Digital to analog converter trim apparatus and method

A digital to analog converter for converting a digital input code into a programmable resistance, comprising:

• a) a plurality of weighted resistors;
• b) a plurality of switches configured to couple said plurality of weighted resistors into an analog network;
• c) an input for receiving an offset code;
• d) an adder for summing the value of said offset code with the digital input code to produce an offset digital code;
• and
• e) a decoder coupling switch control signals to said switches of said analog network as a logical function of said offset digital code;
• wherein said analog network is configured to produce an electrical output resistance that is a stepwise function of said offset digital code;
• and whereby the progression of said steps is selected so that a change in the value of said offset code adjust the output resistance of said analog network by the same factor over a predetermined range of values of the digital input code.

A digital to analog converter for converting a digital input code into an electrical resistance, comprising:

• a) a plurality of weighted resistors coupled end-to-enld as a resistor string having a first end node and a second end node at opposed ends of said resistor string with one resistor node between each of the coupled resistors;
• b) a first output terminal coupled to said first end node;
• c) a plurality of switches coupling each of said resistor nodes and said second end node to a second output terminal;
• d) a memory for storing an offset code;
• e) an adder for summing the value of said offset code with the digital input code to produce an offset digital code;
• and
• f) a decoder coupling switch control signals to said switches of said resistor string as a logical function of said offset digital code;
• wherein the resistance values of said resistors comprising said resistor string have a weight that increases by a multiplicative factor at subsequent resistor nodes of said resistor string so that a change in the value of said offset code adjusts the output resistance between said first and second output terminals by the same factor over a predetermined range of values of the digital input code.

A method of trimming a digital to analog converter converting a digital input code to an analog output, comprising the steps of:

• (a) providing a digital to analog converter having an analog network configured to produce an analog output that is a stepwise function of an offset digital input code received by a decoder, the offset digital input code being produced by a calibration circuit shifting the original digital input code as a function of an offset code;
• and
• (b) selecting the offset code to trim the output of the digital to analog converter,
• wherein the progression of the steps of said analog network is selected so that a shift in the value of the digital input code adjusts the output of said analog network by the same factor over a predetermine range of values of the digital input code thereby trimming said digital to analog converter.

A method of trimming a digital to analog converter, comprising the steps of:

• (a) providing a digital to analog converter having an analog network configured to produce an analog output that is a stepwise function of a digital input code received by a decoder;
• (b) providing a calibration circuit to shift the value of the digital input code decoded by the decoder by an offset code;
• and
• (c) selecting the offset code to trim the output of the digital to analog converter;
• wherein the progression of the steps of said analog network is selected so that a shift in the value of the digital input code adjusts the output of said analog network by the same factor over a predetermined range of values of the digital input code, thereby trimming said digital to analog converter.