–13–

PRELIMINARY TECHNICAL DATA

REV. PrC.

ADE7757

Table III. Maximum Output Frequency on CF

SCF

S1

S0

CF Max for AC Signals (Hz)

1

0

0

128 x F1, F2

= 22.4

0

0

0

64 x F1, F2 = 11.2

1

0

1

64 x F1, F2 = 22.4

0

0

1

32 x F1, F2 = 11.2

1

1

0

32 x F1, F2 = 22.4

0

1

0

16 x F1, F2 = 11.2

1

1

1

16 x F1, F2 = 22.4

0

1

1

2048 x F1, F2 = 2.867 kHz

SELECTING A FREQUENCY FOR AN ENERGY

METER APPLICATION

As shown in Table I, the user can select one of four fre-

quencies. This frequency selection determines the maxi-

mum frequency on F1 and F2. These outputs are intended

for driving an energy register (electromechanical or oth-

ers). Since only four different output frequencies can be

selected, the available frequency selection has been opti-

mized for a meter constant of 100 imp/kWhr with a maxi-

mum current of between 10 A and 120 A. Table IV shows

with a line voltage of 220 V. In all cases the meter con-

stant is 100 imp/kWhr.

Table IV. F1 and F2 Frequency at 100 imp/kWhr

F1 and F2 (Hz)

12.5 A

0.076

25.0 A

0.153

40.0 A

0.244

60.0 A

0.367

80.0 A

0.489

120.0 A

0.733

output frequencies (F1, F2). When designing an energy meter

the nominal design voltage on Channel V2 (voltage) should be

set to half-scale to allow for calibration of the meter constant.

The current channel should also be no more than half-scale

when the meter sees maximum load. This will allow over cur-

rent signals and signals with high crest factors to be accommo-

dated. Table V shows the output frequency on F1 and F2 when

both analog inputs are half-scale. The frequencies listed in

Table V align very well with those listed in Table IV for maxi-

mum load.

Table V. F1 and F2 Frequency with Half-Scale AC Inputs

Frequency on F1 and F2–

S1

S0

CH1 and CH2 Half-Scale AC Inputs

0

0

0.85

0.0438 Hz

0

1

1.7

0.0875 Hz

1

0

3.4

0.175 Hz

1

1

6.8

0.35 Hz

meter constant of 100 imp/kWhr should be compared with

Column 4 of Table V. The closest frequency in Table V

example, if a meter with a maximum current of 25 A is

being designed, the output frequency on F1 and F2 with

a meter constant of 100 imp/kWhr is 0.153 Hz at 25 A and

220 V (from Table IV). Looking at Table V, the closest

frequency to 0.153 Hz in column four is 0.175 Hz. There-

Frequency Outputs

Figure 1 shows a timing diagram for the various frequency

outputs. The outputs F1 and F2 are the low frequency outputs

that can be used to directly drive a stepper motor or elec-

tromechanical impulse counter. The F1 and F2 outputs

provide two alternating low frequency pulses. The

1100 ms (0.909 Hz) the pulsewidth of F1 and F2 is set to

half of their period. The maximum output frequencies for

F1 and F2 are shown in Table II.

The high frequency CF output is intended to be used for

communications and calibration purposes. CF produces a

tional to active power. The CF output frequencies are

given in Table III. As in the case of F1 and F2, if the

set to half the period. For example, if the CF frequency is

20 Hz, the CF pulsewidth is 25 ms.

NOTE: When the high frequency mode is selected, (i.e.,

SCF = 0, S1 = S0 = 1) the CF pulsewidth is fixed at

output frequency on CF.

NO LOAD THRESHOLD

The ADE7757 also includes a “no load threshold” and “start-

up current” feature that will eliminate any creep effects in

the meter. The ADE7757 is designed to issue a minimum

output frequency. Any load generating a frequency lower than

this minimum frequency will not cause a pulse to be issued on

F1, F2 or CF. The minimum output frequency is given as

frequency selections—see Table I. For example, an energy

meter with a meter constant of 100 imp/kWhr on F1, F2

or F2 would be 0.0014% of 3.4 Hz or 4.76 x 10

This would be 3.05 x 10

SCF = S0 = 1, S1 = 0. In this example the no load

threshold would be equivalent to 1.7 W of load or a start-

up current of 8 mA at 220 V. Comparing this value to

the IEC1036 specification which states that the meter

must start up with a load equal to or less than 0.4% Ib.

For a 5A (Ib) meter 0.4% of Ib is equivalent to 20 mA.