AN578_a| Application Note
One Technology Way P.O. Box 9106 Norwood, MA 02062-9106 Tel: 781/329-4700 Fax: 781/326-8703 www.analog.com T
AN-578 APPLICATION NOTE
RMS Calculation for Energy Meter Applications Using the ADE7756
By Etienne Moulin
INTRODUCTION This application note describes the implementation of an rms signal processing algorithm using the ADE7756 and a microcontroller PIC16C63 from Microchip. The solution is implemented in an energy meter based on the ADE7756 (i.e., AN-564 reference design) and, in addition to the active energy measurement, provides the voltage and current rms measurement. The AN-564 describes the implementation of the ADE7756 with a microcontroller. The communication between the ADE7756 and the microcontroller is done through a serial interface (SPI). The SPI port allows the user to calibrate various components of the meter including gain, offset, and phase errors. The microcontroller sends display data to the LCD and manages supervisory functions of the meter. An EEPROM is used to store various calibration parameters of the meter and to store the meter's data during a power-down. The ADE7756 comprises of two ADCs, a reference circuit, and all the signal processing necessary for the calculation of active energy. Ci
rcuitry is provided to null out various system errors including gain, phase, and offset errors. Additional circuitry provides waveform sampling, programmable interrupts, and power line monitoring. All registers of the ADE7756 are available through the SPI port. Please refer to the ADE7756 data sheet for their description. The data sheet provides detailed information on the functionality of the ADE7756 and will be referenced several times in this application note. The AN-564 application note provides detailed information on the implementation of the ADE7756 into a microprocessor based electronics power meter. The waveform sampling mode of the ADE7756 is used to process rms calculation into the microcontroller. This mode is used in conjunction with active energy measurement without any drawback in the performances of the meter. The entire meter is calibrated through an external calibration routine by a PC. DESIGN GOALS Single-phase energy meters are low cost systems that cannot afford any programmable DSP comp
onent. The microcontroller based reference design AN-564 uses a standard 8-bit microcontroller with 192 bytes of RAM, 4K 14 of program memory, and a 4 kbytes external EEPROM. This solution measures the active energy and drives an LCD to display the accumulation of the energy consumed. Besides the active energy measurement, applications also need to measure the rms value of the line voltage and the rms value of the current used. These values can be used to process various electrical parameters, such as power factor and reactive energy. The objective of this document is to demonstrate that a cost-effective architecture, such as the AN-564 reference design, can support these additional measurements with sufficient accuracy when the correct design choices are made. The goal of the solution is to comply within 1% accuracy in the rms measurement display. This accuracy is reached over the current dynamic range of a Class 100 ANSI meter extended to 500 and in the range of 10:1 for the voltage input. In addition, t
his meter is based on the AN-564 and complies with the ANSI C12.16 specifications. This reference design is for a single element, Class 100 meter in a form 2S designation. Although the design in this application is limited to the ANSI standard, the achieved accuracy is well within the accuracy requirements of the IEC1036 standards for a Class 1 meter. Please refer to AN-564 for further details on the accuracy achievements of this reference design.
REV. A
AN578_a Application Note analog Download PDF
Add this permalink to your bookmarks for future download of AN578_a ApplicationNote
Permalink: http://application.emcelettronica.com/analog/AN578_a
