Download AN-1332 APPLICATION NOTE

AN-1332
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 •
www.analog.com
Architecting a Direct, 3-Phase Energy Meter with Shunts Using the
ADE7912/ADE7913
by Petre Minciunescu and Dave Smith
INTRODUCTION
The ADE7912/ADE7913 3-channel, isolated, Σ-Δ analog-todigital converters (ADCs) target polyphase energy metering
applications using shunt current sensors. This application note
expands the Applications Information section of the ADE7912/
ADE7913 data sheet. It provides in depth explanations on how
to use the ADE7912/ADE7913 when developing a direct, 3-phase
meter with shunts.
isolated side. The resistor dividers, R1_x and R2_x, sense the
voltage between the Phase A, Phase B, and Phase C lines and
the neutral line. The current channel ADC of each ADE7912/
ADE7913 measures the voltages across the shunts, while the V1
voltage channel ADC measures the voltages across the R1_A,
R1_B, and R1_C resistors.
METER CASE
ARCHITECTING A DIRECT 3-PHASE METER USING
THE ADE7912/ADE7913
METER BOARD
MICROCONTROLLER
GND_MCU
METER CASE
2
3
4
5
6
7
8
9
10
11
PHASE A
ADE7912/
ADE7913
PHASE B
ADE7912/
ADE7913
PHASE C
ADE7912/
ADE7913
INTERFACE
CIRCUITRY
INTERFACE
CIRCUITRY
INTERFACE
CIRCUITRY
12
PHASE A
GND_A
12665-001
PHASE C
NEUTRAL
GND_B
PHASE B
GND_C
1
SPI
ISOLATION
BARRIER
GND_MCU
R1_C R2_C
R1_B R2_B
R1_A R2_A
Figure 1. Three-Phase, 4-Wire Direct Meter Connections
Figure 2 shows the architecture of a meter using the ADE7912/
ADE7913. Three shunts connect to the meter terminals where
the 3-phase, 4-wire connections are made. Each ADE7912/
ADE7913 manages one phase through an interface circuitry.
The reference terminal of the shunt becomes the ground of the
isolated side of the ADE7912/ADE7913. GND_A is the ground
of the Phase A ADE7912/ADE7913 isolated side, GND_B is the
ground of the Phase B ADE7912/ADE7913 isolated side, and
GND_C is the ground of the Phase C ADE7912/ADE7913
SHUNT
SHUNT
1
2
3
4
5
SHUNT
6
7
8
9
10
11
12
PHASE A
PHASE B
PHASE C
12665-002
Figure 1 shows how a direct, 3-phase energy meter is connected
in a 3-phase, 4-wire system. The Phase A line connects to
Terminal 1 and Terminal 3 of the meter, Phase B connects to
Terminal 4 and Terminal 7, Phase C connects to Terminal 7 and
Terminal 9, and the neutral line connects to Terminal 10 and
Terminal 12. Typically, Terminal 2, Terminal 5, and Terminal 8
of the meter are used for the voltage sensing connections. In the
case of the direct, 3-phase energy meter, they are internally
connected to Terminal 1, Terminal 4, and Terminal 7,
respectively. These are the points of the meter where the lines of
Phase A, Phase B, and Phase C enter the meter.
NEUTRAL
Figure 2. Shunt-Based, Direct, 3-Phase Meter Architecture
Note that when the meter is based on the ADE7912/ADE7913,
Terminal 2, Terminal 5, and Terminal 8 of the meter are not
used. The reference terminals of the shunts (GND_A, GND_B,
and GND_C), being directly connected to the phase lines,
become the reference points for the voltage sensing resistor
dividers.
Rev. 0 | Page 1 of 11
AN-1332
Application Note
There is some interface circuitry between the shunts, the
voltage dividers, and the ADE7912/ADE7913 devices that is
discussed in the Interfacing the ADE7912/ADE7913 with the
Shunt and the Resistor Divider section.
