REV. PrB 08/01

PRELIMINARY TECHNICAL DATA

ADE7752

–8–

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7752 calculates the product of six voltage signals

(on Current channel and Voltage channel) 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 high pulses. The pulse rate at these outputs is relatively

low, e.g., 0.08 Hz maximum for AC signals with SCF = S0

= S1 = 1—see Table III. This means that the frequency at

these outputs is generated from real power information

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.

()

2

5

1

2

.

109

REF

C

CN

B

BN

A

AN

V

F

I

V

I

V

I

V

Freq

−

×

×

+

×

+

×

×

=

where:

Freq

= Output frequency on F1 and F2 (Hz)

age channels (volts)

= Differential rms voltage signal on Current

channels (volts)

=

The reference voltage (2.5 V ± 8%) (volts)

=

One of five possible frequencies selected by

using the logic inputs SCF, S0 and S1—see

Table II.

SCF

S1

S0

XTAL/CLKIN*

1

1

1

0.596

10 MHz/2

24

0

1

1

76.3

10 MHz/2

17

-

1

0

19.07

10 MHz/2

19

-

0

1

4.77

10 MHz/2

21

-

0

0

1.19

10 MHz/2

23

NOTE

CLKIN frequency is altered.

Example 1

Thus if full-scale differential DC voltages of +125 mV and –

125 mV are applied to VA, VB, VC, IA, IB and IC respec-

tively (125 mV is the maximum differential voltage that

can be connected to Current and Voltage channels), the

expected output frequency is calculated as follows:

=

0.596 Hz, SCF = S0 = S1 = 1

= +125 mV dc = 0.125 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%.

Hz

Freq

488

.

0

5

.

2

596

.

0

125

.

0

125

.

0

2

.

109

3

2

=

×

×

×

×

=

Example 2

In this example, with AC voltages of ±500 mV peak ap-

plied to the Voltage channels and Current channels, the

expected output frequency is calculated as follows:

= 0.596 Hz, SCF = S0 = S1 = 1

= 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%.

Hz

Freq

24

.

0

5

.

2

2

2

596

.

0

125

.

0

125

.

0

2

.

109

3

2

=

×

×

×

×

×

×

=

As can be seen from these two example calculations, the

maximum output frequency for AC inputs is always half of

that for DC input signals. The maximum frequency depends

also on the number of phases connected to the ADE7752. In

a 3-phase 3-wire delta service the maximum output frequency

is different from the maximum output frequency in a 3-phase

4-wire Wye service. The reason is that there are only 2 phases

connected to the analog inputs but also that in a delta service,

the Current channel input and Voltage channel input of the

same phase are not in phase in normal operation.

Example 3

In this example, the ADE7752 is connected to a 3-phase 3-

wire delta service as shown in Figure 14. The total real energy

calculation processed in the ADE7752 can be expressed as:

()

(

)

B

C

B

A

C

A

I

V

V

I

V

V

Energy

al

Total

×

−

+

×

−

=

Re

on phase A and B.

As the voltage and current inputs respect Equations 5 and 6,

the Total Real Energy (P) is: