ADE5166_09 datasheet(58/156 Pages) AD

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ADE5166/ADE5169/ADE5566/ADE5569

Rev. B | Page 58 of 156

LPF3

LPF1

VOLTAGE CHANNEL

0x28F5C2

VRMSOS[11:0]

VOLTAGE SIGNAL (V(t))

0x28F5

0x0

0xD70B

216

sgn

215

27 26

Vrms(t)

0x00

Vrms[23:0]

|X|

Figure 63. Voltage Channel RMS Signal Processing

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 ADE5166/ADE5169/ADE5566/ADE5569 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 SFRs (VRMSL, Address 0xD1;

VRMSM, Address 0xD2; and VRMSH, Address 0xD3).

The update rate of the voltage channel rms measurement is

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

VRMS SFRs 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 0.5 V, the

output from the 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 SFRs. The voltage rms

measurement provided in the ADE5166/ADE5169/ADE5566/

ADE5569 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 ADE5166/ADE5169/ADE5566/ADE5569 incorporate the

voltage channel rms offset compensation register (VRMSOS,

Address 0x23). This 12-bit, signed register can be 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 voltage channel 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

(5)

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(

(6)

()

)

sin(

(7)

where:

V is the rms voltage.

I is the rms current.

P(t) = V(t) × I(t)

)

cos(

)

(

−

(8)

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

given by the expression in Equation 9.

∫

)

(

(9)

where:

T is the line cycle period.

P is referred to as the active or real power.

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