Foreign object detection circuit using mutual impedance sensing

1. An apparatus for detecting an object, the apparatus comprising:

a plurality of sense circuits, each of the plurality of sense circuits comprising:

a primary sense coil and a secondary sense coil configured to inductively sense the object and to have a mutual reactance at an operating frequency; and

a capacitor having a reactance at the operating frequency;

a driver circuit configured to selectively drive a current at the operating frequency in the primary sense coil and the capacitor of each of the plurality of sense circuits; and

a measurement circuit configured to selectively measure a voltage across the secondary sense coil and the capacitor of each of the plurality of sense circuits in response to the current in the primary sense coil and the capacitor;

wherein each of the plurality of sense circuits is configured to:

cause the reactance of the capacitor to substantially compensate for the mutual reactance of the primary and secondary sense coils at the operating frequency; and

cause a transimpedance, defined by the ratio of the voltage and the current, to be substantially resistive at the operating frequency.

2. The apparatus of claim 1 , further comprising a detection circuit electrically connected to the measurement circuit, the detection circuit configured to determine whether an object is proximate the primary sense coil or the secondary sense coil of at least one of the plurality of sense circuits based on an output of the measurement circuit.

3. The apparatus of claim 2 , wherein the detection circuit is configured to determine whether the object is proximate to the primary sense coil or the secondary sense coil by using the output of the measurement circuit to determine a change in mutual impedance between the primary sense coil and the secondary sense coil.

4. The apparatus of claim 1 , wherein in each of the plurality of sense circuits, a terminal of the primary sense coil and a terminal of the secondary sense coil each have a direct electrical connection to a same terminal of the capacitor.

5. The apparatus of claim 1 , wherein:

the driver circuit is configured to operate as a current source and to maintain an output; and

changes in an input impedance of each of the plurality of sense circuits or changes in the voltage across the primary sense coil and the capacitor have a substantially negligible impact on the current provided by the driver circuit.

6. The apparatus of claim 1 , wherein the driver circuit comprises a current source circuit including:

an amplifier circuit having an amplifier output; and

a multiplexer electrically connected between the amplifier output and the plurality of sense circuits and configured to selectively connect each of the plurality of sense circuits to the amplifier output.

7. The apparatus of claim 1 , wherein each of the plurality of sense circuits further comprises:

a first shunt inductor electrically connected to a terminal of the primary sense coil, wherein the first shunt inductor is configured in combination with the capacitor provide a first high pass filter; and

a second shunt inductor electrically connected to a terminal of the secondary sense coil wherein the second shunt inductor is configured in combination with the capacitor provide a second high pass filter; and

wherein each of the first and the second high pass filter is configured to attenuate an influence of a wireless power transfer field.

8. The apparatus of claim 1 , wherein:

the driver circuit further comprises an input multiplexer configured to selectively connect each of the plurality of sense circuits to the driver circuit; and

the measurement circuit further comprises an output multiplexer configured to selectively connect each of the plurality of sense circuits to the measurement circuit.

9. The apparatus of claim 1 , wherein the driver circuit further comprises at least one output series resistor mimicking a current source characteristic.

10. The apparatus of claim 1 , wherein the measurement circuit further comprises at least one input series resistor mimicking a voltage measurement characteristic.

11. The apparatus of claim 1 , wherein the primary sense coil and the secondary sense coil of the plurality of sense circuits are positioned to be distributed over a predetermined area at least partially defined by a wireless power transmit coil configured to inductively transfer power.

12. The apparatus of claim 1 , wherein a frequency of a magnetic field generated by the wireless power transmit coil is different than the operating frequency.

13. The apparatus of claim 1 , wherein the primary sense coil is positioned adjacent the secondary sense coil.

14. The apparatus of claim 1 , wherein the primary sense coil is at least partially overlapping with or interleaved with the secondary sense coil.

15. The apparatus of claim 1 , wherein the primary sense coil is at least partially interleaved with the secondary sense coil.

16. The apparatus of claim 1 , wherein the primary sense coil and the secondary sense coil form a bifilar winding structure.

17. The apparatus of claim 1 , wherein the primary sense coil is magnetically tightly coupled with the secondary sense coil.

18. The apparatus of claim 1 , wherein each of the plurality of sense circuits further comprises;

an input multiplexer switch having a first terminal and a second terminal; and

an output multiplexer switch having a third terminal and a fourth terminal, wherein:

the first terminal of the input multiplexer switch is electrically connected to a first terminal of the primary sense coil,

the third terminal of the output multiplexer switch is electrically connected to a first terminal of the secondary sense coil,

the second terminal of the input multiplexer switch is electrically connected to an output of the driver circuit,

the fourth terminal of the output multiplexer switch is electrically connected to an input of the measurement circuit, and

a terminal of the capacitor has a direct electrical connection to circuit ground.