ADE7758 Analog Devices, ADE7758 Datasheet - Page 31

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

Active Power Gain Calibration
Note that the average active power result from the LPF output
in each phase can be scaled by ±50% by writing to the phase’s
watt gain register (AWG, BWG, or CWG). The watt gain registers
are twos complement, signed registers and have a resolution of
0.024%/LSB. The following equation describes mathematically
the function of the watt gain registers.
The output is scaled by −50% when the watt gain registers
contents are set to 0x800 and the output is increased by +50%
by writing 0x7FF to the watt gain register. This register can be
used to calibrate the active power (or energy) calculation in the
ADE7758 for each phase.
Active Power Offset Calibration
The ADE7758 also incorporates a watt offset register on each
phase (AWATTOS, BWATTOS, and CWATTOS). These are
signed twos complement, 12-bit registers that are used to remove
offsets in the active power calculations. An offset may exist in
the power calculation due to crosstalk between channels on the
PCB or in the chip itself. The offset calibration allows the con-
tents of the active power register to be maintained at 0 when no
power is being consumed. One LSB in the active power offset
register is equivalent to 1/16 LSB in the active power multiplier
output. At full-scale input, if the output from the multiplier is
0xCCCCD (838,861d), then 1 LSB in the LPF2 output is
equivalent to 0.0075% of measurement error at −60 dB down of
full scale at current channel. At −60 dB down on full scale (the
input signal level is 1/1000 of full-scale signal inputs), the
average word value from LPF2 is 838.861 (838,861/1,000). One
LSB is equivalent to 1/838.861/16 × 100% = 0.0075% of the
measured value. The active power offset register has a correction
resolution equal to 0.0075% at −60 dB.
Average
⎛
⎜
⎝
1
+
Watt
Power
Gain
2
12
Data
Register
=
LPF2
⎞
⎟
⎠
Output
×
Rev. A | Page 31 of 68
Sign of Active Power Calculation
Note that the average active power is a signed calculation. If the
phase difference between the current and voltage waveform are
more than 90°, the average power becomes negative. Negative
power indicates that energy is being placed back on the grid.
The ADE7758 has a sign detection circuitry for active power
calculation. The REVPAP bit (Bit 17) in the interrupt status
register is set if the average power from any one of the phases
changes sign. The phases monitored are selected by TERMSEL
bits in the COMPMODE register (see Table 17). The TERMSEL
bits are also used to select which phases are included in the
APCF and VARCF pulse outputs. If the REVPAP bit is set in the
mask register, the IRQ logic output goes active low (see the
ADE7758 Interrupts section). Note that this bit is set whenever
there are sign changes, i.e., the REVPAP bit is set for both a
positive-to-negative change or a negative-to-positive change of
the sign bit. The APCFNUM [15:13] indicate reverse power on
each of the individual phases. Bit 15 will be set if the sign of the
power on Phase A is negative, Bit 14 for Phase B, and Bit 13 for
Phase C.
No-Load Threshold
The ADE7758 has an internal no-load threshold on each phase.
The no-load threshold can be activated by setting the NOLOAD
bit (Bit 7) of the COMPMODE register. If the active power falls
below 0.005% of full-scale input, the energy is not accumulated
in that phase. As stated, the average multiplier output with full-
scale input is 0xCCCCD. Therefore, if the average multiplier
output falls below 0x2A, the power is not accumulated to avoid
creep in the meter. The no-load threshold is implemented only
on the active energy accumulation. The reactive and apparent
energies do not have the no-load threshold option.
Active Energy Calculation
As stated earlier, power is defined as the rate of energy flow.
This relationship can be expressed mathematically as Equation 7.
Conversely, Energy is given as the integral of power.
Power =
Energy
=
dEnergy
∫
p
dt
( )
t
dt
ADE7758
(7)
(8)