INTERFACING THE ADE7912/ADE7913 WITH THE
SHUNT AND THE RESISTOR DIVIDER
The neutral line current is not monitored in the meter
presented in Figure 2. Figure 3 presents the architecture of a
direct, 3-phase meter in which the neutral current is monitored.
The only difference is an additional ADE7912 (Phase N) that
senses only the neutral line current using a shunt. The voltage
channel of this chip is not used.
The microcontroller and the primary side of the ADE7912/
ADE7913s have a GND_MCU isolated ground from the neutral
line. This means the power supply of the meter must provide a
supply voltage to the microcontroller and the ADE7912/
ADE7913s that is isolated from the neutral line.
The Phase A, B, C, or N current is sensed with a shunt (see
Figure 4). The antialiasing filters, R/C (1 kΩ/33 nF), between
the IP and IM pins and the shunt terminals have a corner at
4.8 kHz (1/(2 × π × 103 × 33 × 10-9)). The AGND_ADC ground
point where the filter capacitors are biased is connected to the
reference terminal GND_A, GND_B, GND_C, or GND_N of
the shunts.
FROM ONE
OF METER
TERMINALS
1, 4, 7, OR 10
E
PHASE A, B, C, OR N
ADE7912/ADE7913
R
1kΩ
150Ω
E
TO ONE
OF METER
TERMINALS
3, 6, 9, OR 12
GND_A,
GND_B, 150Ω
GND_C,
OR GND_N E
AGND_ADC
C
33nF
R
150Ω
Figure 4. Interfacing the ADE7912/ADE7913 with a Shunt
METER BOARD
MICROCONTROLLER
SPI
ISOLATION
BARRIER
E
METER
TERMINAL 10
GND_MCU
150Ω
PHASE B
ADE7912/
ADE7913
PHASE C
ADE7912/
ADE7913
IP
1kΩ
METER CASE
PHASE A
ADE7912/
ADE7913
IM
C
33nF
12665-004
The neutral line determines the ground of the microcontroller
(GND_MCU) that manages the ADE7912/ADE7913 devices.
This is the same ground of the primary side of the ADE7912/
ADE7913. The microcontroller uses one serial port interface
(SPI) to communicate with the ADE7912/ADE7913s. This
means that the power supply of the meter must provide a supply
voltage to the microcontroller and the ADE7912/ADE7913s
that has the GND_MCU ground on the neutral line.
All the ADE7912/ADE7913s that monitor the phase lines have
the same interface circuitry. This circuitry is composed of two
paths: one that interfaces the ADE7912/ADE7913 with the
shunt (see Figure 4), and one that interfaces the ADE7912/
ADE7913 with the voltage divider (see Figure 5).
R1_A, R1_B, OR R1_C
330kΩ
330kΩ
PHASE A, B, C
ADE7912/ADE7913
330kΩ
R2_A, R2_B, OR R2_C
1kΩ
V1P
C
33nF
AGND_ADC
PHASE N
ADE7912
R
GND_A
INTERFACE
CIRCUITRY
R1_C R2_C
GND_N
R1_A R2_A
SHUNT
1
2
3
4
5
SHUNT
6
7
8
10
11
3  330 K  1 K
0. 5

 350 .37 V
1K
2
12
PHASE A
NEUTRAL
12665-003
PHASE B
PHASE C
VM
33nF
The Phase A, B, or C to neutral voltages are sensed with the
voltage dividers R1_x and R2_x. The resistor R1_x is composed
of three 330 kΩ resistors, and R2_x is equal to 1 kΩ. This
ensures a full scale voltage equal to the following equation,
which is sufficient to monitor 230 V, 3-phase systems:
SHUNT
9
1kΩ
Figure 5. Interfacing the Phase A ADE7912/ADE7913 with a Voltage Divider
R1_B R2_B
SHUNT
AGND_ADC
12665-005
INTERFACE
CIRCUITRY
GND_C
INTERFACE
CIRCUITRY
GND_B
INTERFACE
CIRCUITRY
Figure 3. Shunt-Based, 3-Phase Meter Architecture with Monitored Neutral Line
Capacitor C (33 nF), in parallel with R2_x, creates the
antialiasing filter in the V1P pin path. Between the
AGND_ADC ground point derived from the ground terminal
GND_x of the shunt (see Figure 4) and the VM pin of the
ADE7912/ADE7913, there is an identical antialiasing filter, R/C
(1 kΩ/33 nF). Note that the antialiasing filters in both current
and voltage paths are the same. This ensures that the delays
introduced in the phase current and voltage measurements are
similar.
