circuit diagram for 12- bit adc
also... don`t mind, but...
how can you implement a sample&hold??? its very important so you can measure higher frecuencies...
(if you change the analog signal while its measuring by successive aprox. the result wouldn't be accurated...)
(the sampling rate, will be the 12 times the bit shifting plus init, plus the sample&hold...)
(your DAC should be of 12bits or more.... a high precisition comparator [also fast] so much work... maybe it`s easier to go and find a ADC of 12 bits...)