A SAR converter having enhanced performance by virtue of effectively pre-loading the SAR's most significant bits with a value that makes the associated DAC output almost equal to the
signal to be converted. A normal SAR conversion is then completed with the SAR bits that have not been pre-loaded. The value used to pre-load the most significant bits of the SAR is preferably obtained from a low-resolution, high-speed converter, such as a flash. The range of DAC bits used in the normal SAR part of the conversion may be increased such that errors up to a certain magnitude in the high-speed converter can be corrected. Reducing
power consumption of a SAR
system can be readily accomplished by reducing
comparator supply
voltage. For a SAR converter architecture using a CAPDAC array or CAPDAC (
capacitor array DAC), fairly large variations in
comparator input
voltage can be expected under these circumstances. If the input
voltage variation becomes too large, damage to the
comparator input devices can occur, or inaccuracies may develop. In one embodiment of the invention, the most significant bits are provided by sampling the input
signal through a
flash ADC that does not suffer from the input voltage restriction described above.