ADE7878ACPZ Analog Devices Inc, ADE7878ACPZ Datasheet - Page 39

• Manufacturer Part Number: ADE7878ACPZ
• Description: IC ENERGY METERING 3PH 40LFCSP
• Manufacturer: Analog Devices Inc

Specifications of ADE7878ACPZ

• Input Impedance: 400 KOhm
• Measurement Error: 0.1%
• Voltage - I/o High: 2.4V
• Voltage - I/o Low: 0.4V
• Current - Supply: 22mA
• Voltage - Supply: 3 V ~ 3.6 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 40-WFQFN, CSP Exposed Pad
• Meter Type: 3 Phase
• Supply Voltage Range: 3V To 3.6V
• Operating Temperature Range: -40°C To +85°C
• Digital Ic Case Style: LFCSP
• No. Of Pins: 40
• Msl: MSL 1 - Unlimited
• Peak Reflow Compatible (260 C): Yes
• Supply Voltage Min: 3V
• Rohs Compliant: Yes
• Leaded Process Compatible: Yes
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Current RMS Offset Compensation
The ADE7878 incorporates a current rms offset compensation
register for each phase: AIRMSOS[23:0], BIRMSOS[23:0],
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 I
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% (
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.
where Irms
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.
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.
COMING FROM ADC
CURRENT SIGNAL
Irms
Figure 54. Current MAV Signal Processing for PSM1 Mode
=
0
is the rms measurement without offset correction.
Irms
2
0
+
HPF
128
×
IRMSOS
(
4191
|X|
2
+
128
/
HPF
4191
−
1
)
xIMAV[23:0]
×
100
2
(t).
) of
(14)
Rev. 0 | Page 39 of 92
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.
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.
Figure 55. xIMAV[19:0] Values at Full Scale, 45 Hz to 65 Hz Line Frequencies
212000
210500
210000
209500
209000
208500
208000
207500
207000
211500
211000
31
Figure 56. xIMAV Registers Transmitted as 32-Bit Registers
0000 0000 0000
45
20 19
50
FREQUENCY (Hz)
20-BIT UNSIGNED NUMBER
55
60
ADE7878
65
0