ADE7878ACPZ Analog Devices Inc, ADE7878ACPZ Datasheet - Page 42

• 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

ADE7854/ADE7858/ADE7868/ADE7878
Current RMS Offset Compensation
The ADE7854/ADE7858/ADE7868/ADE7878 incorporate a
current rms offset compensation register for each phase:
AIRMSOS, BIRMSOS, CIRMSOS registers, and the NIRMSOS
register for ADE7878 and ADE7868 only. 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
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 I rms
As stated in the Current Waveform Gain Registers section, the
serial ports of the ADE78xx work on 32-, 16-, or 8-bit words
and the DSP works on 28 bits. Similar to the register presented
in Figure 33, the AIRMSOS, BIRMSOS, CIRMSOS, and
NIRMSOS (ADE7868/ADE7878 only) 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—ADE7868
and ADE7878 Only
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, which is available
to the ADE7868 and ADE7878 only, to allow energy accumu-
lation 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 55 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
COMING FROM ADC
CURRENT SIGNAL
I
rms
Figure 55. Current MAV Signal Processing for PSM1 Mode
=
0
is the rms measurement without offset correction.
I
rms
2
0
+
HPF
128
×
(
IRMSOS
4191
|X|
2
+
128
/
HPF
4191
−
1
)
xIMAV[23:0]
×
100
2
) of
(t).
(14)
Rev. D | Page 42 of 96
values and they 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. As seen in Figure 56,
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 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 ADE7868/ADE7878 work on 32-, 16-, or 8-
bit words. As presented in Figure 57, 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 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 ADE7868/
ADE7878 with nominal currents. The offsets can be estimated
by supplying the ADE7868/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.
Voltage Channel RMS Calculation
Figure 58 shows the detail of the signal processing chain for the
rms calculation on one of the phases of the voltage channel. The
voltage channel rms value is processed from the samples used in
Figure 56. xIMAV Register Values at Full Scale, 45 Hz to 65 Hz Line
212000
211500
211000
210500
210000
209500
209000
208500
208000
207500
207000
31
Figure 57. xIMAV Registers Transmitted as 32-Bit Registers
0000 0000 0000
45
20 19
50
Frequencies
FREQUENCY (Hz)
20-BIT UNSIGNED NUMBER
55
60
65
0