ADE7933 datasheet(55/125 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932/ADE7923

Rev. D | Page 55 of 125

Current RMS Offset Compensation

The ADE7978 incorporates current rms offset compensation

registers for each phase: AIRMSOS, AFIRMSOS, BIRMSOS,

BFIRMSOS, CIRMSOS, CFIRMSOS, and NIRMSOS. These

24-bit signed registers are 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).

The current rms offset compensation register is shifted left by

seven bits and then added to the squared current rms before the

square root is executed. Assuming that the maximum value from

the current rms calculation is 3,761,808 with full-scale ac inputs

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

the following value of the rms measurement at 60 dB down

from full scale:

0.00045% =

100

3761

128

37612

















−

Conduct offset calibration at low current; avoid using currents

equal to zero for calibration purpose.

I rms =

IRMSOS

rms

+128

(23)

where I rms0 is the rms measurement without offset correction.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit

words, whereas the DSP works with 28-bit words. Like the

xIGAIN registers shown in Figure 52, the 24-bit xIRMSOS and

xFIRMSOS registers are sign extended to 28 bits and padded

with four 0s for transmission as 32-bit registers.

VOLTAGE RMS CALCULATION

Figure 76 shows the signal processing chain for the rms calculation

on one phase of the voltage channels. The voltage channel rms

value is processed from the samples used in the voltage channel.

The voltage rms values are signed 24-bit values and are stored

in the AVRMS, AV2RMS, BVRMS, BV2RMS, CVRMS, CV2RMS,

NVRMS, and NV2RMS registers. The rms values of the

fundamental components are stored in the AFVRMS, BFVRMS,

and CFVRMS registers. The update rate of the voltage rms

measurement is 8 kHz.

0xAED2C0 =

–5,320,000

0x512D40 =

+5,320,000

VOLTAGE

SIGNAL

FROM HPF

LPF

xVRMSOS[23:0]

xVRMS[23:0]

Figure 76. Voltage RMS Signal Processing

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

produces an output code that is approximately 5,320,000. The

equivalent rms value of a full-scale sinusoidal signal is 3,761,808,

independent of the line frequency.

The accuracy of the voltage rms is typically 0.1% error from

the full-scale input down to 1/1000 of the full-scale input. This

measurement has a bandwidth of 3.3 kHz.

To ensure rms measurement stability, follow these steps:

1. Read the rms registers at least once per line cycle over 1 sec.

2. Average the readings to obtain the rms value.

The settling time for the voltage rms measurement is 580 ms for

both 50 Hz and 60 Hz input signals. The voltage rms measurement

settling time is the time it takes for the rms register to reflect the

value at the input to the voltage channel when starting from 0.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit words,

whereas the DSP works with 28-bit words. The 24-bit signed

AVRMS, AFVRMS, AV2RMS, BVRMS, BFVRMS, BV2RMS,

CVRMS, CFVRMS, CV2RMS, NVRMS, and NV2RMS registers

are transmitted as 32-bit registers with the eight MSBs padded

with 0s (see Figure 75).

Voltage RMS Offset Compensation

The ADE7978 incorporates voltage rms offset compensation

registers for each phase: AVRMSOS, AFVRMSOS, AV2RMSOS,

BVRMSOS, BFVRMSOS, BV2RMSOS, CVRMSOS, CFVRMSOS,

CV2RMSOS, NVRMSOS, and NV2RMSOS. These 24-bit signed

registers are used to remove offsets in the voltage rms calculations.

An offset can exist in the rms calculation due to input noises that

are integrated in the dc component of V2(t). The voltage rms

offset compensation register is shifted left by seven bits and then

added to the squared voltage rms before the square root is

executed. Assuming that the maximum value from the voltage

rms calculation is 3,761,808 with full-scale ac inputs (50 Hz or

60 Hz), one LSB of the voltage rms offset represents the

following value of the rms measurement at 60 dB down from

full scale:

0.00045% =

100

3761

128

37612

















−

Conduct offset calibration at low current; avoid using voltages

equal to zero for calibration purposes.

V rms =

VRMSOS

rms

+ 128

(24)

where V rms0 is the rms measurement without offset correction.

The serial ports of the ADE7978 work with 32-, 16-, or 8-bit

words, whereas the DSP works with 28-bit words. Like the

xIGAIN registers shown in Figure 52, the 24-bit xVRMSOS,

xV2RMSOS, and xFVRMSOS registers are sign extended to 28

bits and padded with four 0s for transmission as 32-bit registers.

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