Foreign object detection circuit

1. An apparatus for detecting objects, the apparatus comprising:

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

a sense coil configured to inductively sense an object and having a reactance and an equivalent series resistance at an operating frequency; and

a series capacitor electrically connected in series to the sense coil and configured to compensate for the reactance of the sense coil at an operating frequency;

a driver circuit having a plurality of outputs each electrically connected to a first terminal of a respective sense circuit of the plurality of sense circuits and configured to selectively drive a current at the operating frequency in the sense coil and the series capacitor of the respective sense circuit, the driver circuit further comprising:

a driver amplifier having a voltage source output;

a plurality of input multiplexer switches each configured to selectively connect the voltage source output to a selected one of the plurality of outputs of the driver circuit; and

a plurality of first resistors each configured to mimic a current source characteristic at each of the plurality of outputs of the driver circuit and provide a current source output to the respective sense circuit; and

a measurement circuit having a plurality of inputs each electrically connected to a second terminal of the respective sense circuit of the plurality of sense circuits and configured to selectively measure a voltage across the sense coil and the series capacitor of the respective sense circuit in response to the current in the sense coil and the series capacitor, the measurement circuit comprising:

a plurality of second resistors each configured to mimic a voltage measurement circuit characteristic at each of the plurality of inputs of the measurement circuit;

a measurement amplifier having an input with a current measurement circuit characteristic; and

a plurality of output multiplexer switches each configured to selectively connect a selected input of the plurality of inputs of the measurement circuit to the input of the measurement amplifier and provide an output of the respective sense circuit to the input of the measurement amplifier.

2. The apparatus of claim 1 , further comprising a detection circuit electrically connected to the measurement circuit and configured to determine whether an object is proximate to the 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 output of the measurement circuit is indicative of an impedance of the respective sense circuit and wherein proximity of the object to the sense coil of the respective sense circuit is determined based on a change in the impedance of the respective sense circuit.

4. The apparatus of claim 1 , wherein each sense circuit additionally comprises a shunt inductor electrically connected in parallel to the series connection of the sense coil and the series capacitor, the shunt inductor configured to:

have a reactance substantially larger than an impedance of the series connection of the sense coil and the series capacitor; and

attenuate a signal component with a frequency substantially lower than the operating frequency in the voltage at the input of the measurement circuit that is selectively connected to the respective sense circuit.

5. The apparatus of claim 1 , wherein each sense circuit additionally comprises a DC block capacitor configured to attenuate a current with a frequency substantially lower than the operating frequency at the output of the driver circuit that is selectively connected to the respective sense circuit.

6. The apparatus of claim 1 , wherein each sense circuit additionally comprises a DC block capacitor configured to attenuate a voltage with a frequency substantially lower than the operating frequency at the input of the measurement circuit that is selectively connected to the respective sense circuit.

7. The apparatus of claim 1 , wherein the sense coils 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.

8. The apparatus of claim 7 , wherein the operating frequency is higher than a fundamental frequency of a magnetic field generated by the wireless power transmit coil.

9. The apparatus of claim 1 , wherein the impedance of the series connection of each sense coil and series capacitor at the operating frequency is substantially resistive.

10. A method of detecting objects, the method comprising:

in a driver circuit having a plurality of outputs each electrically connected to a first terminal of a respective sense circuit of a plurality of sense circuits each comprising: a sense coil having a reactance and an equivalent series resistance at an operating frequency; and a series capacitor electrically connected in series to the sense coil, selectively driving a current at an operating frequency into the sense coil and series capacitor of the respective sense circuit of the plurality of sense circuits by at least:

producing a voltage at a voltage source output of a driver amplifier;

operating a plurality of input multiplexer switches to selectively connect the voltage source output to a selected output of the plurality of outputs of the driver circuit; and

in a selected resistor of a plurality of first resistors connected to the plurality of outputs of the driver circuit, mimicking a current source characteristic at the selected output and providing a current source output to the respective sense circuit;

in the sense coil of the respective sense circuit, inductively sensing the object;

in the series capacitor, compensating for the reactance of the sense coil at the operating frequency; and

in a measurement circuit having a plurality of inputs each electrically connected to a second terminal of the respective sense circuit, selectively measuring a voltage across the sense coil and the series capacitor of the respective sense circuit in response to the current in the sense coil and the series capacitor, wherein measuring the voltage comprises:

operating a plurality of output multiplexer switches to selectively connect a selected input of the plurality of inputs of the measurement circuit to an input of a measurement amplifier having a current measurement circuit characteristic; and

in a selected resistor of a plurality of second resistors connected to the plurality of inputs of the measurement circuit, mimicking a current voltage measurement circuit characteristic at the selected input.

11. The method of claim 10 , further comprising driving current into and measuring voltage across each of the sense circuits in turn.

12. The method of claim 10 , further comprising, in a detection circuit electrically connected to the measurement circuit, determining whether an object is proximate to the sense coil of at least one of the plurality of sense circuits based on an output of the measurement circuit.

13. The method of claim 12 , wherein the output of the measurement circuit is indicative of an impedance of the respective sense circuit and wherein proximity of the object to the sense coil of the respective sense circuit is determined based on a change in the impedance of the respective sense circuit.

14. The method of claim 10 , further comprising, in the respective sense circuit, attenuating a signal component with a frequency substantially lower than the operating frequency in the voltage at the input of the measurement circuit that is selectively connected to the respective sense circuit via a shunt inductor electrically connected in parallel to the series connection of the sense coil and the series capacitor, the shunt inductor configured to have a reactance substantially larger than an impedance of the series connection of the sense coil and the series capacitor.

15. The method of claim 10 , further comprising, in the respective sense circuit, using a DC block capacitor to attenuate a current with a frequency substantially lower than the operating frequency at the output of the driver circuit that is selectively connected to the respective sense circuit.