EVAL-ADE7878EBZ Analog Devices Inc, EVAL-ADE7878EBZ Datasheet - Page 3

• Manufacturer Part Number: EVAL-ADE7878EBZ
• Description: Energy Meter Eval. Board
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADE7878EBZ

• Silicon Manufacturer: Analog Devices
• Application Sub Type: Energy Meter
• Kit Application Type: Data Converter
• Silicon Core Number: ADE7878
• Features: External Transducers, LED Indicators, Logic Outputs
• Kit Contents: Board
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: Q4472972; Q4684026; Q5015773

Preliminary Technical Data
EVALUATION BOARD POWER SUPPLIES
The board has three different power domains: one that supplies
the microcontroller and one side of the isocouplers, one that
supplies the other side of the optocouplers and one that supplies
ADE7878. The ground of the microcontroller’s power domain is
connected to the ground of the PC through the USB cable. The
ground of ADE7878 power domain is determined by the
ground of the phase voltages VAP, VBP, VCP and VN and must
be different from the ground of the microcontroller’s power
domain.
The microcontroller 3.3V supply is provided at P12 connector.
ADE7878 3.3V supply is provided at P9 connector. The same
supply should also be provided at P10 connector, the connector
that supplies the other side of the isocouplers.
ANALOG INPUTS (P1, P2, P3, P4, P6, P7, AND P8)
Current and voltage signals are connected at the screw
terminals P1 – P4 and P5 - P8 respectively. All analog input
signals are filtered using the on-board anti-aliasing filters before
being connected to ADE7878. The components used on the
board are the recommended values to be used with ADE7878.
Current Sense Inputs (P1, P2, P3, and P4 Connectors)
ADE7878 measures 3 phase currents and the neutral current.
Current transformers or Rogowski coils can be used to sense
the currents, but not mixed together. ADE7878 contains
different internal PGA gains on phase currents and on the
neutral current, so sensors with different ratios can be used.
The only requirement is to have same scale signals at PGAs
outputs, otherwise the mismatch functionality of ADE7878 is
compromised (Please see Neutral Current Mismatch chapter in
ADE7878 data sheet for more details). Figure 2 shows the
structure used for the phase A current: the sensor outputs are
connected to P1 connector. The resistors R1 and R2 are the
burden resistors and by default, they are not populated. They
can also be disabled using JP1A and JP2A jumpers. The RC
networks R9/C9 and R10/C10 are used to provide phase
compensation when a current transformer is being used. They
can be disabled using JP3A and JP4A jumpers. The RC
networks R17/C17 and R18/C18 are the antialiasing filters. The
default corner frequency of these low pass filters is 8.8KHz
(1KΩ/18nF). These filters can easily be adjusted by replacing
the components on the evaluation board.
All the other current channels, that is phase B, phase C and
neutral current have similar input structure.
Using a Current Transformer as the Current Sensor
Figure 3 shows how a current transformer can be used as a
current sensor in one phase of a 3-phase 4-wire distribution
system (Phase A). The other two phases and the neutral current
requires similar connections.
Rev. PrB | Page 3 of 30
The burden resistors R1 and R2 have to be chosen function of
current transformer ratio and maximum current of the system.
The jumpers JP1A and JP2A should be opened if R1 and R2 are
used. The antialiasing filters should be enabled by opening
jumpers J5A and J6A (please see Figure 3).
The transformer’s secondary current is converted to a voltage by
using a burden resistor across the secondary winding outputs.
Care should be taken when using a current transformer as the
current sensor. If the secondary is left open, that is no burden is
connected, a large voltage could be present at the secondary
outputs. This can cause an electric shock hazard and potentially
damage electronic components.
Most current transformers introduce a phase shift that the
manufacturer indicates in the data sheet. This phase shift can
lead to significant energy measurement errors, especially at low
power factors. ADE7878 can correct the phase error using
APHCAL[9:0], BPHCAL[9:0] and CPHCAL[9:0] phase
calibration registers as long as the error stays between -6.732°
and +1.107° at 50Hz. Please see ADE7878 data sheet for more
details. The software supplied with the ADE7878 evaluation
board allows user adjustment of phase calibration registers.
For this particular example, burden resistors of 50 ohm signify
an input current of 7.05 Arms at ADE7878 ADC full scale input
(0.5V). In addition, the PGA gains for the current channel have
to be set at 1. For more information on setting PGA gains,
please see ADE7878 data sheet. The evaluation software allows
the user to configure the current channel gain.
Imax=6 Arms
Figure 2. Phase A Current Input Structure on Evaluation Board
1:2000
IAP
IAN
CT
Figure 3. Example of a Current Transformer Connection
P1
P1
JP4A
100
JP3A
R10
100
R9
JP4A
100
JP3A
R10
100
R9
JP5A
R18
JP6A
EVAL-ADE7878EB
R17
1K
1K
JP5A
R18
JP6A
R17
1K
1K
TP1
TP2
TP1
TP2
IAN
IAP
IAN
IAP
ADE78xx
ADE78xx