ADE7754_15 datasheet(21/44 Pages) AD

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REV. 0

ADE7754

–21–

ACTIVE POWER

PHASE B

ACTIVE POWER

PHASE A

ACTIVE POWER

PHASE C

DFC

CFNUM[11:0]

CFDEN[11:0]

TOTAL ACTIVE

POWER

Figure 29. ADE7754 Energy to Frequency Conversion

A digital to frequency converter (DFC) is used to generate the

CF pulsed output. The DFC generates a pulse each time one

LSB in the active energy register is accumulated. An output

pulse is generated when CFDEN/CFNUM pulses are generated

at the DFC output. Under steady load conditions, the output

frequency is proportional to the active power. The maximum

output frequency (CFNUM = 00h and CFDEN = 00h) with

full scale ac signals on the three phases (i.e., current channel

and voltage channel is approximately 96 kHz).

The ADE7754 incorporates two registers to set the frequency of

CF (CFNUM[11:0] and CFDEN[11:0]). These are unsigned

12-bit registers that can be used to adjust the frequency of CF

to a wide range of values. These frequency scaling registers are

12-bit registers that can scale the output frequency by 1/2

12 to 1

with a step of 1/2

12.

If the value 0 is written to any of these registers, the value 1

would be applied to the register. The ratio CFNUM/CFDEN

should be smaller than 1 to ensure proper operation. If the ratio

of the registers CFNUM/CFDEN is greater than 1, the CF

frequency can no longer be guaranteed to be a consistent value.

For example, if the output frequency is 18.744 kHz and the

contents of CFDEN are zero (000h), then the output frequency

can be set to 6.103 Hz by writing BFFh to the CFDEN register.

The output frequency will have a slight ripple at a frequency

equal to twice the line frequency because of imperfect filtering

of the instantaneous power signal used to generate the active

power signal. See the Active Power Calculation section. Equa-

tion 5 gives an expression for the instantaneous power signal.

This is filtered by LPF2, which has a magnitude response given

by Equation 11.

|( )|

(11)

The active power signal (output of the LPF2) can be rewritten as

()

cos

=−

+ 































()

(12)

where fl is the line frequency (e.g., 60 Hz).

From Equation 8

VIt

()

sin

=−

+ 































()

(13)

Equation 13 shows that there is a small ripple in the energy

calculation due to a sin(2 t) component. This is graphically

displayed in Figure 30. The ripple becomes larger as a percentage

of the frequency at larger loads and higher output frequencies.

Choosing a lower output frequency at CF for calibration can

significantly reduce the ripple. Also, averaging the output fre-

quency by using a longer gate time for the counter achieves the

same results.

–

(

)

sin

E(t)

VIt

( (

Figure 30. Output Frequency Ripple

No Load Threshold

The ADE7754 includes a selectable “no load threshold” or

“startup current” feature that eliminates any creep effects in the

active energy measurement of the meter. When enabled, this

function is independently applied on each phase’s active power

calculation. This mode is selected by default and can be disabled

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