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Data Sheet
Current RMS Offset Compensation
The ADE7880 incorporates a current rms offset compensation
register for each phase: AIRMSOS, BIRMSOS, CIRMSOS, and
NIRMSOS. These are 24-bit signed registers that 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 I 2 (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 3,766,572 with full-scale ac inputs
(50 Hz), one LSB of the current rms offset represents 0.00045%
212000
211500
211000
210500
210000
209500
209000
208500
208000
207500
ADE7880
? ?
? 3767 2 + 128 / 3767 ? 1 ? × 100
? ?
207000
45
50
55
FREQUENCY (Hz)
60
65
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.
2
I rms = I rms 0 + 128 × IRMSOS (15)
where I rms 0 is the rms measurement without offset correction.
As stated in the Current Waveform Gain Registers section, the
serial ports of the ADE7880 work on 32-, 16-, or 8-bit words
and the DSP works on 28 bits. Similar to the register presented
in Figure 43, 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 data path
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. Figure 66 shows the details of the signal
processing chain for the mav calculation on one of the phases of
the current channel.
Figure 67. xIMAV Register Values at Full Scale, 45 Hz to 65 Hz Line
Frequencies
The mav values of full-scale sinusoidal signals of 50 Hz and
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 3.3 kHz. The settling time for
the current mav 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 stated in the Current Waveform Gain Registers section, the
serial ports of the ADE7880 work on 32-, 16-, or 8-bit words.
As presented in Figure 68, the AIMAV, BIMAV, and CIMAV 20-
bit unsigned registers are accessed as 32-bit registers with the 12
MSBs padded with 0s.
31 20 19 0
0000 0000 0000 20-BIT UNSIGNED NUMBER
Figure 68. xIMAV Registers Transmitted as 32-Bit Registers
Current MAV Gain and Offset Compensation
The current rms values stored in the AIMAV, BIMAV, and
CIMAV registers 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 ADE7880
CURRENT SIGNAL
COMING FROM ADC
HPF
|X|
HPF
xIMAV[23:0]
with nominal currents. The offsets can be estimated by
supplying the ADE7880 with low currents, usually equal to the
Figure 66. Current MAV Signal Processing for PSM1 Mode
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
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.
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, BIMAV, and CIMAV
registers. The update rate of this mav measurement is 8 kHz.
Rev. A | Page 41 of 104
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