Patent ID: 6486817
Filing Date: 2002-11-26
Classification: H03M

Abstract:
A digital-analog conversion circuit comprising:a first resistor string; a second resistor string; a third resistor string connected in series between said first resistor string and said second resistor string; a switch group which switches over connection points of output terminals for said first resistor string, said second resistor string, and said third resistor string; a first controller which controls the connection point switchover conducted by said switch group in response to a first digital signal; a first variable voltage source which applies a desired potential to a terminal of said first resistor string which is not connected to said third resistor string; a second variable voltage source which applies a desired potential to a terminal of said second resistor string which is not connected to said third resistor string; and a second controller which keeps a potential difference between the terminal of said first resistor string which is not connected to said third resistor string and the terminal of said second resistor string which is not connected to said third resistor string constant, and changes an output voltage of said first variable voltage source and an output voltage of said second variable voltage source according to a second digital signal; wherein said first variable voltage source comprises a first constant voltage source; a second constant voltage source; a first switch group which switches over the connection point with respect to said first resistor string; a first differential amplifier which outputs a difference between a potential at a connection point of said first switch group with said first resistor string and a potential of said second constant voltage source; and a first transistor which changes a resistance value between said first constant voltage source and the terminal of said first resistor string which is not connected to said third resistor string so as to make the potential at the connection point of said first switch group with said first resistor string equal to the potential of said second constant voltage source according to an output of said first differential amplifier, said second variable voltage source comprises a third constant voltage source; a fourth constant voltage source; a second switch group which switches over the connection point with respect to said second resistor string; a second differential amplifier which outputs a difference between a potential at a connection point of said second switch group with said second resistor string and a potential of said fourth constant voltage source; and a second transistor which changes a resistance value between said third constant voltage source and the terminal of said second resistor string which is not connected to said third resistor string so as to make the potential at the connection point of said second switch group with said second resistor string equal to the potential of said fourth constant voltage source according to an output of said second differential amplifier, and said second controller controls switchover of the connection point of said first switch group with said first resistor string and the connection point of said second switch group with said second resistor string so as to make a resistance value between the connection point of said first switch group with said first resistor string and the connection point of said second switch group with said second resistor string constant.