Rev. 0 | Page 2 of 11
Application Note
AN-1332
The E ferrite beads (150 Ω at 100 MHz) are placed in the shunt
and voltage divider connections to filter the high frequency
noise that may be induced into the wires during electrical fast
transient (EFT) tests.
As the meter architectures considered in Figure 2 and Figure 3
do not present any additional voltages to measure besides the
phase to neutral voltages, Figure 5 does not present the circuitry
required to interface the V2P pin of the ADE7913. However, if
additional voltages are monitored, the V2P circuitry is identical
to the V1P circuitry. The V2P functionality is available only on the
ADE7913. The V2P functionality is not available on the ADE7912.
CS
ADE7912/
ADE7913
SCLK
MOSI
PHASE A MISO
VDD
10kΩ
CS_A
CS
CS_B
SCLK
SCLK
MOSI
MOSI
PHASE B MISO
MISO
CS_C
I/O
ADE7912/
ADE7913
PROVIDING CLOCK SIGNAL TO ADE7912/ADE7913
If the microcontroller is able to generate a clock signal with a
frequency between 3.6 MHz and 4.21 MHz over the −40°C to
+85 °C temperature range, then clock all the ADE7912/
ADE7913s of the meter from it. If the microcontroller cannot
generate the clock, then clock one ADE7912/ADE7913 device
from a crystal and use it to provide clock to the other
ADE7912/ADE7913s of the energy meter. Use the power-up
procedure detailed in the data sheet to accomplish this.
Both clocking schemes ensure all isolated ADCs function
synchronously: they sample signals simultaneously and provide
coherent ADC output samples. See the ADE7912/
ADE7913 data sheet for details.
MICROCONTROLLER
CS
ADE7912/
ADE7913
MOSI
PHASE C MISO
CLKOUT/
DREADY
Figure 6. Connections Between Three ADE7912/ADE7913s and the
Microcontroller
CS
ADE7912/
ADE7913
SCLK
MOSI
PHASE A MISO
VDD
10kΩ
CS_A
COMMUNICATING WITH THE ADE7912/ADE7913
The ADE7912/ADE7913 has an SPI interface, and the
corresponding data sheet provides extensive details on the
protocol used to read and write the device registers.
SCLK
12665-006
The ADE7912/ADE7913 data sheet provides extensive detail on
ways to provide the clock to the ADE7912/ADE7913 devices of
a 3-phase energy meter.
CS
CS_B
SCLK
SCLK
MOSI
MOSI
PHASE B MISO
MISO
CS_C
I/O
CS_N
ADE7912/
ADE7913
In this section, a system perspective of the microcontroller
managing three or four ADE7912/ADE7913s in a 3-phase
energy meter is proposed.
MICROCONTROLLER
CS
The microcontroller communicates with the ADE7912/
ADE7913s at the maximum allowed SPI serial clock frequency
of 5.6 MHz.
ADE7912/
ADE7913
SCLK
MOSI
PHASE C MISO
CLKOUT/
DREADY
CS
ADE7912
SCLK
PHASE N MOSI
MISO
12665-007
Suppose that the microcontroller is connected to the
ADE7912/ADE7913s as shown in Figure 6 or Figure 7. The SPI
port of each ADE7912/ADE7913 is selected using three (see
Figure 6) or four (see Figure 7) input/output (I/O) lines of the
microcontroller: CS_A, CS_B, CS_C, and CS_N. The
CLKOUT/DREADY pin of Phase C of the ADE7912/ADE7913
is connected to an I/O pin of the microcontroller on which the
microcontroller can receive an external interrupt.
