The invention relates to the digital integration of the output of a sigma-delta modulator used in electrical energy metering applications.
There is provided in the art a circuit that includes a sensor that generates a first signal that is proportional to the derivative of a current with respect to time in an electrical power line having a nominal frequency of 60 Hz. This sensor has a non-ideal phase characteristic that includes a phase deviation from 90xc2x0 at the nominal line frequency. An underdamped integrator integrates the first signal, compensating for the phase deviation in the non-ideal phase characteristic of the current sensor. The integrator has a transfer function with a magnitude peak at a frequency below the line frequency and with a phase shift deviating from 90xc2x0 at the line frequency. This sensor interfaces directly to a flux sensor, thus making it an inefficient design. The system refers to an off-chip opamp circuit which does the integration. These extra analog components are subject to much variation, such as opamp DC off-set, allowing inaccuracies and manufacturing variations to be introduced.
Another technique is using a flux sensor for current measurement. The integration is done digitally and an additional digital filter is added to compensate for analog domain filters that condition the analog output of the flux sensor. However, these filters introduce various errors that must be compensated.
According to one aspect of the invention, there is provided a power metering measurement device. The power metering measurement device includes a first modulator that receives a first analog voltage associated with a current and outputs a first digitized signal. A second modulator receives a second voltage and outputs a second digitized signal. A first lowpass filter receives the first digitized signal and filters out high frequency noise associated with the first signal and decimates the frequency of the first digitized signal. The first lowpass filter outputs a first filtered signal. An interpolator receives a signal associated with the first filtered signal and performs up sampling of the signal associated with the first filtered signal, the interpolator outputting a first up sampled signal. An integrator receives the first up sampled signal and integrates the up sampled signal. The integrator outputs an integrated signal, and also compensates for phase errors introduced to the first up sampled signal and the integrator. A first multiplier multiplies the second digitized signal and integrated signal, and outputs a multiplied signal. The multiplied signal is used to measure power.
In yet another aspect of the invention, there is provided a filter system used in a power metering system. The filter system includes a first lowpass filter filters out high frequency noise associated with the first signal and decimates the frequency of the first digitized signal. The first lowpass filter outputs a first filtered signal. An interpolator receives a signal associated with the first filtered signal and performs up sampling of the signal associated with the first filtered signal. The interpolator outputs a first up sampled signal. An integrator receives the first up sampled signal and integrates the up sampled signal. The integrator outputs an integrated signal, and also compensates for phase errors introduced to the first up sampled signal and integrator.
In yet another aspect of the invention, there is provided an integrator used in metering systems. The integrator includes a core filter that receives a digitized signal and performs integration on the digitized signal. The core filter outputs an integrated signal. A compensator receives the integrated signal and filters phase errors introduced to the integrated signal, thus outputting a reliable integrated digitized signal.