-C DAC mismatch. A 12-bit resolution is achieved in 7-5 digits distribution structure of SAR ADC which operates with 1.8 V analog power and 1.8V digital power, is realized in 0.18 μm CMOS 1P6M technology. The SAR ADC draws only 0.54 mW of power and has a maximum conversion frequency of 50 KS/s and SFDR of 81 dB.