Figure 7. Connections Between Four ADE7912/ADE7913s and the
Microcontroller
Rev. 0 | Page 3 of 11
AN-1332
Application Note
The time to read these registers from three ADE7912/
ADE7913s (see the case shown in Figure 6) is
The ADE7912/ADE7913s are set to provide the ADC outputs at
8 kHz, that is, a 125 μs period. Set Bits[5:4] (ADC_FREQ) in
the CONFIG register (Register 0x8) to 00 on all ADE7912/
ADE7913s.
3 
All the ADE7912/ADE7913s function synchronously, that is,
the ADCs sample simultaneously on all three phases and the
ADC outputs are coherent.
8  3  24  16  8  16
5.6  10 6
 65 s
In reality, this timing is usually longer because the
microcontroller management of the SPI communication
includes some C programming inefficiency, but there are 125 –
65 = 60 μs left to accommodate them every ADC output period.
DREADY functionality is selected at Phase C on the
ADE7912/ADE7913 CLKOUT/DREADY pin: Bit 0
(CLKOUT_EN) in the CONFIG register cleared to 0. Every
8 kHz, the CLKOUT/DREADY pin goes low for 64 CLKIN
cycles (15.625 μs at CLKIN = 4.096 MHz).
Figure 3 presents the case of an energy meter containing four
ADE7912/ADE7913s. Considering that the ADE7912 managing
Phase N senses only the neutral line current, the time to read
these registers from all ADE7912/ADE7913s is
Figure 8 presents the result of the microcontroller receiving an
external interrupt because the CLKOUT/DREADY pin of the
ADE7912/ADE7913 managing Phase C has a high to low
transition.
4 
8  3  24  16  8  16
5.6  10 6
 86 s
The timing is well within the 125 μs allotted time. In this case,
125 – 86 = 39 μs are available to accommodate eventual C
programming inefficiencies.
The microcontroller executes a burst read of the
ADE7912/ADE7913 output registers: IWV, V1WV, V2WV (if
ADE7913 is used), ADC_CRC, STATUS0, and
CNT_SNAPSHOT. Reading ADC_CRC, STATUS0, and
CNT_SNAPSHOT is not obligatory, but they are included in
the calculation as the worst-case scenario.
PHASE C
ADE7912/ADE7913
DREADY
125µs
65µs WHEN ADE7913s ARE USED, OR
52µs WHEN ADE7912s ARE USED
CS_A
CS_B
CS_C
PHASE A
ADE7912/ADE7913
IS ACCESSED
PHASE B
ADE7912/ADE7913
IS ACCESSED
BURST READ
COMMAND BYTE:
0x04
PHASE C
ADE7912/ADE7913
IS ACCESSED
MOSI
MISO
60µs OR 39 µs TO ACCOMMODATE
C PROGRAMMING INEFFICIENCIES
BURST REGISTERS :
IWV, V1WV, V2WV (IF ADE7913s ARE USED), ADC_CRC,
STATUS0 AND CNT_SNAPSHOT
Figure 8. System Perspective of Communication Between the Microcontroller and ADE7912/ADE7913s in an Energy Meter
Rev. 0 | Page 4 of 11
12665-008
PHASE C
PHASE B
PHASE A
ADE7912/ADE7913 ADE7912/ADE7913 ADE7912/ADE7913
BURST
BURST
BURST
REGISTERS
REGISTERS
REGISTERS
Application Note
AN-1332
The top and bottom layers are identical to a 2-layer PCB design,
and the inner layers create a stitching capacitor. The presence of
the stitching capacitor is essential in order to reduce the
emissions generated by the ADE7912/ADE7913 dc-to-dc
converter, and to meet the standard requirements.
ENERGY METERING SOFTWARE PROGRAM
REMARKS
This section contains observations related to the
ADE7912/ADE7913. It is recommended that the energy
metering program residing in the microcontroller take these
observations into account.
