ADE7758 Analog Devices, ADE7758 Datasheet - Page 41

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

ENERGY REGISTERS SCALING
The ADE7758 provides measurements of active, reactive, and
apparent energies that use separate signal paths and filtering for
calculation. The differences in the data paths can result in small
differences in LSB weight between the active, reactive, and
apparent energy registers. These measurements are internally
compensated so that the scaling is nearly one to one. The
relationship between the registers is shown in Table 11.
Table 11. Energy Registers Scaling
Frequency of 60 Hz
Integrator Off
VAR = 1.004 × WATT
VA = 1.00058 × WATT
Integrator On
VAR = 1.0059 × WATT
VA = 1.00058 × WATT
WAVEFORM SAMPLING MODE
The waveform samples of the current and voltage waveform, as
well as the active, reactive, and apparent power multiplier out-
puts, can all be routed to the WAVEFORM register by setting
the WAVSEL[2:0] bits (Bit 2 to Bit 4) in the WAVMODE
register. The phase in which the samples are routed is set by
setting the PHSEL[1:0] bits (Bit 0 and Bit 1) in the WAVMODE
register. All energy calculation remains uninterrupted during
waveform sampling. Four output sample rates may be chosen by
using Bit 5 and Bit 6 of the WAVMODE register (DTRT[1:0]).
The output sample rate may be 26.0 kSPS, 13.0 kSPS, 6.5 kSPS,
or 3.3 kSPS (see Table 16).
By setting the WSMP bit in the interrupt mask register to
Logic 1, the interrupt request output IRQ goes active low
when a sample is available. The 24-bit waveform samples are
transferred from the ADE7758 one byte (8 bits) at a time, with
the most significant byte shifted out first.
The interrupt request output IRQ stays low until the interrupt
routine reads the reset status register (see the ADE7758 Interrupts
section).
VAR = 1.0054 × WATT
VA = 1.0085 × WATT
VAR = 1.0064 × WATT
VA = 1.00845 × WATT
Frequency of 50 Hz
Rev. A | Page 41 of 68
CALIBRATION
A reference meter or an accurate source is required to calibrate
the ADE7758 energy meter. When using a reference meter, the
ADE7758 calibration output frequencies, APCF and VARCF,
are adjusted to match the frequency output of the reference
meter under the same load conditions. Each phase must be
calibrated separately in this case. When using an accurate source
for calibration, one can take advantage of the line cycle accu-
mulation mode and calibrate the three phases simultaneously.
There are two objectives in calibrating the meter: to establish
the correct impulses/kW-hr constant on the pulse output and to
obtain a constant that relates the LSBs in the energy and rms
registers to Watt/VA/VAR hours, amps, or volts. Additionally,
calibration compensates for part-to-part variation in the meter
design as well as phase shifts and offsets due to the current
sensor and/or input networks.
Calibration Using Pulse Output
The ADE7758 provides a pulsed output proportional to the
active power accumulated by all three phases, called APCF.
Additionally, the VARCF output is proportional to either the
reactive energy or apparent energy accumulated by all three
phases. The following section describes how to calibrate the
gain, offset, and phase angle using the pulsed output
information. The equations are based on the pulse output from
the ADE7758 (APCF or VARCF) and the pulse output of the
reference meter or CF
Figure 76 shows a flow chart of how to calibrate the ADE7758
using the pulse output. Since the pulse outputs are proportional
to the total energy in all three phases, each phase must be cali-
brated individually. Writing to the registers is fast in order to
reconfigure the part for calibrating a different phase, therefore
Figure 76 shows a method that calibrates all phases at a given
test condition before changing the test condition.
EXPECTED
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ADE7758