ADE7758_15 datasheet(31/72 Pages) AD

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Data Sheet

ADE7758

Rev. E | Page 31 of 72

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. Equation 16 describes

mathematically the function of the watt gain registers.

⎟

⎠

⎞

⎜

⎝

⎛ +

gister

Gain

Watt

Output

LPF

Data

Power

Average

(16)

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 21). 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

section). Note that this bit is set whenever there are sign

changes, that is, the REVPAP bit is set for both a positive-to-

negative change and a negative-to-positive change of the sign

bit. The response time of this bit is approximately 176 ms for a

full-scale signal, which has an average value of 0xCCCCD at the

low pass filter output. For smaller inputs, the time is longer.

Interrupts

The output is scaled by −50% by writing 0x800 to the watt gain

registers and increased by +50% by writing 0x7FF to them.

These registers can be used to calibrate the active power (or

energy) calculation in the ADE7758 for each phase.

CLKIN

Value

Average

Time

sponse

⎥

⎦

⎤

⎢

⎣

⎡

≅

(17)

Active Power Offset Calibration

The APCFNUM [15:13] indicate reverse power on each of the

individual phases. Bit 15 is set if the sign of the power on Phase A is

negative, Bit 14 for Phase B, and Bit 13 for Phase C.

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 can

exist in the power calculation due to crosstalk between channels

on the PCB or in the chip itself. The offset calibration allows the

contents 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 from full scale on the 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

(838,861/1000). 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.

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 previously stated, power is defined as the rate of energy flow.

This relationship can be expressed mathematically as

dEnergy

Power =

(18)

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 is

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.

Conversely, Energy is given as the integral of power.

()dt

∫

Energy

(19)

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