Figure 9 presents the structure of the 4-layer PCB. The top layer
contains the components. The second layer creates one plate of
the stitching capacitor using the GNDISO isolated ground of the
ADE7912/ADE7913s. The third layer creates the other plate of
the stitching capacitor using the GND_MCU primary ground
of the ADE7912/ADE7913s. Note that the PCB thickness
between the plates is 0.5 mm. Because the standard IEC 62052-31
requires a minimum thickness of 0.4 mm when line to neutral
voltages are below 300 V, a thickness of 0.5 mm is used to allow
any PCB production variation.
Use the power-up and initialization procedure outlined in the
ADE7912/ADE7913 data sheet to ensure all ADE7912/
ADE7913s the board start correctly and function
synchronously.
The ADE7912/ADE7913 specifications table of the data sheet
states that the current channel ADC offset error is typically
−2 mV and the voltage channels ADC offset error is typically
−35 mV. The ADC offset drift over temperature is ±500 ppm/°C,
maximum. The ac energy metering calculations need to
eliminate this offset. Because the offset drifts with temperature,
implement a high-pass filter (HPF) to eliminate it. A simple
offset calibration at room temperature is not recommended as it
does not fully compensate the temperature influence.
It is not recommended to invert the second and third layers,
that is, setting the GND_MCU plane on the second layer and
the GNDISO plates on the third layer, because the GND_MCU
plane creates a parasitic capacitance with the shunt related PCB
traces on the top layer. This parasitic capacitance creates a
voltage that adds to the voltage across the shunt and is
proportional and in phase with the voltage between the GNDISO
and GND_MCU. This also affects the accuracy of the metrology
measurements. By setting the GNDISO plates on the second layer
and GND_MCU plane on the third, the GNDISO plates shield
the GND_MCU plane and remove the coupling to the shunt IP
and IM PCB traces on the top layer.
Independent of how the shunts are mounted on the printed
circuit board (PCB), their electric circuits introduce a phase
error in the metrology calculations. Therefore, it is recommended
to use a phase compensation procedure that aligns the currents
and voltages on each phase.
LAYOUT GUIDELINES
Figure 10 presents the schematic of the metrology section of a
direct, 3-phase meter using three ADE7913s, identical to the
schematic of the ADE7913 evaluation board. The second
voltage channel, V2, is shown with a voltage divider in order to
provide the most comprehensive layout example possible.
Figure 11, Figure 12, Figure 13, and Figure 14 present the
recommended 4-layer layout of the metrology section from
Figure 10. It is the layout implemented in the ADE7913
evaluation board.
A 4-layer PCB is required when using ADE7912/ADE7913. A
3-phase meter using three ADE7913s passes the Class B
CISPR22 standard specification with a sufficient margin only
when a 4-layer PCB is used (see the Radiated Emission Tests
Results section for details on these results).
