Metering circuit including a time-varying reference and method

1. A metering circuit comprising:

a comparator including a first input to receive an input signal having a ringing waveform from a resonant circuit, and including a second input and a comparator output, the comparator configured to detect peaks in the input signal; and

a reference source including a reference output coupled to the second input of the comparator to provide a time-varying reference signal that has an amplitude that decreases over time during a peak counting operation, the time-varying reference signal configured to enable the comparator to discriminate between damped and undamped ringing waveforms.

2. The metering circuit of claim 1 , wherein the reference source provides a linearly decreasing reference signal as the time-varying reference signal.

3. The metering circuit of claim 2 , wherein the linearly decreasing reference signal includes an initial value that is greater than a voltage level of an asymptote of an undamped envelope of the input signal.

4. The metering circuit of claim 2 , wherein the linearly decreasing reference signal includes a slope that is less than zero and greater than minus one.

5. The metering circuit of claim 1 , wherein the reference source provides an exponentially decreasing reference signal as the time-varying reference signal.

6. The metering circuit of claim 5 , wherein the exponentially decreasing reference signal includes an initial value that is less than a peak amplitude of an undamped envelope of the input signal.

7. The metering circuit of claim 5 , wherein the exponentially decreasing reference signal includes an attenuation that is less than an attenuation of an undamped envelope of the input signal.

8. The metering circuit of claim 5 , wherein the exponentially decreasing reference signal is programmable.

9. The metering circuit of claim 1 , wherein the reference source comprises a digital-to-analog converter.

10. The metering circuit of claim 1 , wherein the reference source comprises a variable resistor and a capacitor coupled in parallel between the first input and a power supply.

11. A metering circuit comprising:

a first comparator including a first input, a second input, and an output, the first input configured to receive a resonant signal from a resonant circuit, the second input to receive, from a programmable reference circuit, a time-varying reference signal that decreases over time to discriminate between damped and undamped resonant signals;

a counter to count pulses at the output of the first comparator; and

a second comparator to determine a state of a system based on a count at an output of the counter.

12. The metering circuit of claim 11 , further comprising the programmable reference circuit including a digital-to-analog converter to provide the time-varying reference signal to the second input of the comparator.

13. The metering circuit of claim 12 , further comprising a finite state machine to control the digital-to-analog converter to provide the time-varying reference signal.

14. The metering circuit of claim 11 , wherein the second comparator includes a first input coupled to the output of the counter, a second input to receive a discriminator threshold, and an output to provide a signal indicating the state of the system based on the count.

15. The metering circuit of claim 11 , wherein the time-varying reference signal comprises a linearly decreasing reference signal.

16. The metering circuit of claim 11 , wherein the time-varying reference signal comprises an exponentially decreasing reference signal.

17. The metering circuit of claim 11 , further comprising:

a temperature compensation circuit to receive a reference attenuation signal and a temperature signal and to provide an output signal indicating a frequency;

a low power oscillator to receive the frequency and to provide a clock signal proportional to the frequency;

a table counter to output data in response to the clock signal;

a table look up to provide values to the input of the digital-to-analog converter to produce the time-varying reference signal.

18. The metering circuit of claim 11 , further comprising:

a capacitor coupled between the second input of the comparator and a power supply;

a variable resistor coupled between the second input of the comparator and a node; and

a transistor coupled between the node and the power supply, the transistor including a gate responsive to a control signal to couple the variable resistor to the power supply to produce the time-varying reference signal by discharging the capacitor.

19. A method of providing a time-varying signal comprises:

receiving a resonant signal from a resonant circuit at a first input of a comparator;

receiving a time-varying reference signal from a programmable reference circuit at a second input of a comparator, the time-varying reference signal decreasing over time to discriminate between damped and undamped resonant signals;

counting a number of peaks of the resonant signal that exceed the time-varying reference signal using a counter coupled to an output of the comparator; and

determining a state of a system based on the count.

20. The method of claim 19 , wherein the time-varying reference signal comprises an exponentially decreasing reference signal.