Circuit and method for dynamically controlling op-amp offset for photodetector applications

1. A circuit for controlling op-amp offset in a photodetector application comprising:

a photodetector configured to produce a current in response to detecting light;

an op-amp having an output terminal, a positive trim terminal, a negative trim terminal, and first and second input terminals, wherein at least one of the first and second input terminals is coupled to the photodetector and wherein an output produced at the output terminal of the op-amp when operational is based on the current produced by the photodetector;

a variable trimming circuit configured to receive a current I, wherein the variable trimming circuit includes a plurality of current branches each configured to provide a portion of the current I to an output node of the variable trimming circuit;

a test node; and

a selector having an input portion coupled to the output node and an output portion that can be coupled to both the test node and a transmission node, wherein the transmission node is configured to couple to one of the positive and negative trim terminals, and wherein the selector is configurable to direct current received from the output node to one of the test node and the transmission node.

2. The circuit of claim 1 further comprising a current amplifier positioned between the output portion of the selector and the test node.

3. The circuit of claim 1 wherein at least one of the plurality of current branches is configured to provide a variable amount of current.

4. The circuit of claim 1 wherein the plurality of current branches are configured in a binary arrangement.

5. The circuit of claim 1 wherein a current branch that is configured to provide the smallest current of the plurality of current branches defines a resolution of a trimming current that can be provided to the transmission node.

6. The circuit of claim 1 further comprising a packaging material covering the op-amp, variable trimming circuit, test node, transmission node, and selector, wherein the test node is accessible only before the application of the packaging material.

7. The circuit of claim 1 further comprising a packaging material covering the op-amp, variable trimming circuit, test node, transmission node, and selector, wherein the test node is accessible after the application of the packaging material.

8. The circuit of claim 1 further comprising:

a register coupled to the variable trimming circuit; and

at least one input terminal coupled to the variable trimming circuit, wherein the input terminal is configured to carry configuration parameters to the variable trimming circuit that can be stored in the register for the plurality of current branches.

9. The circuit of claim 8 wherein a feedback loop couples the output terminal of the op-amp to the variable trimming circuit.

10. An integrated circuit package for an optical sensing application comprising:

a photodetector configured to produce a current in response to detecting light;

an op-amp having an output terminal, a positive trim terminal, a negative trim terminal, and first and second input terminals, wherein at least one of the first and second input terminals is coupled to the photodetector and wherein an output produced at the output terminal of the op-amp is based on the current produced by the photodetector;

a variable trimming circuit configured to receive a current I, wherein the variable trimming circuit includes a plurality of current branches each configured to provide a portion of the current I to an output node of the variable trimming circuit;

a selector having an input portion coupled to the output node and an output portion that can be coupled to both a test node and a transmission node, wherein the transmission node is configured to couple to one of the positive and negative trim terminals, and wherein the selector is configurable to direct current received from the output node to one of the test node and the transmission node; and

a packaging material covering the op-amp, variable trimming circuit, transmission node, and selector.

11. The integrated circuit package of claim 10 further comprising:

a register coupled to the variable trimming circuit; and

at least one input terminal coupled to the variable trimming circuit, wherein the input terminal is configured to carry configuration parameters to the variable trimming circuit that can be stored in the register for the plurality of current branches.

12. The integrated circuit package of claim 11 wherein the packaging material further covers the test node.

13. The integrated circuit package of claim 12 wherein the test node is accessible via the at least one input terminal.

14. A method comprising:

testing a plurality of current branches in a variable trimming circuit to determine whether each of the current branches is capable of transmitting a defined amount of current, wherein each current branch provides a fraction of a total current I received by the variable trimming circuit to an output node of the variable trimming circuit;

identifying an offset value for an operational amplifier (op-amp) that receives an input signal from a photodetector, wherein the offset value causes an output level of the op-amp when no signal is being received from the photodetector;

identifying at least one of the current branches needed to provide a trimming current to the output node, wherein the trimming current is selected to minimize the offset value; and

selecting the at least one identified current branch to provide the trimming current to the op-amp.

15. The method of claim 14 further comprising selecting the trimming current as the closest match to the offset value that can be produced by the variable trimming circuit.

16. The method of claim 14 wherein testing the plurality of current branches includes:

selecting a single one of the plurality of current branches;

providing current to the selected single current branch; and

identifying whether an output current provided by the variable trimming circuit matches the fraction of the total current I to be provided by that current branch.

17. The method of claim 14 wherein testing the plurality of current branches includes:

selecting a plurality of current branches;

providing current to the selected plurality of current branches; and

identifying whether an output current provided by the variable trimming circuit matches the fraction of the total current I to be provided by the selected plurality of current branches.

18. The method of claim 14 further comprising:

identifying a second offset value for the op-amp;

identifying at least one of the current branches needed to provide a second trimming current to the output node, wherein the second trimming current is the closest match to the second offset value that can be produced by the variable trimming circuit; and

selecting the at least one identified current branch to provide the second trimming current to the op-amp.

19. The method of claim 18 wherein the steps of identifying, identifying, and selecting are repeated each time a new offset value is needed.

20. The method of claim 18 further comprising retesting the plurality of current branches prior to identifying at least one of the current branches needed to provide the second trimming current to the output node.