REV. A

AD7751

–14–

Fault with Active Input Greater than Inactive Input

If V1A is the active current input (i.e., is being used for billing),

and the signal on V1B (inactive input) falls by more than 12.5%

of V1A, the fault indicator will go active. Both analog inputs are

filtered and averaged to prevent false triggering of this logic

output. As a consequence of the filtering, there is a time delay of

approximately one second on the logic output FAULT after the

fault event. The FAULT logic output is independent of any activ-

ity on outputs F1 or F2. Figure 13 illustrates one condition under

which FAULT becomes active. Since V1A is the active input and it

is still greater than V1B, billing is maintained on VIA, i.e., no swap

to the V1B input will occur. V1A remains the active input.

V1B < 87.5% OF V1A

0V

V1A

V1B

V1A

V1A

V1N

AGND

V1B

V1B

FILTER

AND

COMPARE

TO

MULTIPLIER

FAULT

A

B

Figure 13. Fault Conditions for Inactive Input Less than

Active Input

Fault with V1B Greater than V1A

Figure 14 illustrates another fault condition. If V1A is the active

input (i.e., is being used for billing), and the voltage signal on

V1B (inactive input) becomes greater than 114% of V1A, the

FAULT indicator goes active and there is also a swap over to

the V1B input. The analog input V1B has now become the

active input. Again there is a time delay of about 1.2 second

associated with this swap. V1A will not swap back to being the

active channel until V1A becomes greater than 114% of V1B.

However, the FAULT indicator will become inactive as soon as

V1A is within 12.5% of V1B. This threshold eliminates poten-

tial chatter between V1A and V1B.

V1A < 87.5% OF V1B

OR

V1B > 114% OF V1A

0V

V1A

V1B

V1A

V1A

V1N

AGND

V1B

V1B

FILTER

AND

COMPARE

TO

MULTIPLIER

FAULT

A

B

Figure 14. Fault Conditions for Inactive Input Greater than

Active Input

Calibration Concerns

Typically, when a meter is being calibrated, the voltage and current

circuits are separated as shown in Figure 15. This means that

current will only pass through the phase or neutral circuit. Figure

15 shows current being passed through the phase circuit. This is

the preferred option since the AD7751 starts billing on the input

V1A on power-up. The phase circuit CT is connected to V1A in

the diagram. Since there is no current in the neutral circuit the

FAULT indicator will come on under these conditions. However,

this does not affect the accuracy of the calibration and can be

used as a means to test the functionality of the fault detection.

Ib

V

240Vrms

NOTE:

Ra

Rb

Rb

V1A

0V

V1B

CT

V1A

AGND

CT

NEUTRAL

PHASE

V1N

Rb

VR

V2P

V2N

TEST

CURRENT

Ib

Ra

Figure 15. Fault Conditions for Inactive Input Greater than

Active Input

If the neutral circuit is chosen for the current circuit in the arrange-

ment shown in Figure 15, it may have implications for the

calibration accuracy. The AD7751 will power up with the V1A

input active as normal. However, since there is no current in the

phase circuit, the signal on V1A is zero. This will cause a FAULT

to be flagged and the active input to be swapped to V1B (Neutral).

The meter may be calibrated in this mode but the phase and

neutral CTs may differ slightly. Since under no-fault condi-

tions all billing is carried out using the phase CT, the meter

should be calibrated using the phase circuit. Of course, both

phase and neutral circuits may be calibrated.

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The AD7751 calculates the product of two voltage signals (on

Channel 1 and Channel 2) and then low-pass filters this product

to extract real-power information. This real-power information

is then converted to a frequency. The frequency information is

output on F1 and F2 in the form of active low pulses. The pulse

rate at these outputs is relatively low, e.g., 0.34 Hz maximum for

ac signals with S0 = S1 = 0 (see Table III). This means that the

frequency at these outputs is generated from real-power informa-

tion accumulated over a relatively long period of time. The result is

an output frequency that is proportional to the average real

power. The averaging of the real-power signal is implicit to the

digital-to-frequency conversion. The output frequency or pulse

rate is related to the input voltage signals by the following equation.

Freq

V

V

Gain

F

=

××

×

×

574

1

2

14

2

.

–

(7)