PRELIMINARY TECHNICAL DATA

EVAL-ADE7756EB

–3–

REV. PrB 01/01

PRELIMINARYTECHNICALDATA

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.

JP1

JP3

V1P

V1N

TP1

TP2

ADE7756

JP15

JP2

JP25

JP4

33nF

33nF

1k

Ω

1k

Ω

100

Ω

100

Ω

Full Scale

differential input = 0.5V

Gain = 16

16mV

rms

200

µΩ

Twisted pair

connection

80A

33nF

33nF

BVM-D-R0002-5.0

Figure 3 — Shunt connection to Current Channel

The shunt used in this example is a 200

µΩ 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

JP9

JP3

R57

R54

C54

C53

V2N

V2P

TP5

TP4

ADE7756

JP8

R53

33nF

33nF

1k

Ω

JP10

255k

Ω

255k

Ω

Attenuation

Network

R56

1k

Ω

JP7

200 - 300 mV

rms

100 - 250 V rms

JP51

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 (1k

Ω) should not be altered as the phase

response of Channel 2 should match that of Channel 1—

see AN-559 (Attenuation Network).