ade7880 Analog Devices, Inc., ade7880 Datasheet - Page 38

• Manufacturer Part Number: ade7880
• Description: Polyphase Multifunction Energy Metering Ic With Harmonic Monitoring
• Manufacturer: Analog Devices, Inc.

ADE7880
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
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% (
60 dB down from full scale. Conduct offset calibration at low
current; avoid using currents equal to zero for this purpose.
where I rms
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 18, 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



Figure 39. 24-bit AIRMS, BIRMS, CIRMS and NIRMS registers Transmitted as
3767
I
rms
31
2
+
=
128
0
is the rms measurement without offset correction.
0000
I
/
rms
3767
0000
0
2
+
−
128
1



24
32-Bit Words
×
×
100
IRMSOS
23
CURRENT SIGNAL
) of the rms measurement at
FROM HPF OR
INTEGRATOR
(IF ENABLED)
2
24-BIT NUMBER
(t). One LSB of the current
Figure 40. Current RMS Signal Processing
0
x
0xAEB86F=
0x514791=
-5,326,737
5,326,737
2
Rev. PrE | Page 38 of 103
(14)
0V
LPF
2
xIRMSOS[23:0]
7
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 41 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, BIMAV, and CIMAV
registers. The update rate of this mav measurement is 8 kHz.
COMING FROM ADC
CURRENT SIGNAL
Σ
Figure 42. xIMAV Register Values at Full Scale, 45 Hz to 65 Hz Line
212000
211500
211000
210500
210000
209500
209000
208500
208000
207500
207000
Figure 41. Current MAV Signal Processing for PSM1 Mode
45
Preliminary Technical Data
xIRMS[23:0]
HPF
50
Frequencies
FREQUENCY (Hz)
|X|
55
HPF
60
xIMAV[23:0]
65