The SAR method of successive approximation, is a method used in A/D converters for converting the analog signal into a digital signal. It is based on the comparison of the analog input voltage with a reference voltage and is also called weighing method. In successive approximation, the comparison is made step by step and is repeated continuously, changing the reference voltage so that it becomes closer and closer to the input voltage.
After an initial comparison in the comparator, the reference voltage is adjusted to the voltage of the analog signal in a larger digital step. Then, in the following comparison, the digital step of the reference voltage is reduced. After a new comparison, a further reduction takes place, and so on. For each step, i.e. each new comparison, a separate clock cycle is required in each case. This means that for a successive approximation with a resolution of 12 bits, 12 clock cycles are required.
The comparison between the input voltage and the reference voltage is performed in a comparator.
The successive approximation in the example
For example, in the first step, if the input voltage of the A/D converter is lower than the comparison voltage, then the Most Significant Bit ( MSB) is set to the Lo value (0), otherwise to the Hi value (1), and the reference voltage is halved. The renewed comparison for the next bit shows that the reference voltage is lower than the input voltage and the following bit is set to "1", the reference voltage is halved again and compared again and so it goes on cyclically until all bits are set, up to the Least Significant Bit( LSB).
The number of bits set is the measure of resolution.
SAR converters are characterized by an extremely high resolution. Top products achieve resolutions of 20 bits, which corresponds to 1,048, 576 steps.