ADE7878 datasheet(39/92 Pages) AD

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ADE7878

Rev. 0 | Page 39 of 92

Current RMS Offset Compensation

The ADE7878 incorporates a current rms offset compensation

CIRMSOS[23:0], and NIRMSOS[23:0]. These are 24-bit signed

registers and 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).

One LSB of the current rms offset compensation register is

equivalent to one LSB of the current rms register. Assuming that

the maximum value from the current rms calculation is 4,191,400

with full-scale ac inputs (50 Hz), one LSB of the current rms

offset represents 0.00037% (

(

)

100

4191

128

4191

−

) of

the rms measurement at 60 dB down from full scale. Conduct

offset calibration at low current; avoid using currents equal to

zero for this purpose.

IRMSOS

Irms

128

(14)

where Irms0 is the rms measurement without offset correction.

As previously stated, the serial ports of the ADE7878 work on

32-, 16-, or 8-bit words, and the DSP works on 28 bits. Similar

to the register presented in Figure 32, the AIRMSOS,

BIRMSOS, CIRMSOS, and NIRMSOS 24-bit signed registers

are accessed as 32-bit registers with four MSBs padded with 0s

and sign extended to 28 bits.

Current Mean Absolute Value Calculation

This section presents the second approach to estimate the rms

values of all phase currents using the mean absolute value (mav)

method. This approach is used in PSM1 mode to allow energy

accumulation based on current rms values when the missing

neutral case demonstrates to be a tamper attack. This datapath

is active also in PSM0 mode to allow for its gain calibration. The

gain is used in the external microprocessor during PSM1 mode.

The mav value of the neutral current is not computed using this

method.

CURRENT SIGNAL

COMING FROM ADC

xIMAV[23:0]

HPF

|X|

Figure 54. Current MAV Signal Processing for PSM1 Mode

Figure 54 shows the details of the signal processing chain for the

mav calculation on one of the phases of the current channel.

The current channel mav value is processed from the samples

used in the current channel waveform sampling mode. The

samples are passed through a high-pass filter to eliminate the

eventual dc offsets introduced by the ADCs, and the absolute

values are computed. The outputs of this block are then filtered

to obtain the average. The current mav values are unsigned 20-bit

values and they are stored in the AIMAV[19:0], BIMAV[19:0], and

CIMAV[19:0] registers. The update rate of this mav measure-

ment is 8 kHz.

207000

207500

208000

208500

209000

209500

210000

210500

211000

211500

212000

FREQUENCY (Hz)

Figure 55. xIMAV[19:0] Values at Full Scale, 45 Hz to 65 Hz Line Frequencies

The mav values of full-scale sinusoidal signals of 50 Hz and

60 Hz are 209,686 and 210,921, respectively. As seen in Figure 55,

there is a 1.25% variation between the mav estimate at 45 Hz

and the one at 65 Hz for full-scale sinusoidal inputs. The

accuracy of the current mav is typically 0.5% error from the

full-scale input down to 1/100 of the full-scale input.

Additionally, this measurement has a bandwidth of 2 kHz. The

settling time for the IMAV measurement, that is, the time it

takes for the mav register to reflect the value at the input to the

current channel within 0.5% error, is 500 ms.

As previously stated, the serial ports of the ADE7878 work on

32-, 16-, or 8-bit words. As presented in Figure 56, the AIMAV,

BIMAV, and CIMAV 20-bit unsigned registers are accessed as

32-bit registers with the 12 MSBs padded with 0s.

20 19

20-BIT UNSIGNED NUMBER

0000 0000 0000

Figure 56. xIMAV Registers Transmitted as 32-Bit Registers

Current MAV Gain and Offset Compensation

The current rms values stored in AIMAV, BIMAV, and CIMAV

can be calibrated using gain and offset coefficients corresponding

to each phase. It is recommended to calculate the gains in PSM0

mode by supplying the ADE7878 with nominal currents. The

offsets can be estimated by supplying the ADE7878 with low

currents, usually equal to the minimum value at which the

accuracy is required. Every time the external microcontroller

reads the AIMAV, BIMAV, and CIMAV registers, it uses these

coefficients stored in its memory to correct them.

ADE7878 数据表(PDF) 39 Page - Analog Devices

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