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A new multiple-differential-voltage input, MOS, sampled-data, `charge-balance' comparator which can `weight' or scale each of many input voltage pairs has been developed. This comparator easily allows a differential analog input voltage capability on a monolithic A/D converter and greatly reduces the required number of resistors and decoding switches of a potentiometric successive approximation register (SAR) A/D design. An 8 bit converter has been built which uses 20 Rs and 32 switches as compared to the 256 Rs and 512 switches of a standard 2/SUP N/R ladder design. Measurements made on the 8 bit A/D converter are reported and indicate that at least 12 bit converters are possible with this technique. Therefore, a 13 bit converter has been designed which exhibits even greater component reductions-33 Rs and 64 switches instead of 8192 Rs and 16384 switches. A simple interface to microprocessors is provided for both converters which makes use of the standard logic signals of the control bus where the A/D is designed to appear as memory or an I/O port to the microprocessor. A new flexible reference voltage circuit is presented which, in combination with the analog differential input voltage feature, can accommodate arbitrary analog input voltage spans with any desired zero scale offset.
Date of Publication: Dec. 1979