More accurate readings of total energy with metering ICs

28 January 2004 Power Electronics / Power Management Products & Solutions

Analog Devices' next-generation metering chips now measure 'reactive' energy for more precise readings of energy use. These new energy metering ICs - the ADE7753 and ADE7758 - are based on ADI's high-resolution ADC technologies combined with a fixed-function DSP. Reactive energy, caused by nonlinear loads, requires energy suppliers to supply more volt-amps to customers than they can measure with a conventional watt-hour (active energy) meter.

The DSP performs all the necessary energy measurement calculations, allowing a low-cost microprocessor to be used for data management and supervisory functions. This architecture simplifies meter designs that previously relied on microprocessors to perform complex computations. The reduction in extensive code development significantly reduces cost and time-to-market.

The ADE7758, which features second-order sigma-delta ADCs, is designed for mid-range 'three-phase' energy meters. For each phase, the chip measures reactive, active, and apparent energy; as well as rms voltage and rms current. These measurements are accessed via a serial interface that allows a fully automated digital calibration. The ADE7758 interfaces with a variety of sensors, including current transformers and di/dt current sensors, such as Rogowski coils. Like other products in the ADE family, the ADE7758 provides accurate active energy measurements with less than 0,1% gain error over a current dynamic range of 1000:1. Its reactive energy measurement also surpasses the Class 2 IEC61268 accuracy requirement of VAR-hr meters.

Key features of both chips include:

* Power line supervisory features, such as line period measurement, SAG, peak detection, peak level recording and missing zero-crossing.

* Zero-crossing detection.

* Active power no-load threshold, offset compensation and pulse output.

* Reactive power offset compensation and reactive power pulse output (ADE7758 only).

The ADE7758 consumes 70 mW, while the ADE7753, for 'single-phase' connections, consumes only 25 mW. Both devices are designed to operate over a temperature range of minus 40 to 85°C and require a single 5 V supply.

For more information contact Analog Data Products, a division of Avnet Kopp, 011 809 6100.

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