Device for extending the resolution of a N-bit resistive digital-to-analog converter to a (N+P)-bit digital-to-analog converter

Device for extending the resolution of a N-bit digital-to-analog DAC converter having N-digital inputs (A6..A3) and one output (7) to a (N+P)-bit DAC having (N+P) digital inputs (A6..A0) set to a first or second binary value (0 or 1) and one output, for converting a (N+P)-bit word into an analog voltage generated on the (N+P)-bit DAC,

• said N-bit DAC comprising a first string (2) made of 2.sup.N resistive elements (R1 to R16), each having a first and a second taps,
• said string being mounted in series between a first terminal connected to a first voltage (V1) and a second terminal connected to a second voltage (V2), and
• a first set of at least 2.sup.N switches (10,12), each switch being mounted between the first tap of a resistive element and the N-bit DAC output and being responsive to one of the 2.sup.N combinations of the values of the N digital inputs to establish a first conductive path between the first tap of a selected one of said resistive elements and the output of the N-bit DAC,
• said device being characterized in that it comprises:
• a second string (18) comprising at least 2.sup.P -2 elements and being connected in series between a first node (M1) connected to the N-bit DAC output, and a second node (M7),
• said elements having first and second pads,
• a second set of switches (14), each switch being mounted between the second tap of a resistive element of the first string and said second node and being responsive to the same combinations of the values of the N digital inputs as the first set of switches to establish a second conductive path between said second node (M7) and the second tap of the selected one of said resistive elements (R1 to R16),
• means (22, SW21) responsive to a specific combination of the P-bits to disconnect the second string from the first and second nodes (M1, M7) and provide the voltage generated at the N-bit DAC output to the output of the (N+P)-bit DAC when all said P-bits are set to the first value.