Common mode stabilization in a fully differential amplifier

A system comprising:

• an analog to digital converter (ADC);
• a processor that is coupled to the ADC;
• and
• a fully differential amplifier circuit that receives an input signal and generates a reference voltage for the ADC by amplifying the input signal, the fully differential amplifier circuit including:
• an input stage that receives the input signal and provides an intermediate signal in differential form on a pair of paths, wherein the intermediate signal having a differential component and a common mode component, and wherein the differential component is of a higher gain compared to the common mode component;
• an output stage that buffers the intermediate signal and provides the amplified output signal;
• a feedback block that provides a feedback signal to the input stage, wherein the feedback signal indicates a deviation of the common mode component in the amplified output signal from a desired level;
• and
• a stabilization block that measures the common mode component and injects a current that is proportional to the common mode component into each of the pair of paths to stabilize a feedback loop formed by the input stage, the output stage and the feedback block.

A method of stabilizing a common mode loop containing an input stage, an output stage and a feedback stage in a fully differential amplifier, the method comprising:

• measuring a common mode voltage at the output of the input stage;
• buffering the measured common mode voltage;
• and
• injecting a current to each of a pair of paths forming the output of the input stage, wherein the current is generated based on the buffered measured voltage.

An apparatus comprising:

• an input stage having a first input terminal, a second input terminal, an first output terminal, and a second output terminal, wherein the input stage receives an input signal and outputs an intermediate signal;
• a stabilization circuit having:
• an measuring circuit that is coupled to the first output terminal of the input stage and the second output terminal of the input stage, wherein the measuring circuit measures a common mode component of the intermediate signal;
• a first current source that is coupled to the first output terminal of the input stage, wherein the first current source injects a first current into the first output terminal that is proportional to the common mode component;
• a second current source that is coupled to the second output terminal of the input stage, wherein the second current source injects a second current into the second output terminal that is proportional to the common mode component;
• an output stage that is coupled to the first and second output terminals of the input stage;
• common-mode feedback circuit that is coupled to the output stage and to the input stage.