ISOLATED SIDE GROUND
PLANE ON TOP LAYER
ADE7913
C23
20mils = 0.508mm MIN
50mils = 1.27mm
8mm
ISOLATED SIDE GROUND
PLANE ON LAYER 2
PRIMARY SIDE GROUND
PLANE ON LAYER 3
PRIMARY SIDE GROUND
PLANE ON BOTTOM LAYER
Figure 9. Four-Layer Structure with Stitching Capacitor
Rev. 0 | Page 5 of 11
12665-009
62mils ±10% =
1.574mm NOM
PRIMARY SIDE GROUND
PLANE ON TOP LAYER
AN-1332
Application Note
1
10µF
150Ω
FERRITE
FROM ONE OF
METER TERMINALS
1, 4, OR 7
FROM
METER TERMINAL 10
1kΩ
GNDISO
IP
VDD
150Ω
FERRITE
33nF
GND
AGND_ADC
6
CLKOUT/DREADY
150Ω
FERRITE
330kΩ
330kΩ
CS
IM
1kΩ
1kΩ
AUXILIARY
VOLTAGE
TERMINAL
7
VDDISO
150Ω
FERRITE 33nF
19
20
3.3V
100nF
10µF
GND_MCU
12
TO MCU
18
FROM MCU
ADE7913C
330kΩ
1kΩ
150Ω
FERRITE
2
4
V1P
MISO
33nF
33nF
5
MOSI
VM
SCLK
15
16
17
AGND_ADC
AGND_ADC
330kΩ 330kΩ 330kΩ
1kΩ
3
33nF
4.7µF
AGND_ADC
XTAL1
V2P
8
4.7µF
XTAL2
LDO
100nF
10
100nF
9
GNDISO
GND
REF
13
14
11
GND_MCU
3.3V
1
2
3
4
5
SAME AS IN
ADE7913 C
6
7
8
9
10
VDDISO
CLKOUT/DREADY
GNDISO
CS
V2P
MISO
V1P
MOSI
VM
ADE7913B
SCLK
IM
VDD
IP
XTAL1
LDO
XTAL2
REF
GND
GNDISO
GND
VDDISO
CLKOUT/DREADY
GNDISO
CS
12
18
FROM MCU
10kΩ
15
TO MCU
16
FROM MCU
17
FROM MCU
19
SAME AS IN
ADE7913 C
13
14
11
20
GND_MCU
1
2
3
4
5
SAME AS IN
ADE7913 C
6
7
8
9
10
MISO
V2P
V1P
VM
ADE7913A
MOSI
SCLK
IM
VDD
IP
XTAL1
LDO
XTAL2
REF
GND
GNDISO
GND
12
18
FROM MCU
15
16
17
19
SAME AS IN
ADE7913 C
13
14
11
4.096MHz
20
22pF
GND_MCU
Figure 10. Direct, 3-Phase Meter Schematic of Metrology Section
Rev. 0 | Page 6 of 11
22pF
GND_MCU
11115-008
TO ONE
OF TERMINALS
3, 6, 9, OR 12
100nF
AN-1332
12665-011
Application Note
12665-012
Figure 11. Top Layer Layout of the Direct, 3-Phase Meter Metrology Section
12665-013
Figure 12. Layer 2 Layout of the Direct, 3-Phase Meter Metrology Section
Figure 13. Layer 3 Layout of the Direct, 3-Phase Meter Metrology Section
Rev. 0 | Page 7 of 11
Application Note
12665-014
AN-1332
Figure 14. Bottom Layer Layout of the Direct, 3-Phase Meter Metrology Section
In the following sections, the various parts of the schematic are
analyzed and commented upon, relative to their associated layout.
Decoupling Capacitors Layout Guidelines



Place the 100 nF capacitors closest to the chip.
Make the connections between the capacitors and the
VDD, VDDISO, LDO, and REF pins as short as possible.
Make the connections between the capacitors and the
GND, GNDISO, LDO, and REF pins as short as possible.
12665-016
Follow these recommendations for the decoupling capacitors
between the VDD and GND (10 μF and 100 nF), VDDISO and
GNDISO (10 μF and 100 nF), LDO and GNDISO (4.7 μF and
100 nF), and REF and GNDISO (4.7 μF and 100 nF) pins of the
ADE7912/ADE7913:
Figure 16. Layout of Capacitors Between the VDDISO and GNDISO Pins
This improves the electromagnetic compatibility (EMC)
immunity of the chips and lowers the emissions generated by
the ADE7912/ADE7913 dc-to-dc converter.
12665-017
Figure 15, Figure 16, Figure 17, and Figure 18 present the
various layouts of these capacitors.
