ADE7761ARS-REF datasheet(21/28 Pages) AD

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ADE7761

Rev. A | Page 21 of 28

V1B

V1N

V1A

*RB + VR = RF

VR*

RB*

RA*

V2P

V2N

LOAD

240V RMS

POWER

GENERATOR

Figure 29. Missing Neutral System Diagram

The ADE7761 detects a missing neutral condition by continu-

ously monitoring the voltage channel input (V2P–V2N). The

FAULT pin is held high when a missing neutral condition is

detected. In this mode, the ADE7761 continues to bill the

energy based on the signal level on the current channel (see

Figure 30). The billing rate or frequency outputs can be adjusted

by changing the dc level on the MISCAL pin.

V1A

V1N

V1B

MISSING NEUTRAL

GAIN ADJUSMENT

DIGITAL-

TO-

FREQUENCY

CONVERTERS

CF F1

ZERO

CROSSING

DETECTION

A > B

B > A

B <> A

ADC

MISCAL

LPF

HPF

Figure 30. Energy Calculation in Missing Neutral Mode

Important Note for Billing of Active Energy

The ADE7761 provides pulse outputs—CF, F1, and F2—

intended to be used for the billing of active energy. Pulses are

generated at these outputs in two different situations.

Case 1:

When the analog input V2P – V2N complies with the

conditions described in Figure 32, CF, F1, and F2 frequencies

are proportional to active power and can be used to bill active

energy.

Case 2:

When the analog input V2P – V2N does not comply with

the conditions described in Figure 32, the ADE7761 does not

measure active energy, but a quantity proportional to kAh. This

quantity is used to generate pulses on the same CF, F1, and F2.

This situation is indicated when the FAULT pin is high.

Analog Devices Inc. cautions users of the ADE7761:

•

Billing active energy in Case 1 is consistent with the

understanding of the quantity represented by pulses on

CF, F1, and F2 outputs (watt-hour).

•

Billing active energy while the ADE7761 is in Case 2 must

be decided knowing that the entity measured by the

ADE7761 in this case is ampere-hour and not watt-hour.

Users should be aware of this limitation and decide if the

ADE7761 is appropriate for their application.

Missing Neutral Detection

The ADE7761 continuously monitors the voltage input and

detects a missing neutral condition when the voltage input peak

value is smaller than 9% of the analog full scale or when no zero

crossings are detected on this input (see Figure 31).

9% OF FS

FILTER AND

THRESHOLD

V2P

V2N

AGND

ADC

MISSING

NEUTRAL

|V2|PEAK < 9% OF FULL SCALE

V2P–V2N

NO ZERO-CROSSING ON V2

Figure 31. Missing Neutral Detection

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