ADE7932 datasheet(54/120 Pages) AD

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ADE7978/ADE7933/ADE7932

Data Sheet

Rev. 0 | Page 54 of 120

ACTIVE POWER OFFSET CALIBRATION

The ADE7978 includes a 24-bit watt offset register for each

phase and each active power. The AWATTOS, BWATTOS, and

CWATTOS registers (Address 0x439C to Address 0x439E) compen-

sate offsets in the total active power calculations. The AFWATTOS,

BFWATTOS, and CFWATTOS registers (Address 0x43A3 to

Address 0x43A5) compensate offsets in the fundamental active

power calculations. These signed twos complement registers are

used to remove offsets in the active power calculations.

An offset can exist in the power calculation due to crosstalk

between channels on the PCB or in the chip itself. One LSB in

the active power offset register is equivalent to 1 LSB in the

active power multiplier output. With full-scale current and

voltage inputs, the LPF2 output is PMAX = 26,991,271. At

−80 dB down from full scale (active power scaled down 104

times), one LSB of the active power offset register represents

0.037% of PMAX.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit

words, whereas the DSP works with 28-bit words. Like the

xIGAIN registers shown in Figure 44, the 24-bit xWATTOS

and xFWATTOS registers are sign extended to 28 bits and

padded with four 0s for transmission as 32-bit registers.

SIGN OF ACTIVE POWER CALCULATION

The average active power is a signed calculation. If the phase

difference between the current and voltage waveforms is more

than 90°, the average power becomes negative. Negative power

indicates that energy is being injected back onto the grid. The

ADE7978 has sign detection circuitry for active power calcula-

tions; this circuitry can monitor the total active powers or the

fundamental active powers.

As described in the Active Energy Calculation section, the

active energy accumulation is performed in two stages. Each

time a sign change is detected in the energy accumulation at the

end of the first stage—that is, after the energy accumulated in the

internal accumulator reaches the WTHR register threshold—a

dedicated interrupt is triggered. The sign of each phase active

power can be read in the PHSIGN register (Address 0xE617).

Bit 0 (REVAPSEL) in the MMODE register (Address 0xE700)

specifies the type of active power that is monitored. When

REVAPSEL is cleared to 0 (the default value), the total active

power is monitored. When REVAPSEL is set to 1, the funda-

mental active power is monitored.

Bits[8:6] (REVAPC, REVAPB, and REVAPA) in the STATUS0

the power selected by Bit 0 (REVAPSEL) in the MMODE register.

Bits[2:0] (CWSIGN, BWSIGN, and AWSIGN) in the PHSIGN

and REVAPA bits in the STATUS0 register. The xWSIGN bits

indicate the sign of the power. When these bits are set to 0, the

corresponding power is positive. When the bits are set to 1, the

corresponding power is negative.

The REVAPx bits in the STATUS0 register and the xWSIGN

bits in the PHSIGN register refer to the total active power of

Phase x and the power type selected by Bit 0 (REVAPSEL) in

the MMODE register.

Interrupts attached to Bits[8:6] (REVAPC, REVAPB, and

REVAPA) in the STATUS0 register can be enabled by setting

Bits[8:6] in the MASK0 register. When enabled, the IRQ0 pin

goes low and the status bit is set to 1 when a change of sign occurs.

To identify the phase that triggered the interrupt, read the PHSIGN

status bit is cleared and the IRQ0 pin is returned high by writing

a 1 to the appropriate bits in the STATUS0 register.

ACTIVE ENERGY CALCULATION

Active energy is defined as the integral of active power.

Energy =

∫

p(t)

(30)

The ADE7978 achieves the integration of the active power

signal in two stages (see Figure 76). The process is identical for

total active power and fundamental active power.

The first stage accumulates the instantaneous phase total or

fundamental active power at 1.024 MHz (the DSP computes

these values at an 8 kHz rate). Each time a threshold is reached,

a pulse is generated, and the threshold is subtracted from the

internal register. The sign of the energy at this moment is

considered the sign of the active power (see the Sign of Active

Power Calculation section for more information).

INTERNAL

ACCUMULATOR

WATTACC BITS IN

ACCMODE[7:0]

THRESHOLD

WTHR

27 26

32-BIT REGISTER

AWATTHR[31:0]

REVAPA BIT IN

STATUS0[31:0]

INSTANTANEOUS

PHASE A ACTIVE POWER

COMPUTED IN DIGITAL

SIGNAL PROCESSOR

Figure 76. Total Active Energy Accumulation

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