EVAL-ADE7756EB Datasheet, PDF (2/15 Pages) Analog Devices – Evaluation Board Documentation AD7756 Energy metering IC

PRELIMINARY TECHNICAL DATA
EVAL-ADE7756EB
ANALOG INPUTS (SK1 AND SK2)
Voltage and current signals are connected at the screw termi-
nals SK1 and SK2 respectively. All analog input signals are
filtered using the on-board anti-alias filters before being
presented to the analog inputs of the ADE7756. The
default component values which are shipped with the
evaluation board are the recommended values to be used
with the ADE7756. The user can easily change these
components, however this is not recommended unless the
user is familiar with sigma-delta converters and also the
criteria used for selecting the component values for the
analog input filters—see AN-559 for a more comprehen-
sive description of the anti-alias filters and their function.
Current sense inputs (SK2)
SK2 is a three-way connection block which allows the
ADE7756 to be connected to a current transducer. Figure
1 shows the connector SK2 and the filtering network
which is provided on the evaluation board.
The resistors SH1A and SH1B are by default not popu-
lated. They are intended to be used as burden resistors
when a CT is used as the current transducer—see using a
CT as a the current transducer.
The RC networks R41/C11 and R42/C21 are used to
provide phase compensation when a shunt is being used as
the current transducer—see using a shunt as the current
transducer. These RC networks are easily disabled by
placing JP15 & JP25 and removing C11 & C21 (sock-
eted).
The RC networks R50/C50 & R51/C51 are the anti-alias
filters which are required by the on-chip ADCs. The
default corner frequency for these LPFs (Low Pass
Filters) is selected as 4.8kHz (1kΩ & 33nF). These filters
can easily be adjusted by replacing the components on the
evaluation board. However before adjusting the compo-
nent values of R50, R51, C50 or C51 the user should first
review application note AN-559.
SK2 1
SK2 2
SK2 3
SH1A
JP15
R41
JP2 ^100Ω
JP25
R42
SH1B
100Ω
JP4
JP1
R50
C11 ^1kΩ
33nF _JP3
R51
C21 ^1kΩ
33nF
ADE7756
TP1
C50
33nF
TP2
C51
33nF
V1P
V1N
Figure 1 — Current Channel on the ADE7756 evaluation
board
Using a CT as the current transducer
Figure 2 shows how a CT can be used as a current
transducer in a signal phase 3-wire distribution system.
This is how electrical energy is distributed to residential
users in the United States. Phase A and Phase B are
nominally 180° out of phase. The vector addition of the
two currents is easily achieved by using two primary turns
of opposite polarity on the CT.
I max = 80A
CT
1:1800
SH1A
4Ω
JP15
JP2 ^100Ω
JP25
SH1B
4Ω
100Ω
JP4
ADE7756
JP1
1kΩ
JP3
TP1
33nF
TP2
V1P
355mV
rms
V1N
1kΩ
33nF
Full Scale
differential input = 1V
Gain = 2
Figure 2 — CT connection to Current Channel
The CT secondary current is converted to a voltage by
using a burden resistance across the secondary winding
outputs. Care should be taken when using a CT as the
current transducer. If the secondary is left open, i.e., no
burden is connected, a large voltage could be present at
the secondary outputs. This can cause an electrical shock
hazard and potentially damage electronic components.
Warning!
Using a CT without a burden resistor
can lead to electrical shock.
When using a CT as the current sensor, the phase com-
pensation network for a shunt application should be
disabled. This is achieved by closing jumpers JP15/JP25
and removing C11/C21.
The anti-alias filters should be enabled by opening
jumpers JP1/JP3—see Figure 2.
Most CTs will have an associated phase shift of between
0.1° and 1° at 50Hz/60Hz. This phase shift or phase error
can lead to significant energy measurement errors, espe-
cially at low power factors—see AN-559 for more infor-
mation. However this phase error can be corrected by
writing to the Phase Calibration register (PHCAL) in the
ADE7756. The software supplied with the ADE7756
evaluation board allows user adjustment of the Phase
Calibration register. See the Evaluation Software Description
for more information.
For this example, notice that the maximum analog input
range on Channel 1 is set to 1V. And the Gain for Chan-
nel 1 has be set to 2. The maximum analog input range
and gain are set via the Gain register (GAIN)—see the
ADE7756 data sheet. The evaluation software allows the
user to configure the channel range and gain. This means
that the maximum peak differential signal on Channel 1 is
0.5V.
–2–
REV. PrB 01/01