ADE7758_15 datasheet(29/72 Pages) AD

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Data Sheet

ADE7758

Rev. E | Page 29 of 72

Current RMS Offset Compensation

The ADE7758 incorporates a current rms offset compensation

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

offsets in the current rms calculations. An offset can exist in the

rms calculation due to input noises that are integrated in the dc

component of I2(t). Assuming that the maximum value from

the current rms calculation is 1,914,753d with full-scale ac

inputs (60 Hz), one LSB of the current rms offset represents

0.94% of the measurement error at 60 dB down from full scale.

The IRMS measurement is undefined at zero input. Calibration

of the offset should be done at low current and values at zero

input should be ignored. For details on how to calibrate the

current rms measurement, see the Calibration section.

IRMSOS

IRMS







16384

(9)

where IRMS0 is the rms measurement without offset correction.

Table 7. Approximate IRMS Register Values

Frequency (Hz)

Integrator Off (d)

Integrator On (d)

1,921,472

2,489,581

1,914,752

2,067,210

Voltage Channel RMS Calculation

Figure 64 shows the details of the signal path for the rms

estimation on Phase A of the voltage channel. This voltage rms

estimation is done in the ADE7758 using the mean absolute

value calculation, as shown in Figure 64.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 because apparent power is the product of the

voltage and current rms values. The voltage rms values are

stored in 24-bit registers (AVRMS, BVRMS, and CVRMS). One

LSB of a voltage waveform sample is approximately equivalent to

256 LSBs of the voltage rms register. 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, that is, ±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

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 260 Hz. It is

recommended to read the rms registers synchronous to the

voltage zero crossings to ensure stability. The IRQ can be used

to indicate when a zero crossing has occurred (see the

Interrupts section).

VAN

AVRMSGAIN[11:0]

0x2748

LPF OUTPUT

WORD RANGE

0x0

60Hz

0xD8B8

0x2797

LPF OUTPUT

WORD RANGE

0x0

50Hz

0xD869

LPF1

VOLTAGE SIGNAL–V(t)

0.5

GAIN

0x193504

50Hz

0x0

0x1902BD

60Hz

0x0

|X|

AVRMS[23:0]

LPF3

SGN 216 215 214

VRMSOS[11:0]

Figure 64. Voltage RMS Signal Processing

Table 8 shows the settling time for the VRMS measurement,

which is the time it takes for the rms register to reflect the value

at the input to the voltage channel.

Table 8. Settling Time for VRMS Measurement

63%

100%

100 ms

960 ms

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 can 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.

VRMS

= VRMS0 + VRMSOS × 64

(10)

where VRMS0 is the rms measurement without the offset

correction.

Table 9. Approximate VRMS Register Values

Frequency (Hz)

Value (d)

1,678,210

1,665,118

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