ADE7854AACPZ View Datasheet(PDF) - Analog Devices
Part Name
Description
MFG CO.
'ADE7854AACPZ' PDF : 96 Pages View PDF
Data Sheet
ADE7854A/ADE7858A/ADE7868A/ADE7878A
Current RMS Offset Compensation
The ADE7854A/ADE7858A/ADE7868A/ADE7878A incor-
porate a current rms offset compensation register for each phase:
AIRMSOS, BIRMSOS, CIRMSOS; the NIRMSOS register is
provided in the ADE7878A and ADE7868A only. These 24-bit
signed registers remove offsets in the current rms calculations.
An offset can exist in the rms calculation caused by input noises
that are integrated in the dc component of i2(t). The current rms
offset register is multiplied by 128 and added to the squared current
rms before the square root is executed. Assuming that the
maximum value from the current rms calculation is 4,191,910
with full-scale ac inputs (50 Hz), one LSB of the current rms
offset represents the following value of the rms measurement at
60 dB down from full scale:
0.00037%
=
41912 128
4191
1 100
Conduct offset calibration at low current; avoid using currents
equal to zero for calibration purposes.
I rms I rms02 128 IRMSOS
(20)
where I rms0 is the rms measurement without offset correction.
The serial ports of the ADE7854A, ADE7858A, ADE7868A,
and ADE7878A work with 32-, 16-, or 8-bit words, whereas the
DSP works with 28-bit words. Like the xIGAIN registers shown
in Figure 34, the 24-bit AIRMSOS, BIRMSOS, CIRMSOS, and
NIRMSOS (ADE7868A/ADE7878A only) registers are sign
extended to 28 bits and padded with four 0s for transmission as
32-bit registers.
Current Mean Absolute Value Calculation—ADE7868A
and ADE7878A Only
This section describes 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, which is
available to the ADE7868A and ADE7878A only, to allow energy
accumulation based on current rms values when the missing
neutral case is identified as a tamper attack. This datapath is
also active in PSM0 mode to allow for its gain calibration. The
external microprocessor uses the gain during PSM1 mode. The
MAV value of the neutral current is not computed using this
method. Figure 62 shows the signal processing chain for the MAV
calculation on one phase of the current channel.
CURRENT SIGNAL
COMING FROM ADC
HPF
|X|
xIMAV[23:0]
LPF
Figure 62. 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 pass through a high-pass filter to eliminate the eventual
dc offsets introduced by the ADCs and the absolute values are
computed.
Next, to obtain the average, outputs of this block are filtered. The
current MAV values are unsigned 20-bit values and are stored
in the AIMAV, BIMAV, and CIMAV registers. The update rate
of this MAV measurement is 8 kHz.
The MAV values of full-scale sinusoidal signals of 50 Hz and
60 Hz are 209,686 and 210,921, respectively. There is a 1.25%
variation between the MAV estimate at 45 Hz and the one at
65 Hz for full-scale sinusoidal inputs (see Figure 63).
212000
211500
211000
210500
210000
209500
209000
208500
208000
207500
207000
45
50
55
60
65
FREQUENCY (Hz)
Figure 63. xIMAV Register Values at Full Scale, 45 Hz to 65 Hz Line
Frequencies
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 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 ADE7868A/ADE7878A work on 32-, 16-, or
8-bit words. As shown in Figure 64, 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 64. 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 corre-
sponding to each phase. Calculate the gains in PSM0 mode by
supplying the ADE7868A/ADE7878A with nominal currents.
Estimate the offsets by supplying the ADE7868A/ADE7878A
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.
Rev. C | Page 45 of 96
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