12665-015
Figure 17. Layout of Capacitors Between the LDO and GNDISO Pins
Figure 15. Layout of Capacitors Between the VDD and GND Pins
Rev. 0 | Page 8 of 11
Application Note
AN-1332
12665-018
The stitching capacitor is created by plates on the second and
third PCB layers. Figure 21 shows the layout of one plate
created underneath the Phase A ADE7912/ADE7913. The vias
within the red circles connect this plate to the GNDISO plane
from the top layer. Figure 22 shows the layout of the GND_MCU
plane in the third layer. It creates the plate of three stitching
capacitors, one for each ADE7912/ADE7913. The vias within
the red circles are the GNDISO vias. There is more than 1 mm of
space between these vias and the GND_MCU plate in order to
comply with the IEC 62052-31 standard that requires a 0.4 mm
minimum distance between adjacent conductors when the line
to neutral voltages are below 300 V.
Figure 18. Layout of Capacitors Between the REF and GNDISO Pins
Crystal and Load Capacitors Layout Guidelines
The crystal load capacitors must be placed closest to the
Phase A ADE7912/ADE7913 to improve the EMC immunity of
the chip, whereas the crystal can be placed in close proximity
(Figure 19). The distance between the crystal and the load
capacitors is not as critical as the distance between the
capacitors and the XTAL1 and XTAL2 pins.
The minimum stitching capacitor area recommended to pass
Class B CISPR22/EN-55022 standard specification is 825 mm2,
giving a capacitor value of 63 pF (see the Radiated Emission
Tests Results section for more details). The expression used to
compute this 63 pF capacitor value is:
C  0  r 
A
d
where:
Ɛ0 = 8.854 × 10-12 F/m and is the vacuum permittivity.
Ɛr = 4.3 and is the relative permittivity of the PCB FR4 material.
A = 825 mm2 and is the area of the capacitor plates.
d = 0.5 mm and is the distance between the plates.
12665-019
The larger the area of the plates, the larger the capacitor value,
the greater the margin in passing the Class B CISPR22/EN55022 standard. In the ADE7913 evaluation board, a distance of
2.54 mm is designed between the GNDISO plates of various
ADE7913s on Layer 2, although the IEC 62052-31 standard
requires a 0.4 mm minimum distance between adjacent
conductors when the line to neutral voltages are below 300 V
(see Figure 23). The capacitor value is 84 pF for an area of the
plates equal to 1100 mm2.
Figure 19. Layout of Crystal and Load Capacitors
Isolated Ground Layout Guidelines
Figure 20. Layout of the GNDISO Connections to the Isolated Ground Plane
and Related Vias
12665-021
12665-020
The ADE7912/ADE7913 have two GNDISO isolated ground
pins. They are connected through a ferrite bead to the reference
terminal of the shunt and determine the isolated ground plane
on the top layer of the PCB. The connections between the
GNDISO pins and the isolated ground plane must be very short.
The vias used to connect the isolated ground plane from the top
layer to the stitching capacitor plate on the second layer need to
be close to the GNDISO pins in order to have the lowest inductance
possible. Limit the number of vias to the minimum as they take
area from the stitching capacitor plates and reduce its capacitance.
Figure 21. Section of PCB Layer 2 Creating Stitching Capacitor GNDISO Plate
Rev. 0 | Page 9 of 11
AN-1332
Application Note
1.27mm
1.27mm
12665-022
1.65mm
Figure 22. Section of Layer 3 Creating Stitching Capacitor GNDMCU Plate
2.54mm
12665-022
2.54mm
Figure 23. Section of Layer 2 Showing All Three Stitching Capacitors
RADIATED EMISSION TESTS RESULTS
We tested the ADE7913 evaluation board for compliance with
the Class B CISPR22 standard. The board has been placed in a
three phase meter case and was powered from a battery. A 5 m,
3-phase cable without termination was mounted on the meter
and then placed under the floor in the anechoic chamber
(Figure 24).
As already stated, we do not recommend PCB designs that have
stitching capacitors smaller than 63 pF (that is with the area of
the plates smaller than 825 mm2). For example, with a stitching
capacitor of 42 pF (that is an area of the plates of 550 mm2), an
ADE7978 evaluation board was over the Class B CISPR22 limit
by 4 dBμV/m at 380 MHz.
