ADE7761B datasheet(19/24 Pages) AD

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ADE7761B

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 ADE7761B 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 ADE7761B 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

A ≠ B

ADC

MISCAL

LPF

HPF

Figure 32. Energy Calculation in Missing Neutral Mode

Important Note for Billing of Active Energy

The ADE7761B provides pulse outputs, CF, F1, and F2, that are

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, the CF, F1, and F2 frequencies are propor-

tional 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 ADE7761B 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 cautions users of the ADE7761B about the

following:

• Billing active energy in Case 1 is consistent with the under-

standing of the quantity represented by pulses on the CF, F1,

and F2 outputs (watthour).

• Billing active energy while the ADE7761B is in Case 2 must

be decided knowing that the entity measured by the ADE7761B

in this case is ampere-hour and not watthour. Users should

be aware of this limitation and decide if the ADE7761B is

appropriate for their application.

Missing Neutral Detection

The ADE7761B 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

NO ZERO CROSSING ON V2

Figure 33. Missing Neutral Detection

The ADE7761B 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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