ADE7566 datasheet(50/136 Pages) AD

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ADE7566/ADE7569

Preliminary Technical Data

Rev. PrA | Page 50 of 136

Vrms[23:0]

LPF3

LPF1

VOLTAGE CHANNEL

0x28F5C2

0x00

VRMOS[11:0]

VOLTAGE SIGNAL (V(t))

0x28F5

0x0

0xD70B

216

sgn

215

27 26

Figure 49. Voltage Channel RMS Signal Processing

Voltage Channel RMS Calculation

Figure 49 shows details of the signal processing chain for the

rms calculation on the voltage channel. The voltage channel

rms value is processed from the samples used in the voltage

channel waveform sampling mode and is stored in the unsigned

24-bit Vrms register.

The update rate of the voltage channel rms measurement is

MCLK/5. To minimize noise in the reading of the register, the

Vrms register can also be configured to update only with the zero

crossing of the voltage input. This configuration is done by

setting ZXRMS bit in the MODE2 Register (0x0C).

With the specified full-scale ac analog input signal of 0.5 V, the

output from the LPF1 in Figure 49 swings between 0x28F5 and

0xD70B at 60 Hz (see the Voltage Channel ADC section). The

equivalent rms value of this full-scale ac signal is approximately

0d1,898,124 (0x1CF68C) in the Vrms register. The voltage rms

measurement provided in the ADE7566/ADE7569 is accurate to

within ±0.5% for signal input between full scale and full scale/20.

The conversion from the register value to volts must be done

externally in the microprocessor using a V/LSB constant.

Voltage Channel RMS Offset Compensation

The ADE7566/ADE7569 incorporate a voltage channel rms

offset compensation register (VRMSOS). This is a 12-bit signed

rms calculation. An offset can exist in the rms calculation due

to input noises and dc offset in the input samples. The offset

calibration allows the contents of the Vrms register to be

maintained at 0 when no voltage is applied. One LSB of the

voltage channel rms offset is equivalent to 64 LSBs of the rms

channel rms calculation is 0d1,898,124 with full-scale ac inputs,

then 1 LSB of the voltage channel rms offset represents 3.37% of

measurement error at −60 dB down of full scale.

Vrms = Vrms0 + 64 × VRMSOS

(7)

where Vrms0 is the rms measurement without offset correction.

ACTIVE POWER CALCULATION

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

to load. It is the product of the voltage and current waveforms.

The resulting waveform is called the instantaneous power signal

and is equal to the rate of energy flow at every instant of time.

The unit of power is the watt or joules/sec. Equation 10 gives an

expression for the instantaneous power signal in an ac system.

()

)

sin(

(8)

()

)

sin(

(9)

where:

v is the rms voltage.

i is the rms current.

)

(

)

(

)

(

)

cos(

)

(

−

(10)

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

given by the expression in Equation 11.

∫

)

(

(11)

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 component of the

instantaneous power signal p(t) in Equation 11, that is, VI. This

is the relationship used to calculate active power in the

ADE7566/ADE7569. The instantaneous power signal p(t) is

generated by multiplying the current and voltage signals. The dc

component of the instantaneous power signal is then extracted

by LPF2 (low-pass filter) to obtain the active power

information. This process is illustrated in Figure 50.

INSTANTANEOUS

POWER SIGNAL

p(t) = v × i – v × i × cos(2ωt)

ACTIVE REAL POWER

SIGNAL = v × i

0x19999A

0xCCCCD

0x00000

CURRENT

i(t) = √2 × i × sin(ωt)

VOLTAGE

v(t) = √2 × v × sin(ωt)

Figure 50. Active Power Calculation

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