Digital to analog converter with dual resistor string

A digital-to-analog converter comprising:

• first and second high reference voltage sources and first and second low reference voltage sources:
• a first resistor string coupled between said first high reference voltage source and said first low reference voltage source, said first resistor string having a plurality of first resistors;
• a second resistor string having a plurality of second resistors;
• a first switching means having a plurality of switch pairs with each switch pair corresponding to one of said plurality of first resistors, said first switching means being effective for selectively coupling said second resistor string in parallel to any one of said plurality of first resistors by means of a corresponding switch pair;
• a current source coupled between said second high reference voltage source and the upper-most node of said second resistor string, said current source being effective for supplying a first current into said second resistor string;
• and
• a current drain coupled between said second low voltage source and the lower-most node of said second resistor string, said current drain being effective for drawing a second current from said second resistor string.

A method of constructing an N-bit digital-to-analog converter comprising:

• subdividing said N-bits into c most significant bits and f least significant bits where f=N-c;
• coupling 2.sup.c first resistors of substantially similar magnitude Rc in series to form a first resistor string;
• coupling said first resistor string between a first high reference voltage V.sub.REF and a first low reference voltage V.sub.REPN to establish a first current Ic within said first resistor string;
• coupling 2.sup.f second resistors of substantially similar magnitude Rf in series to form a second resistor string;
• coupling a current source to supply a second current I.sub.BIAS through the entirety of said second resistor string;
• coupling a current drain for drawing said second current I.sub.BIAS from said second resistor string;
• implementing a switch network effective for selectively coupling said second resistor string in parallel to any one of said 2.sup.c first resistors.