ADE7758 datasheet(29/68 Pages) AD

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ADE7758

Rev. A | Page 29 of 68

equivalent to one LSB of the current waveform sample. The

update rate of the current rms measurement is CLKIN/12.

SGN 225 224 223

217 216 215

CURRENT SIGNAL

FROM HPF OR

INTEGRATOR

(IF ENABLED)

0x1D3781

0x00

0x2851EC

0x0

0xD7AE14

LPF3

AIRMS[24:0]

AIRMSOS[11:0]

Figure 62. Current RMS Signal Processing

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

ADC produces an output code that is approximately ±2,642,412d

(see the Current Channel ADC section). The equivalent rms

values of a full-scale sinusoidal signal at 60 Hz is 1,914,753

(0x1D3781).

The accuracy of the current rms is typically 0.2% error

from the full-scale input down to 1/500 of the full-scale input.

Additionally, this measurement has a bandwidth of 14 kHz.

Current RMS Offset Compensation

The ADE7758 incorporates a current rms offset compensation

for each phase (AIRMSOS, BIRMSOS, and CIRMSOS). These

are 12-bit signed registers that can be used to remove offsets in

the current rms calculations. An offset may exist in the rms

calculation due to input noises that are integrated in the dc

component of I2(t). The offset calibration allows the contents of

the IRMS registers to be maintained at 0 when no current is

being consumed. One LSB of the current rms offset register is

equivalent to 16,384 (decimal) of the square of the current rms

rms calculation is 1,868,467d with full-scale ac inputs, one LSB

of the current rms offset represents 0.94% of the measurement

error at −60 dB down of full scale. For details on how to

calibrate the current rms measurement, see the Calibration

section.

IRMSOS

IRMS

384

where IRMS0 is the rms measurement without offset correction.

Voltage Channel RMS Calculation

Figure 63 shows the details of the signal path for the rms

calculation on Phase A of the voltage channel. The voltage

channel rms value is processed from the waveform samples

after the low-pass filter LPF1. The output of the voltage channel

ADC can be scaled by ±50% by changing VRMSGAIN[11:0]

registers to perform an overall rms voltage calibration. The

VRMSGAIN registers scale the rms calculations as well as the

apparent energy calculation, since apparent power is the

product of the voltage and current rms values. The voltage rms

values are stored in unsigned 24-bit registers (AVRMS, BVRMS,

and CVRMS). The 256 LSBs of the voltage rms register is

approximately equivalent to one LSB of a voltage waveform

sample. The update rate of the voltage rms measurement is

CLKIN/12.

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

LPF1 produces an output code that is approximately 63% of its

full-scale value , i.e., ±9,372d, at 60 Hz (see the Voltage Channel

ADC section). The equivalent rms value of a full-scale ac signal

is approximately 1,639,101 (0x1902BD) in the VRMS register.

The accuracy of the VRMS measurement is typically 0.5% error

from the full-scale input down to 1/20 of the full-scale input.

Additionally, this measurement has a bandwidth of 160 Hz.

VAN

AVRMSGAIN[11:0]

0x249C

LPF OUTPUT

WORD RANGE

0x0

60Hz

0xDB64

0x25A2

LPF OUTPUT

WORD RANGE

0x0

50Hz

0xDA5E

LPF1

VOLTAGE SIGNAL–V(t)

0.5

GAIN

0x193504

50Hz

0x0

0x1902BD

60Hz

0x0

AVRMS[23:0]

LPF3

SGN 216 215 214

VRMSOS[11:0]

Figure 63. Voltage RMS Signal Processing

Voltage RMS Offset Compensation

The ADE7758 incorporates a voltage rms offset compensation

for each phase (AVRMSOS, BVRMSOS, and CVRMSOS).

These are 12-bit signed registers that can be used to remove

offsets in the voltage rms calculations. An offset may exist in the

rms calculation due to input noises and offsets in the input

samples. It should be noted that the offset calibration does not

allow the contents of the VRMS registers to be maintained at 0

when no voltage is applied. This is caused by noise in the

voltage rms calculation, which limits the usable range between

full scale

and 1/50th of full scale. One LSB of the voltage rms offset is

equivalent to 64 LSBs of the voltage rms register.

Assuming that the maximum value from the voltage rms

calculation is 1,639,101d with full-scale ac inputs, then 1 LSB

of the voltage rms offset represents 0.042% of the measurement

error at 1/10 of full scale.

VRMSOS

VRMS

where VRMS0 is the rms measurement without the offset

correction.

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