Measurements of the radiated emissions were executed for
frequencies between 30 MHz and 1 GHz and between 1 GHz
and 2 GHz.
To test the radiated emissions when the stitching capacitor is
smaller, we built an ADE7978 evaluation board with a stitching
capacitor of 63 pF (825 mm2). As an ADE7933 isolated ADC
used on this board is equivalent to an ADE7913 from emissions
perspective, we may say the ADE7978 evaluation board behaves
in the same way as an equivalent ADE7912/ADE7913 based
design. This board passed Class B CISPR22 specification by a
9 dBμV/m quasi peak margin for frequencies between 30 MHz
and 1 GHz (Figure 26) and 7 dBμV/m average results margin
for frequencies between 1 GHz and 2 GHz (Figure 28).
12665-024
With a stitching capacitor of 84 pF (1100 mm2 plates), a
minimum 10 dBμV/m quasi peak margin to Class B CISPR22
limits was obtained for frequencies between 30 MHz and 1 GHz
(Figure 25). For frequencies between 1 GHz and 2 GHz, a
minimum 11 dBμV/m average results margin to Class B
CISPR22 limits was obtained (Figure 26).
Figure 24. Three-Phase Meter with Battery in the Anechoic Chamber
Rev. 0 | Page 10 of 11
Application Note
AN-1332
100
100
90
90
80
80
60
50
40
CISPR22 CLASS B AT 10m QP
30
40
20
10
10
100M
1G
0
30M
1G
Figure 27. Anechoic Chamber Emissions from ADE7978 Evaluation Board,
63 pF Stitching Capacitor (Quasi Peak Points are in Blue, 30 MHz to 1 GHz)
100
100
90
90
80
80
70
LEVEL (dBµV/m)
CISPR 22 CLASS B AT 3m PEAK
60
50 CISPR 22 CLASS B AT 3m AVERAGE
40
CISPR 22 CLASS B AT 3m PEAK
60
50 CISPR 22 CLASS B AT 3m AVERAGE
40
30
30
FREQUENCY (GHz)
12665-026
0
2
1
2
FREQUENCY (GHz)
Figure 26. Anechoic Chamber Emissions from the ADE7913 Evaluation
Board, 84 pF Stitching Capacitor (Average Points are in Green, 1 GHz to
2 GHz)
12665-028
10
10
1
TRACE3: MEASURED AVERAGE
TRACE1: MEASURED PEAK
20
TRACE3: MEASURED AVERAGE
TRACE1: MEASURED PEAK
20
0
100M
FREQUENCY (Hz)
Figure 25. Anechoic Chamber Emissions from the ADE7913 Evaluation
Board, 84 pF Stitching Capacitor (Quasi Peak Points are in Blue, 30 MHz to
1 GHz)
70
CISPR22 CLASS B AT 10m QP
30
FREQUENCY (Hz)
LEVEL (dBµV/m)
50
20
0
30M
TRACE3: MEASURED AVERAGE
TRACE2: MEASURED QUASI PEAK
TRACE1: MEASURED PEAK
60
12665-027
LEVEL (dBuV/m)
70
TRACE3: MEASURED AVERAGE
TRACE2: MEASURED QUASI PEAK
TRACE1: MEASURED PEAK
12665-025
LEVEL (dBµV/m)
70
Figure 28. Anechoic Chamber Emissions from ADE7978 Evaluation Board,
63 pF Stitching Capacitor (Average Points are in Green, 1 GHz to 2 GHz)
CONCLUSION
This application note has shown how to develop a direct three
phase meter with shunts using the ADE7912/ADE7913 isolated
ADC. It provided recommendations on the schematic and
layout in order to pass the Class B CISPR22 radiated emissions
specification by at least 10 dBμV/m margin. It showed how
much bandwidth a microcontroller has to reserve to managing
the SPI communication with up to four ADE7912/ADE7913s.
I2C refers to a communications protocol originally developed by Philips Semiconductors (now NXP Semiconductors).
©2014 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN12665-0-10/14(0)
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