ADE7754_15 datasheet(17/44 Pages) AD

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REV. 0

ADE7754

–17–

If the VGAIN registers are used for apparent power calibration

(WATMOD bits in VAMODE register = 1 or 2), the voltage

rms values are changed by voltage gain register value as described

in the expression

Voltage rms

Phase A

rms

AVGAIN

=× +





























For example, when 7FFh is written to the voltage gain register,

the ADC output is scaled up by +50%. 7FFh = 2047d, 2047/

12 = 0.5. Similarly, 800h = –2047d (signed twos complement)

and ADC output is scaled by –50%. These two examples are

illustrated in Figure 21.

Voltage RMS Offset Compensation

The ADE7754 incorporates a voltage rms offset compensation

for each phase (AVRMSOS, BVRMSOS, and CVRMSOS).

These are 12-bit twos complement signed registers that can be

used to remove offsets in the voltage rms calculations. An offset

may exist in the rms calculation due to input noises and offsets

in the input samples. The offset calibration allows the contents

of the VRMS registers to be maintained at zero when no voltage

is applied.

n LSB of the voltage rms offset are equivalent to 64

n LSB of

the voltage rms register. Assuming that the maximum value from

the voltage rms calculation is 1,898,124 decimal with full-scale

ac inputs, then 1 LSB of the voltage rms offset represents 0.07%

of measurement error at –26 dB below full scale.

VRMSOS

rms

where Vrmso is the rms measurement without offset correction.

The voltage rms offset compensation should be done by testing

the rms results at two non-zero input levels. One measurement

can be done close to full scale and the other at approximately

full scale/10. The voltage offset compensation can then be derived

from these measurements. See the Calibration of a 3-Phase

Meter Based on the ADE7754 Application Note AN-624.

ACTIVE POWER CALCULATION

Electrical power is defined as the rate of energy flow from source

to load. It is given by the product of the voltage and current

waveforms. The resulting waveform is called the instantaneous

power signal and it is equal to the rate of energy flow at every

instant of time. The unit of power is the watt or joules/sec. Equa-

tion 5 gives an expression for the instantaneous power signal in

an ac system.

()

sin(

)

= 2

(3)

()

sin(

)

= 2

(4)

where V = rms voltage and I = rms current.

v t

i t

VI VI

()

cos(

)

=×

=−

(5)

The average power over an integral number of line cycles (n) is

given by the expression in Equation 6.

pt dt

∫

()

(6)

where T is the line cycle period. P is referred to as the active or

real power. Note that the active power is equal to the dc compo-

nent of the instantaneous power signal p(t) in Equation 5 (i.e.,

VI). This is the relationship used to calculate active power in the

ADE7754 for each phase. The instantaneous power signal p(t)

is generated by multiplying the current and voltage signals in

each phase. The dc component of the instantaneous power signal

in each phase (A, B, and C) is then extracted by LPF2 (low-pass

filter) to obtain the active power information on each phase. This

process is illustrated in Figure 22. In a polyphase system, the total

electrical power is simply the sum of the real power in all active

phases. The solutions available to process the total active power

are discussed in the following section.

VOLTAGE

v(t) = 2V sin( t)

CURRENT

i(t) = 2I sin( t)

INSTANTANEOUS

POWER SIGNAL

ACTIVE REAL POWER

SIGNAL = V

V. I.

D1B717h

00000h

1A36E2Eh

p(t) = V

I – V

I cos(2 t)

Figure 22. Active Power Calculation

Since LPF2 does not have an ideal brick wall frequency

response (see Figure 23), the active power signal has some

ripple due to the instantaneous power signal. This ripple is

sinusoidal and has a frequency equal to twice the line frequency.

Since the ripple is sinusoidal in nature, it is removed when the

active power signal is integrated to calculate the energy. See the

Energy Calculation section.

FREQUENCY (Hz)

–4

–8

–12

–16

–20

–24

100

8Hz

Figure 23. Frequency Response of the LPF Used

to Filter Instantaneous Power in Each Phase

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