ADE7566 datasheet(49/136 Pages) AD

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Preliminary Technical Data

ADE7566/ADE7569

Rev. PrA | Page 49 of 136

Current Channel RMS Calculation

Each ADE7566/ADE7569 simultaneously calculates the rms values

for the current and voltage channels in different registers. Figure 48

shows the detail of the signal processing chain for the rms

calculation on the current channel. The current channel rms

value is processed from the samples used in the current channel

waveform sampling mode and is stored in an unsigned 24-bit

equivalent to one LSB of a current channel waveform sample.

The update rate of the current channel rms measurement is

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

Irms 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 in the MODE2 Register (0x0C).

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

ADC produces an output code that is approximately

±0d2,684,354 (see the Current Channel ADC section). The

equivalent rms value of a full-scale ac signal is 0d1,898,124

(0x1CF68C). The current rms measurement provided in the

ADE7566/ADE7569 is accurate to within 0.5% for signal inputs

between full scale and full scale/1000. The conversion from the

microprocessor using an amps/LSB constant.

Current Channel RMS Offset Compensation

The ADE7566/ADE7569 incorporate a current channel rms

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

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

to input noises that are integrated in the dc component of V2(t).

The offset calibration allows the content of the Irms register to be

maintained at 0 when no input is present on current channel.

One LSB of the current channel rms offset is equivalent to

16,384 LSBs of the square of the current channel rms register.

Assuming that the maximum value from the current channel

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

1 LSB of the current channel rms offset represents 0.23% of

measurement error at −60 dB down of full scale.

32768

IRMSOS

rms

(6)

where Irms0 is the rms measurement without offset correction.

Irms(t)

0x00

IRMSOS[11:0]

Irms[23:0]

226

225

sgn

227

217 216

218

HPF

MODE1[5]

HPF

CURRENT CHANNEL

WAVEFORM

DATA RANGE WITH

INTEGRATOR OFF

0xD70A3E

0x000000

0x28F5C2

60Hz

0xD487B0

0x2B7850

0x000000

CURRENT CHANNEL

WAVEFORM

DATA RANGE WITH

INTEGRATOR ON (60Hz)

HPF1

DIGITAL

INTEGRATOR*

LPF3

Figure 48. Current Channel RMS Signal Processing

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