REV. A

ADE7757

–13–

S1

S0

OSC Relation

1

OSC (Hz)

2

00

0.86

01

1.72

10

3.44

11

6.86

NOTES

Example

In this example, with ac voltages of

±30 mV peak applied to V1

and

±165 mV peak applied to V2, the expected output frequency

is calculated as follows:

= OSC/2

√2 V

√2 V

= 2.5 V (nominal reference value)

NOTE: If the on-chip reference is used, actual output frequencies may vary

from device to device due to reference tolerance of

± 8%.

Freq

F

F

=

××

×

××

=×

=

515 85

0 03

0 165

22

2 5

0 204

0 175

1

2

1

...

.

..

Table II. Maximum Output Frequency on F1 and F2

Max Frequency

*

S1

S0

OSC Relation

for AC Inputs (Hz)

0

0

0.204

× F

1

0.175

0

1

0.204

0.35

1

0

0.204

× F

3

0.70

1

1

0.204

× F

4

1.40

*Values are generated using the nominal frequency of 450 kHz

Frequency Output CF

The pulse output CF (calibration frequency) is intended for

calibration purposes. The output pulse rate on CF can be up to

frequency selected, the higher the CF scaling (except for the

high frequency mode SCF = 0, S1 = S0 = 1). Table III shows

how the two frequencies are related, depending on the states of

the logic inputs S0, S1, and SCF. Due to its relatively high

pulse rate, the frequency at CF logic output is proportional to

the instantaneous real power. As with F1 and F2, CF is derived

from the output of the low-pass filter after multiplication. How-

ever, because the output frequency is high, this real power

information is accumulated over a much shorter time. There-

fore, less averaging is carried out in the digital-to-frequency

conversion. With much less averaging of the real power signal,

the CF output is much more responsive to power fluctuations

(see the Signal Processing Block in Figure 3).

Table III. Maximum Output Frequency on CF

SCF

S1

S0

CF Max for AC Signals (Hz)

*

10

0

128

× F1, F2 = 22.4

00

0

64

× F1, F2 = 11.2

10

1

64

× F1, F2 = 22.4

00

1

32

× F1, F2 = 11.2

11

0

32

× F1, F2 = 22.4

01

0

16

× F1, F2 = 11.2

11

1

16

× F1, F2 = 22.4

01

1

2048

× F1, F2 = 2.867 kHz

*Values are generated using the nominal frequency of 450 kHz.

SELECTING A FREQUENCY FOR AN ENERGY METER

APPLICATION

As shown in Table I, the user can select one of four frequencies.

This frequency selection determines the maximum frequency on

F1 and F2. These outputs are intended for driving an energy

register (electromechanical or others). Since only four different

output frequencies can be selected, the available frequency

selection has been optimized for a meter constant of 100 imp/kWh

with a maximum current of between 10 A and 120 A. Table IV

with a line voltage of 220 V. In all cases, the meter constant is

100 imp/kWh.

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

F1 and F2 (Hz)

12.5

0.076

25.0

0.153

40.0

0.244

60.0

0.367

80.0

0.489

120.0

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 overcurrent

signals and signals with high crest factors to be accommodated.

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 maximum load.