EVAL-ADE7756EB Datasheet, PDF (3/15 Pages) Analog Devices – Evaluation Board Documentation AD7756 Energy metering IC

PRELIMINARY TECHNICAL DATA
PRELIMINARY TECHNICAL DATA
EVAL-ADE7756EB
Using a shunt resistor as the current transducer
Figure 3 shows how a shunt resistance can be used to
perform the current to voltage conversion required for the
ADE7756. A shunt is a very cost effective way to perform
the current to voltage conversion in a two-wire, single-
phase application. No isolation is required in a two-wire
application and the shunt has advantages over the CT
arrangement. For example a shunt does not suffer from dc
saturation problems and the phase response of the shunt is
linear over a very wide dynamic range. Although the shunt
is predominately resistive, it does have parasitic reactive
elements (inductance) which can become significant, even
at 50Hz/60Hz. This means that there can be a small phase
shift associated with the shunt. However once it is under-
stood the phase shift is easily compensated with the filter
network R41/C11 and R42/C21—see AN-559 for a
detailed discussion of this issue.
200µΩ
80A
Twisted pair
connection
JP15
JP2 ^100Ω
JP25
100Ω
JP4
BVM-D-R0002-5.0
JP1
1kΩ
33nF _JP3
1kΩ
33nF
ADE7756
TP1
33nF
TP2
33nF
V1P
16mV
rms
V1N
Full Scale
differential input = 0.5V
Gain = 16
Figure 3 — Shunt connection to Current Channel
The shunt used in this example is a 200µΩ manganin type.
The resistance of the shunt should be as low as possible in
order to avoid excessive power dissipation in the shunt.
Although the shunt is fabricated from a special alloy
(manganin) which has a very low temperature coefficient
of resistance, excessive heating due to power dissipation
can cause measurement inaccuracies when operating at
heavy loads over extended periods of time.
The manganin shunt used in this example (BVM-D-
R0002-5.0) is designed specifically for energy metering
applications and is supplied by Isotek Corp.
(http://www.isotekcorp.com).
This shunt is PCB mountable with a current carrying
ability of 70A rms. The technical data supplied by Isotek
Corp. gives detailed information regarding PCB layout.
Figure 3 shows how the shunt can be connected to the
evaluation board. Two sense wired should be soldered to
the shunt at the copper/manganium junctions as shown.
These sense wires should be formed into a twisted pair to
reduce the loop area which will reduce antenna effects. A
connection for the common mode voltage can be made at
the connection point for the current carrying conductor—
see Figure 3.
Voltage sense inputs
The voltage input connections on the ADE7756 evalua-
tion board can be directly connected to the line voltage
source. The line voltage is attenuated using a simple
resistor divider network before it is presented to the
ADE7756. Because of the relatively large signal on this
channel and the small dynamic range requirement, the
voltage channel can be configured in a single-ended
configuration. Figure 4 shows a typical connection for the
line voltage.
SK1 1
SK1 2
JP8
100 - 250 V rms
JP9
R57
JP10 1kΩ
JP7
JP3
R53
R54
255kΩ
255kΩ
R56
Attenuation
1kΩ
Network
ADE7756
TP5
C54
33nF
V2N
JP51 ^TP4
V2P
C53
33nF
200 - 300 mV
rms
Figure 4 — Voltage Channel on the ADE7756 evaluation
board
Note that the analog inputs V2N is connected to AGND
via the anti-alias filter R57/C54 using JP10. Jumper JP9
should be left open.
The voltage attenuation network is made up of R53, R54
and R56. The maximum signal level permissible at V2P is
1V peak. Although the ADE7756 analog inputs can
withstand ±6V without risk of permanent damage, the
signal range should not exceed ±1V with respect to
AGND, for specified operation.
The attenuation network can be easily modified by the
user to accommodate any input signal levels. However the
value of R56 (1kΩ) should not be altered as the phase
response of Channel 2 should match that of Channel 1—
see AN-559 (Attenuation Network).
REV. PrB 01/01
–3–