REV. 0

ADE7751

–15–

Table IV.

CF Max for AC Signals

SCF

S1

S0

(Hz)

(Hz)

10

0

1.7

128

× F1, F2 = 43.52

00

0

1.7

64

× F1, F2 = 21.76

10

1

3.4

64

× F1, F2 = 43.52

00

1

3.4

32

× F1, F2 = 21.76

11

0

6.8

32

× F1, F2 = 43.52

01

0

6.8

16

× F1, F2 = 21.76

11

1

13.6

16

× F1, F2 = 43.52

01

1

13.6

8

× F1, F2 = 21.76

SELECTING A FREQUENCY FOR AN ENERGY METER

APPLICATION

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

This frequency selection determines the maximum frequency

on F1 and F2. These outputs are intended to be used to drive

the energy register (electromechanical or other). Since only four

different output frequencies can be selected, the available

frequency selection has been optimized for a meter constant of

100 imp/kWhr with a maximum current of between 10 A and

120 A. Table V shows the output frequency for several maximum

meter constant is 100 imp/kWhr.

Table V.

F1 and F2 (Hz)

12.5 A

0.076

25 A

0.153

40 A

0.244

60 A

0.367

80 A

0.489

120 A

0.733

output frequencies on F1 and F2. When designing an energy

meter, the nominal design voltage on Channel 2 (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 VI

shows the output frequency on F1 and F2 when both analog

inputs are half scale. The frequencies listed in Table VI align very

well with those listed in Table V for maximum load.

Table VI.

Frequency on F1 and F2 –

CH1 and CH2

S1

S0

Half-Scale AC Inputs

00

1.7

0.085 Hz

01

3.4

0.17 Hz

10

6.8

0.34 Hz

11

13.6

0.68 Hz

Example 1

If full-scale differential dc voltages of +660 mV and –660 mV are

applied to V1 and V2 respectively (660 mV is the maximum

differential voltage that can be connected to Channel 1 and

Channel 2), the expected output frequency is calculated as follows.

Gain

=

1, G0 = G1 = 0

=

1.7 Hz, S0 = S1 = 0

V1

=

+660 mV dc = 0.66 V (rms of dc = dc)

V2

=

–660 mV dc = 0.66 V (rms of dc = |dc|)

=

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

Hz

Hz

=

××× ×

=

574

0 66

066 1 17

25

068

2

...

.

.

.

(8)

Example 2

In this example, if ac voltages of

±660 mV peak are applied to

V1 and V2, the expected output frequency is calculated as follows.

Gain

= 1, G0 = G1 = 0

= 1.7 Hz, S0 = S1 = 0

V1

= rms of 660 mV peak ac = 0.66/

√2 V

V2

= rms of 660 mV peak ac = 0.66/

√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

Hz

Hz

=

××× ×

××

=

574

0 66

066

1

17

22

2 5

034

2

...

.

.

.

(9)

As shown in these two example calculations, the maximum

output frequency for ac inputs is always half of that for dc

input signals. Table III shows a complete listing of all maxi-

mum output frequencies.

Table III.

Max Frequency

Max Frequency

S1

S0

for DC Inputs (Hz)

for AC Inputs (Hz)

00

0.68

0.34

01

1.36

0.68

10

2.72

1.36

11

5.44

2.72

Frequency Output CF

The pulse output CF (calibration frequency) is intended for use

during calibration. The output pulse rate on CF can be up to 128

selected the higher the CF scaling. Table IV shows how the two

frequencies are related depending on the states of the logic inputs

S0, S1, and SCF. Because of its relatively high-pulse rate, the

frequency at this logic output is proportional to the instantaneous

real power. As is the case with F1 and F2, the frequency is derived

from the output of the low-pass filter after multiplication. However,

because the output frequency is high, this real power information

is accumulated over a much shorter time. Hence, 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 Figure 2).