EVAL-ADE7756EB

–2–

REV. PrB 01/01

PRELIMINARYTECHNICALDATA

ANALOG INPUTS (SK1 AND SK2)

ANALOG INPUTS (SK1 AND SK2)

ANALOG INPUTS (SK1 AND SK2)

ANALOG INPUTS (SK1 AND SK2)

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 (1k

Ω & 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

JP1

JP3

R50

R51

C50

C51

V1P

V1N

TP1

TP2

ADE7756

SH1A

SH1B

JP15

JP2

JP25

JP4

R41

R42

C11

C21

33nF

33nF

33nF

33nF

1k

Ω

1k

Ω

100

Ω

100

Ω

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.

JP1

JP3

V1P

V1N

TP1

TP2

ADE7756

SH1A

SH1B

JP15

JP2

JP25

JP4

33nF

33nF

1k

Ω

1k

Ω

100

Ω

100

Ω

4

Ω

Full Scale

differential input = 1V

Gain = 2

4

Ω

CT

I max = 80A

355mV

rms

1:1800

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.

PRELIMINARYTECHNICALDATA