Dynamic compensation of aging drift in current sense resistor

1. A current sense resistor circuit comprising

a primary current sense resistor that drifts with age;

a secondary current sense resistor that drifts with age in substantial unison with the primary current sense resistor;

a calibration resistor that does not drift with age in substantial unison with the primary current sense resistor; and

a compensation circuit configured to compensate for aging drift in the resistance of the primary current sense resistor based on a comparison of the calibration resistor with the secondary current sense resistor.

2. The current sense resistor circuit of claim 1 wherein the secondary current sense resistor is in parallel with the primary current sense resistor, except when the compensation circuit is comparing the calibration resistor with the secondary current sense resistor.

3. The current sense resistor circuit of claim 2 wherein the primary and the secondary sense resistors have substantially the same current densities.

4. The current sense resistor circuit of claim 3 wherein the resistance of the secondary current sense resistor is between 1 and 1,000,000 times the resistance of the primary sense resistor.

5. The current sense resistor circuit of claim 2 wherein the secondary current sense resistor is configured such that it is always at substantially the same temperature as the primary sense resistor.

6. The current sense resistor circuit of claim 2 wherein the secondary sense resistor and the calibration resistor are substantially the same value.

7. The current sense resistor circuit of claim 6 wherein the compensation provided by the compensation circuit is a function of the difference between the resistances of the calibration resistor and the secondary current sense resistor.

8. The current sense resistor circuit of claim 2 further comprising an electronic switch configured to controllably connect the secondary sense resistor in parallel with the primary sense resistor.

9. The current sense resistor circuit of claim 8 further comprising an electronic switch compensation circuit configured such that the on resistance of the electronic switch tracks the resistance of the primary current sense resistor.

10. The current sense resistor circuit of claim 9 wherein the electronic switch compensation circuit includes a feedback circuit that includes a resistor voltage divider network.

11. The current sense resistor circuit of claim 10 wherein the primary sense resistor functions as the resistor voltage divider network.

12. The current sense resistor circuit of claim 2 further comprising a current source circuit configured to deliver a substantially constant current to the secondary current sense resistor when the compensation circuit compares the calibration resistor to the secondary current sense resistor.

13. The current sense resistor circuit of claim 2 further comprising a current source circuit configured to deliver a substantially constant current to the calibration resistor when the compensation circuit compares the calibration resistor to the secondary current sense resistor.

14. The current sense resistor circuit of claim 2 wherein the secondary sense resistor includes a plurality of resistors that are:

connected in parallel when the compensation circuit is not comparing the calibration resistor with the secondary current sense resistor; and

connected in series when the compensation circuit is comparing the calibration resistor with the secondary current sense resistor.

15. The current sense resistor circuit of claim 14 wherein the resistance of the calibration resistor is substantially equal to the resistance of the secondary sense resistor when the plurality of resistors are connected in series.

16. The current sense resistor circuit of claim 1 wherein the compensation circuit is configured to compensate for aging drift in the resistance of the primary current sense resistor by providing a compensation current that substantially corrects for the aging drift.

17. The current sense resistor circuit of claim 1 wherein the compensation circuit is configured to compensate for aging drift in the resistance of the primary current sense resistor by changing the resistance of the primary current sense resistor to substantially correct for the aging drift.

18. The current sense resistor circuit of claim 17 wherein:

the primary current sense resistor includes a set of resistors connected in series having a selectable voltage sensing tap point; and

the compensation circuit is configured to change the resistance of the primary current sense resistor by changing the selectable voltage sensing tap point.

19. The current sense resistor circuit of claim 17 wherein the compensation circuit is configured to compensate for aging drift in the resistance of the primary current sense resistor by changing the resistance of the secondary current sense resistor to match the change in the resistance of the primary current sense resistor.

20. The current sense resistor circuit of claim 19 wherein:

the secondary current sense resistor includes a set of resistors connected in series having a selectable voltage sensing tap point; and

the compensation circuit is configured to change the resistance of the secondary current sense resistor by changing the selectable voltage sensing tap point.

21. A current sense resistor circuit comprising

a primary current sense resistor that drifts with age;

a secondary current sense resistor;

a calibration resistor; and

a compensation circuit configured to compensate for aging drift in the resistance of the primary current sense resistor based on a comparison of the calibration resistor with the secondary current sense resistor.

22. The current sense resistor circuit of claim 21 wherein the secondary current sense resistor is in parallel with the primary current sense resistor, except when the compensation circuit is comparing the calibration resistor with the secondary current sense resistor.