ADE7569 datasheet(60/136 Pages) AD

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ADE7566/ADE7569

Preliminary Technical Data

Rev. PrA | Page 60 of 136

APPARENT POWER CALCULATION

Apparent power is defined as the maximum power that can be

delivered to a load. Vrms and Irms are the effective voltage and

current delivered to the load, respectively. Therefore, the apparent

power (AP) = Vrms × Irms. This equation is independent from the

phase angle between the current and the voltage.

Equation 31 gives an expression of the instantaneous power

signal in an ac system with a phase shift.

()

sin( )

rms

(28)

()

)

sin(

rms

(29)

)

(

)

(

)

(

(30)

()

)

cos(

)

cos(

−

rms

(31)

Figure 62 illustrates the signal processing for the calculation of

the apparent power in the ADE7566/ADE7569.

The apparent power signal can be read from the waveform register

by setting the WAVMODE Register (0x0D) and setting the

WFSM bit in the Interrupt Enable Register 3 SFR (MIRQENH,

0xDB). Like the current and voltage channel waveform sampling

modes, the waveform data is available at sample rates of 27.9

kSPS, 14 kSPS, 7 kSPS, or 3.5 kSPS.

The gain of the apparent energy can be adjusted by using the

multiplier and by writing a twos complement, 12-bit word to the

VAGAIN register (VAGAIN[11:0]). Equation 32 shows how the

gain adjustment is related to the contents of the VAGAIN register.

Output VAGAIN =

⎟

⎠

⎞

⎜

⎝

⎛

⎭

⎬

⎫

⎩

⎨

⎧ +

VAGAIN

Power

Apparent

(32)

For example, when 0x7FF is written to the VAGAIN register, the

power output is scaled up by 50% (0x7FF = 2047d, 2047/212 = 0.5).

Similarly, 0x800 = –2047d (signed twos complement) and power

output is scaled by –50%. Each LSB represents 0.0244% of the

power output. The apparent power is calculated with the current

and voltage rms values obtained in the rms blocks of the

ADE7566/ADE7569.

Apparent Power Offset Calibration

Each rms measurement includes an offset compensation register to

calibrate and eliminate the dc component in the rms value (see the

Current Channel RMS Calculation section and Voltage Channel

RMS Calculation section). The voltage and current channels

rms values are then multiplied together in the apparent power

signal processing. Because no additional offsets are created in

the multiplication of the rms values, there is no specific offset

compensation in the apparent power signal processing. The

offset compensation of the apparent power measurement is

done by calibrating each individual rms measurement.

Vrms

Irms

0x1A36E2

APPARENT POWER

SIGNAL (P)

CURRENT RMS SIGNAL – i(t)

VOLTAGE RMS SIGNAL – v(t)

0x00

0x1CF68C

0x00

0x1CF68C

VAGAIN

DIGITAL-TO-FREQUENCY

CONVERTER

VARMSCFCON

Figure 62. Apparent Power Signal Processing

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