ADE7569_15 datasheet(62/152 Pages) AD

62 / 152 page

ADE7116/ADE7156/ADE7166/ADE7169/ADE7566/ADE7569

Rev. B | Page 62 of

152

Voltage Channel RMS Calculation

Figure 63 shows details of the signal processing chain for the rms

calculation on the voltage channel. This voltage rms estimation is

done in the ADE7116/ADE7156/ADE7166/ADE7169/

ADE7566/ADE7569 using the mean absolute value calculation,

as shown in Figure 63. 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

the ZXRMS bit (Bit 2) in the MODE2 register (Address 0x0C).

With the specified full-scale ac analog input signal of 0.4 V, the

output from LPF1 in Figure 63 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 ADE7116/ADE7156/ADE7166/

ADE7169/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 ADE7116/ADE7156/ADE7166/ADE7169/ADE7566/

ADE7569 incorporate a voltage channel rms offset compensation

used to remove offset in the voltage channel rms calculation. An

offset can exist in the rms calculation due to input noises and dc

offset in the input samples. One LSB of the voltage channel rms

offset is equivalent to 64 LSBs of the rms register. Assuming that

the maximum value from the voltage 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 from 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/second. Equation 8 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 ADE7116/

ADE7156/ADE7166/ADE7169/ADE7566/ADE7569. The instan-

taneous 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 64.

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 64. Active Power Calculation

Because LPF2 does not have an ideal brick wall frequency

response (see Figure 65), 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.

Because of its sinusoidal nature, the ripple is removed when the

active power signal is integrated to calculate energy (see the

Active Energy Calculation section).

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