Smart Metering ICs Support Accurate Power Monitoring
Four new energy-metering ICs from Analog Devices, Inc. (ADI) improve the accuracy and performance of commercial, industrial, and residential smart meters. The devices in this smart-meter IC family — which includes the ADE7878, ADE7868, ADE7858, and ADE7854 — accurately measure the amount of electricity consumed or generated, and communicate data for power monitoring, billing, and other purposes.
These ICs enable smart electricity meters to deliver improved user billing accuracy, advanced power-quality monitoring, and reduced operating costs for utility companies. The Table lists the system capabilities intended for these ICs in polyphase configurations, including three- and four-wire wye and delta services. The devices are available in a 40-lead, RoHS-compliant lead-frame CSP.
The metering ICs can measure total active and reactive energy with accuracy and dynamic range exceeding Class 0.2 specifications for energy meters.
The four new devices measure reactive and active energy with 0.1% accuracy over a dynamic range of 1,000:1, and 0.2% accuracy for active and reactive energy measurements over a dynamic range of 3,000:1. The products also deliver 0.1% accuracy over a 1,000:1 dynamic range for rms current and voltage measurements. The ADE7878 can simultaneously measure energy of only the fundamental frequency and total energy, which is critical for power-quality measurements.
BASIC METER FEATURES
Fig. 1 is a simplified block diagram of the ADE7854 that operates with a single 3.3-V supply and is specified from -40° to 85°C. It is a high-accuracy, three-phase electrical energy measurement IC with serial interfaces and three flexible pulse outputs.
The ADE7854 incorporates second-order, sigma-delta (-Δ) analog-to-digital converters (ADCs), a digital integrator, reference circuitry, and all the signal processing required to perform total active and apparent energy measurement and rms calculations. The ADE7858 adds reactive energy calculations.
All ADCs in the ADE7854 produce the same 24-bit signed output code for the same input signal level. With a full-scale input signal of ±0.5 V and an internal reference of 1.2 V, the ADC output code is nominally 5,928,256 (0x5A7540). The ADC code can vary between 0x800000 (-8,388,608) and 0x7FFFFF (+8,388,607); this is equivalent to an input signal level of ±0.707 V. However, for specified performance, it is recommended not to exceed the nominal range of ±0.5 V. ADC performance is guaranteed only for input signals less than ±0.5 V.
The ADE7854 has six analog inputs that form current and voltage channels. All inputs have a programmable gain amplifier (PGA) with gain options of 1, 2, 4, 8, or 16.
Current channels consist of three pairs of fully differential voltage inputs, with each pair having a maximum differential signal of ±0.5 V. Current measurements are achieved with current transformers or Rogowski coils for each of the three phases. The maximum common-mode signal allowed on the inputs is ±25 mV. Fig. 2 shows the current channel signal path.
The voltage channel has three single-ended voltage inputs that have a maximum input voltage of ±0.5 V with respect to the neutral voltage, VN. The voltage inputs pass through a voltage-sensing box that acts as a voltage divider. In addition, the maximum signal level on the analog inputs for VN and VxP (where x = a or b or c) is ±0.5 V with respect to AGND. The maximum common-mode signal allowed on the inputs is ±25 mV. Fig. 3 illustrates the voltage channel data path.
The ADE7854 provides system calibration features for each phase; that is, rms offset correction, phase calibration, and gain calibration. The device provides a wide choice of power information, such as power-quality measures to the meter MCU.
The ADE7854 includes waveform sample registers that enable access to all ADC outputs. The device also incorporates power-quality measurements, such as short-duration low- or high-voltage detections, short-duration high-current variations, line-voltage period measurement, and angles between phase voltages and currents.
The SPI and I2C serial interfaces provide bidirectional communication with the ADE7854. A dedicated high-speed interface — its high-speed data capture port — can be used with I2C to provide access to the ADC outputs and real-time power information.
The nominal reference voltage for the ADCs in the ADE7854 is 1.2 V (±1%). The voltage of the ADE7854 reference drifts slightly with temperature, but changes are usually very small, typically much smaller than the drift of other components on a meter. Alternatively, the meter can be calibrated at multiple temperatures.
A fixed-function DSP in the ADE7854 computes all power and rms values. It contains program memory ROM and RAM, and data-memory RAM.
The program used for the power and rms computations is stored in the program memory ROM, and it is executed by the processor every eight kHz. All smart-meter ICs share the ability to perform angle measurements, angle sequencing, and remap waveforms.
TAMPER-PROOF
The ADE7868 and ADE7878 can detect various tamper conditions while continuing to operate properly — reducing field calls and meter troubleshooting times. These two ICs have seven sigma-delta ADCs, whereas the other two devices have six ADCs for current and voltage inputs.
The seventh ADC detects a neutral-current mismatch. Fig. 4 shows the neutral-current signal path used to detect errors when the sum of the three power-line phases does not equal the neutral current. This allows the ICs to detect and respond to either benign or malicious wiring errors.
In addition, it enables them to remotely reconfigure meters to correct wiring errors. Besides per-phase and neutral-energy measurements, the ADE7868 and ADE7878 can monitor energy-quality parameters such as sag, peak, period, angle measurements, and phase sequence.
All versions are capable of performing phase-angle measurements and detecting phase sequences.
OPERATION MODES
The ADE7878 and ADE7868 ICs have four power modes of operation — normal power, reduced power, low power, and sleep — which are determined by the state of its PM0 and PM1 pins that have internal pull-up resistors. The ADE7854 has two modes of operation — normal power and sleep — that are determined by the state of its PM1 pin, which provides complete control of ADE7854.
The PM1 pin features internal pull-up resistors. For the ADE7854, ADE7868, and ADE7878, the PM0 and PM1 pins may be connected to an external microprocessor I/O.
