DESCRIPCIÓN GENERAL

The ADE7758 is a high accuracy, 3-phase electrical energy measurement IC with a serial interface and two pulse outputs. The ADE7758 incorporates second-order Σ-Δ ADCs, a digital integrator, reference circuitry, a temperature sensor, and all the signal processing required to perform active, reactive, and apparent energy measurement and rms calculations. The ADE7758 is suitable to measure active, reactive, and apparent energy in various 3-phase configurations, such as WYE or DELTA services, with both three and four wires. The ADE7758 provides system calibration features for each phase, that is, rms offset correction, phase calibration, and power calibration. The APCF logic output gives active power information, and the VARCF logic output provides instantaneous reactive or apparent power information.The ADE7758 has a waveform sample register that allows access to the ADC outputs. The part also incorporates a detection circuit for short duration low or high voltage variations. The voltage threshold levels and the duration (number of half-line cycles) of the variation are user programmable. A zero-crossing detection is synchronized with the zero-crossing point of the line voltage of any of the three phases. This information can be used to measure the period of any one of the three voltage inputs. The zero-crossing detection is used inside the chip for the line cycle energy accumulation mode. This mode permits faster and more accurate calibration by synchronizing the energy accumulation with an integer number of line cycles.  Data is read from the ADE7758 via the SPI serial interface. The interrupt request output (IRQ) is an open-drain, active low logic output. The IRQ output goes active low when one or more interrupt events have occurred in the ADE7758. A status register indicates the nature of the interrupt. The ADE7758 in a 24-lead SOIC package.

 

CARACTERÍSTICAS

Highly accurate; supports IEC 60687, IEC 61036, IEC 61268, IEC 62053-21, IEC 62053-22, and IEC 62053-23

Compatible with 3-phase/3-wire, 3-phase/4-wire, and other 3-phase services

Less than 0.1% active energy error over a dynamic range of 1000 to 1 at 25°C

Supplies active/reactive/apparent energy, voltage rms, current rms, and sampled waveform data

Two pulse outputs, one for active power and the other selectable between reactive and apparent power with programmable frequency

Digital power, phase, and rms offset calibration

On-chip, user-programmable thresholds for line voltage SAG and overvoltage detections

An on-chip, digital integrator enables direct interface-tocurrent sensors with di/dt output

A PGA in the current channel allows direct interface to current transformers

An SPI-compatible serial interface with IRQ Proprietary ADCs and DSP provide high accuracy over large variations in environmental conditions and time

Reference 2.4 V (drift 30 ppm/°C typical) with external overdrive capability

Single 5 V supply, low power (70 mW typical)

 

TEORÍA DE FUNCIONAMIENTO

ANTIALIASING FILTER

This filter prevents aliasing, which is an artifact of all sampled systems. Input signals with frequency components higher than half the ADC sampling rate distort the sampled signal at a frequency below half the sampling rate. This happens with all ADCs, regardless of the architecture. The combination of the high sampling rate ∑-Δ ADC used in the ADE7758 with the relatively low bandwidth of the energy meter allows a very simple lowpass filter (LPF) to be used as an antialiasing filter. A simple RC filter (single pole) with a corner frequency of 10 kHz produces an attenuation of approximately 40 dB at 833 kHz. This is usually sufficient to eliminate the effects of aliasing.

ANALOG INPUTS

The ADE7758 has six analog inputs divided into two channels: current and voltage. The current channel consists of three pairs of fully differential voltage inputs: IAP and IAN, IBP and IBN, and ICP and ICN. These fully differential voltage input pairs have a maximum differential signal of ±0.5 V. The current channel has a programmable gain amplifier (PGA) with possible gain selection of 1, 2, or 4. In addition to the PGA, the current channels also have a full-scale input range selection for the ADC. The ADC analog input range selection is also made using the gain register (see Figure 38). As mentioned previously, the maximum differential input voltage is ±0.5 V. However, by using Bit 3 and Bit 4 in the gain register, the maximum ADC input voltage can be set to ±0.5 V, ±0.25 V, or ±0.125 V on the current channels. This is achieved by adjusting the ADC reference (see the Reference Circuit section).