ADE7761AARS-REF datasheet(19/24 Pages) AD

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ADE7761A

Rev. 0 | Page 19 of 24

V1B

V1N

V1A

1RB + VR = RF.

VR1

RB1

RA1

V2P

V2N

LOAD

244V rms

POWER

GENERATOR

Figure 31. Missing Neutral System Diagram

The ADE7761A detects a missing neutral condition by

continuously monitoring the voltage channel input (V2P − V2N).

The FAULT pin is held high when a missing neutral condition is

detected. In this mode, the ADE7761A continues to bill the

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

Figure 32). The billing rate or frequency outputs can be adjusted

by changing the dc level on the MISCAL pin.

V1A

V1N

V1B

MISSING NEUTRAL

GAIN ADJUSMTENT

DIGITAL-TO-

FREQUENCY

CONVERTERS

CF F1 F2

ZERO

CROSSING

DETECTION

A > B

B > A

B <> A

ADC

MISCAL

LPF

HPF

Figure 32. Energy Calculation in Missing Neutral Mode

Important Note for Billing of Active Energy

The ADE7761A 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 34, 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 34, the ADE7761A 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 ADE7761A about the

following:

• 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 ADE7761A is in Case 2 must

be decided knowing that the entity measured by the

ADE7761A in this case is ampere-hour and not watt-hour.

Users should be aware of this limitation and decide if the

ADE7761A is appropriate for their application.

Missing Neutral Detection

The ADE7761A 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 33).

9% OF FS

FILTER AND

THRESHOLD

V2P

V2N

AGND

ADC

MISSING

NEUTRAL

|V2|PEAK < 9% OF FULL SCALE

V2P –V2N

V2P – V2N

NO ZERO CROSSING ON V2

Figure 33. Missing Neutral Detection

The ADE7761A leaves the missing neutral mode for normal

operation when both conditions are no longer valid, that is, a

voltage peak value of greater than 9% of full scale and zero

crossing on the voltage channel is detected (see Figure 34).

FILTER AND

THRESHOLD

V2P

V2N

AGND

ADC

MISSING

NEUTRAL

|V2|PEAK > 9% OF FULL SCALE

AND

ZERO CROSSING ON V2

+9% OF FS

–9% OF FS

V2P – V2N

Figure 34. Return to Normal Mode after Missing Neutral Detection

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