ADE75 datasheet(53/148 Pages) AD

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Preliminary Technical Data

ADE75xx/ADE71xx

Rev. PrE | Page 53 of 148

LSB of the current channel rms offset represents 0.23% of

measurement error at –60 dB down of full scale.

IRMS =

32768

+ IRMSOS

IRMS

(4)

where IRMS0 is the rms measurement without offset correction.

Voltage channel RMS Calculation

Figure 36 shows the details of the signal processing chain for

the rms calculation on Voltage channel. The Voltage channel

rms value is processed from the samples used in the Voltage

channel waveform sampling mode. Voltage channel rms value 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 be updated 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 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 ADE75XX/ADE71XX 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 volts/LSB

constant.

Voltage channel RMS Offset Compensation

The ADE75XX/ADE71XX incorporates a voltage channel rms

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

rms calculation. An offset could 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 LSB of the rms

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

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

of measurement

error at –60 dB down of full scale.

VRMS = VRMS0 + 64 x VRMSOS

(6)

where VRMS0 is the rms measurement without offset correction.

VRMS[23:0]

LPF3

LPF1

VOLTAGE CHANNEL

0x28F5C2

0x00

VRMOS[11:0]

VOLTAGE SIGNAL (V(t))

216

sgn

215

28 27 26

0x28F5

0x0

0xD70B

Figure 36. Voltage channel RMS Signal Processing

ACTIVE POWER CALCULATION

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

to load. It is defined as 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 9 gives an expression for the instantaneous power

signal in an ac system.

v(t) =

)

sin(

(7)

i(t) =

)

sin(

(8)

where:

V is the rms voltage.

I is the rms current.

)

(

)

(

)

(

)

cos(

)

(

−

(9)

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

given by the expression in Equation 10.

P =

∫

)

(

(10)

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 9, i.e., VI. This is

the relationship used to calculate active power in the

ADE75XX/ADE71XX. 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 37.

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