Capacitance sensing systems and methods

1. An apparatus for measuring capacitance of a load, the apparatus comprising:

a ground connector configured to couple to ground;

an output connector configured to couple to the load;

a floating time-varying signal source;

a DC power source, wherein:

the DC power source and the floating time-varying signal source are arranged in a series conduction path between the ground connector and the output connector;

the DC power source is configured to apply a DC voltage onto the conduction path; and

the floating time-varying signal source is coupled between the DC power source and the output connector, and configured to inject a time-varying signal onto the conduction path;

a current monitor configured to measure current in the conduction path; and

a capacitance module coupled to the current monitor, the capacitance module configured to determine the capacitance based upon the measured current.

2. The apparatus of claim 1 , wherein the capacitance module is configured to determine the capacitance by processing a digital signal representation of the measured current.

3. The apparatus of claim 1 , wherein the floating time-varying signal source couples the time-varying signal to the conduction path via a transformer.

4. The apparatus of claim 1 , wherein:

the floating time-varying signal source is a floating AC signal source configured to inject an AC signal onto the conduction path; and

the floating AC signal source includes a direct digital synthesizer to produce a DC representation of the AC signal and a digital-to-analog converter to convert the DC representation to the AC signal, wherein the direct digital synthesizer and the digital-to-analog converter receive power and control signals via galvanically-isolating coupling.

5. The apparatus of claim 1 , wherein the floating time-varying signal source includes an oscillator to produce the time-varying signal, wherein the oscillator receives power and control signals via galvanically-isolating coupling.

6. The apparatus of claim 1 , wherein the DC power source is coupled between the floating time-varying signal source and the output connector.

7. The apparatus of claim 1 , wherein the DC power source is a switch mode power supply.

8. The apparatus of claim 1 , wherein the time-varying signal source includes a linear amplifier.

9. The apparatus of claim 1 , wherein the current monitor includes one or more of a flux gate sensor, a resistive shunt sensor, a hall effect sensor, or a current mirror circuit.

10. The apparatus of claim 1 , wherein the current monitor includes one or more of a peak to peak detector, root-mean-square detector, or a synchronous detector to measure AC current in the conduction path.

11. An apparatus for measuring capacitance of a load, the apparatus comprising:

a ground connector configured to couple to ground;

an output connector configured to couple to the load;

a time-varying signal source configured to inject a time-varying voltage signal onto a conduction path between the ground connector and the output connector;

a DC power source configured to apply a DC offset to the time-varying voltage signal; wherein the DC power source and the time-varying signal source are arranged in series on the conduction path between the ground connector and the output connector, and the time-varying signal source is a floating time-varying signal source coupled between the DC power source and the output connector that couples the time-varying voltage signal to the conduction path via a transformer;

a current monitor configured to measure time-varying current in the conduction path; and

a capacitance module configured to determine the capacitance based upon at least one of the time-varying current and a frequency of the time-varying voltage signal.

12. The apparatus of claim 11 , wherein the time-varying signal source is coupled between the ground connector and the DC power source.

13. The apparatus of claim 11 , further comprising:

a resonant sensing module configured to vary the frequency of the time-varying voltage signal to achieve resonance;

wherein the capacitance module is configured to determine the capacitance based upon the frequency of the time-varying voltage signal when resonance is achieved.

14. The apparatus of claim 11 , further comprising:

a voltage monitor configured to measure a voltage applied by the time-varying signal source;

wherein the capacitance module is configured to determine the capacitance based upon a ratio of the time-varying current to the voltage applied by the time-varying signal source.

15. The apparatus of claim 11 , further comprising:

a second time-varying signal source configured to apply a second time-varying voltage signal to a reference load to monitor variations in a dielectric constant of a dielectric of the load due to temperature;

wherein the capacitance module is configured to determine the capacitance of the load based upon the dielectric constant and at least one of the time-varying current and the frequency of the time-varying voltage signal.

16. The apparatus of claim 11 , further comprising:

an autoranging module configured to:

control the time-varying signal source to maintain a substantially constant amplitude of the time-varying voltage signal; and

control the time-varying signal source to vary the frequency of the time-varying voltage signal to maintain a substantially constant amplitude of the time-varying current in the conduction path;

wherein the capacitance module is configured to determine the capacitance based upon the frequency of the time-varying voltage signal.

17. The apparatus of claim 11 , further comprising:

an autoranging module configured to:

control the time-varying signal source to maintain a substantially constant amplitude of the time-varying voltage signal; and

control the time-varying signal source to vary the time-varying current to maintain a substantially constant frequency of the time-varying voltage signal in the conduction path;

wherein the capacitance module is configured to determine the capacitance based upon the time-varying current.

18. A method for determining capacitance of a load, the method comprising:

applying a time-varying voltage signal with a time-varying signal source to the load;

galvanically isolating the time-varying signal source from ground;

applying a DC offset to the load with a DC power source;

fixing an amplitude of the time-varying voltage signal;

measuring a time-varying current that is provided to the load;

controlling the time-varying signal source to vary a frequency of the time-varying voltage signal to maintain a substantially constant amplitude of the time-varying current provided to the load; and

determining the capacitance based upon the frequency of the time-varying voltage signal.