Successive approximation analog to digital converter with automatic offset

A method for converting an analog value to a digital value with an offset having a given degree of accuracy using a successive approximation analog to digital converter, comprising the steps of:

• measuring an analog value, wherein the analog value being measured is a resistance value;
• converting the analog value to a digital value using a successive approximation analog to digital converter, wherein a multiplying digital to analog portion of the analog to digital converter is comprised of the following elements:
• a plurality of n binarily weighted current sources connected to a first supply voltage and connected in parallel to each other, where n represents the resolution in bits of the successive approximation A/D converter;
• a plurality of n switches, with each binarily weighted current source of the plurality of n binarily weighted current sources connected to a corresponding switch of the plurality of n switches, where n represents the resolution in bits of the successive approximation A/D converter and wherein when the corresponding switch is closed a corresponding binarily weighted current source of the plurality of binarily weighted current sources is connected to the supply voltage;
• a first operational amplifier having a current input signal, a reference input signal, and a voltage output signal;
• a first current source and a second current source, wherein the first current source and the second current source are matched and each are connected to the first supply voltage;
• a first resistive element connected in parallel to the first operational amplifier, wherein the first operational amplifier and the first resistive element operate to convert the current input signal to the voltage output signal;
• a second resistive element connected to the first current source and a second supply voltage;
• a third resistive element connected to the second supply voltage;
• a fourth resistive element connected to the second supply voltage;
• a second operational amplifier having a first input signal equal to a first node defined by the connection of the first current source, the second resistive element, and the reference input signal of the first operational amplifier;
• a second input signal connected to the third resistive element and a emitter of a transistor, wherein the transistor is connected to the first supply voltage;
• and
• a third operational amplifier having a first input signal equal to the voltage output signal of the first operational amplifier, and a second input signal equal to a second node defined as the connection of the second current source and the fourth resistive element;
• and
• subtracting an offset value from the analog value to generate a digital conversion result representative of the analog value with respect to a measurement range, having a minimum value and a maximum value, wherein the offset value achieved by the successive approximation A/D converter is proportional to an external reference value and wherein the digital conversion result is equal to k(X.sub.unknown -X.sub.offset), where k is a constant, X.sub.known is the analog value being measured, and X.sub.offset is the offset value.

A successive approximation analog to digital converter which converts an analog value to a digital value with an offset having a given degree of accuracy, comprising:

• a successive approximation analog to digital converter capable of measuring an analog value, converting the analog value to a digital value, and subtracting an offset value from the analog value to generate a digital conversion result representative of the analog value with respect to a measurement range, having a minimum value and a maximum value, a multiplying digital to analog portion of the analog to digital converter comprised of the following elements:
• a plurality of n binarily weighted current sources connected to a supply voltage and connected in parallel to each other, where n represents the resolution in bits of the successive approximation A/D converter;
• a plurality of n switches, with each binarily weighted current sources of the plurality of n binarily weighted current sources connected to a corresponding switch of the plurality of n switches, where n represents the resolution in bits of the successive approximation A/D converter and wherein when the corresponding switch is closed a corresponding binarily weighted current source of the plurality of binarily weighted current sources is connected to the supply voltage;
• an offset current source representative of the offset value which is connected in parallel to the plurality of binarily weighted current sources and which is connected across the supply voltage;
• an operational amplifier having a current input signal and a voltage output signal, wherein the current input signal is equal to the offset value produced by the offset current source and a current produced by those binarily weighted current sources of the plurality of binarily weighted current sources whose corresponding switch is closed;
• and
• a resistive element connected in parallel to the operational amplifier, wherein the operational amplifier and the resistive element operate to convert the current input signal to the voltage output signal.

A successive approximation analog to digital converter which converts an analog value to a digital value with an offset having a given degree of accuracy, comprising:

• a successive approximation analog to digital converter capable of measuring an analog value, converting the analog value to a digital value, and subtracting an offset value from the analog value to generate a digital conversion result representative of the analog value with respect to a measurement range, having a minimum value and a maximum value, a multiplying digital to analog portion of the analog to digital converter comprised of the following elements:
• a plurality of n binarily weighted current sources connected to a first supply voltage and connected in parallel to each other, where n represents the resolution in bits of the successive approximation A/D converter;
• a plurality of n switches, with each binarily weighted current sources of the plurality of n binarily weighted current sources connected to a corresponding switch of the plurality of n switches, where n represents the resolution in bits of the successive approximation A/D converter and wherein when the corresponding switch is closed a corresponding binarily weighted current source of the plurality of binarily weighted current sources is connected to the supply voltage;
• a first operational amplifier having a current input signal, a reference input signal, and a voltage output signal;
• a first current source and a second current source, wherein the first current source and the second current source are matched and each are connected to he first supply voltage;
• a first resistive element connected in parallel to the first operational amplifier, wherein the first operational amplifier and the first resistive element operate to convert the current input signal to the voltage output signal;
• a second resistive element connected to the first current source and a second supply voltage;
• a third resistive element connected to the second supply voltage;
• a fourth resistive element connected to the second supply voltage;
• a second operational amplifier having a first input signal equal to a first node defined by the connection of the first current source, the second resistive element, and the reference input signal of the first operational amplifier;
• a second input signal connected to the third resistive element and a emitter of a transistor, wherein the transistor is connected to the first supply voltage;
• and
• a third operational amplifier having a first input signal equal to the voltage output signal of the first operational amplifier, and a second input signal equal to a second node defined as the connection of the second current source and the fourth resistive element.