Analog amplifier having DC offset cancellation circuit and method of offset cancellation for analog amplifiers
An amplifier having DC offset compensation includes at least one input node and a pair of differential output nodes, a biasing circuit coupled to the input node; and a plurality of current sources. Selected ones of said current sources are coupled to the input node to adjust a DC voltage at the input node to provide DC offset compensation for the amplifier
1. An apparatus comprising:
an amplifier (e.g., 110 ) having a pair of differential input nodes (e.g., INP, INN) and a pair of differential output nodes (e.g., V ON , V OP );
first and second biasing circuits (e.g., R bias , R cm , i bias ) coupled to first and second input nodes of the pair of differential input nodes, respectively; and
a first group of two or more selectable current sources (e.g., 116 ) coupled to the first input node, wherein the first group selectably adjusts DC voltage at the first input node to compensate for DC offset between the pair of differential output nodes; and
a second group of one or more selectable current sources (e.g., 116 ), different from the first group, coupled to the second input node, wherein the second group selectably adjusts DC voltage at the second input node to compensate for the DC offset between the pair of differential output nodes.
2. The invention of claim 1 , further comprising control circuitry (e.g., 204 , 208 , 206 ) that compares signals at the pair of differential output nodes and generates control signals that control the two or more selectable current sources in the first group and the one or more selectable current sources in the second group.
3. The invention of claim 1 , wherein:
the first group is permanently connected to the first input node; and
the second group is permanently connected to the second input node.
4. The invention of claim 1 , wherein the second group has only one selectable current source.
5. The invention of claim 4 , wherein:
each selectable current source has a corresponding weighting;
the two or more selectable current sources of the first group are binary weighted with respect to one another; and
the weighting of the one selectable current source of the second group is greater than a sum of the weightings of the two or more selectable current sources in the first group.
6. The invention of claim 1 , further comprising an offset detection circuit (e.g., 204 ) coupled to the differential output nodes and having an offset detection output (e.g., OUT), wherein the offset detection circuit comprises a comparator.
7. The invention of claim 1 , wherein the apparatus is fabricated on an integrated circuit chip.
8. The invention of claim 1 , wherein at least one of the selectable current sources comprises a constant current source coupled in series with a switch.
9. The invention of claim 1 , wherein each biasing circuit comprises:
a first resistive element coupled as a bias resistive element between a power supply node and a respective intermediate node;
a bias current source coupled between the intermediate node and a ground node; and
a second resistive element coupled between the intermediate node and a respective input node,
wherein:
each of the selectable current sources in the first group is coupled between the first input node and the ground node; and
each of the selectable current sources in the second group is coupled between the second input node and the ground node.
10. The invention of claim 1 , wherein the apparatus comprises a decision circuit that slices a signal based on a differential signal presented at the differential output nodes.
11. The invention of claim 1 , wherein the apparatus is part of a serializer deserializer (SERDES) system.
12. A method for compensating DC offset at a pair of differential output nodes of an amplifier, the method comprising:
(a) monitoring outputs at the pair of differential output nodes to characterize DC offset corresponding to a configuration of a plurality of selectable current sources, the plurality of selectable current sources comprising:
a first group of two or more selectable current sources coupled to a first differential input node of the amplifier; and
a second group of one or more selectable current sources, different from the first group, coupled to a second differential input node of the amplifier; and
(b) adjusting, based on the characterization in step (a), the configuration of the plurality of selectable current sources to reduce the DC offset.
13. The invention of claim 12 , further comprising (c) repeating steps (a) and (b) for one or more additional iterations, wherein, for each additional iteration, step (a) comprises:
(a1) comparing an output of a first output node of the pair of differential output nodes to an output of a second output node of the differential output nodes;
(a2) generating an existing comparison value for the iteration based on the comparison in step (a1); and
(a3) comparing the existing comparison value to a comparison value for a previous iteration to determine whether or not DC offset is reduced from the previous iteration to the current iteration.
14. The invention of claim 13 , wherein:
in step (c), steps (a) and (b) are repeated for the one or more additional iterations until a configuration of selectable current sources is determined that minimizes DC offset; and
the method further comprises (d) programming the apparatus to use, during an operating phase of the amplifier, the configuration of selectable current sources that minimizes DC offset.
15. A calibration system for a decision circuit comprising an amplifier having DC offset compensation, the amplifier comprising at least one input node, a pair of differential output nodes, and a DC offset compensation circuit responsive to a control signal to adjust a DC voltage at the input node to provide the DC offset compensation for the amplifier, wherein the offset compensation adjustment circuit comprises a plurality of selectable current sources, selected ones of the current sources being selected for coupling to the input node of the amplifier by the control signal to adjust the DC voltage at the input node to provide the DC offset compensation for the amplifier, the system comprising:
an offset detection module having a pair of inputs corresponding to the output nodes, the offset detection module providing an offset detection output representative of the DC offset; and
an offset compensation control module for providing the control signal to the offset compensation adjustment circuit for adjusting the DC offset, wherein the amplifier is formed in an integrated circuit and the offset compensation control module is external to the integrated circuit, wherein the control signal programs the DC offset into the integrated circuit.
16. The invention of claim 15 , wherein the plurality of selectable current sources are binary weighted.