–11–

PRELIMINARY TECHNICAL DATA

REV. PrC.

ADE7757

DIGITAL-TO-FREQUENCY CONVERSION

As previously described, the digital output of the low-pass filter

after multiplication contains the real power information. How-

ever, since this LPF is not an ideal “brick wall” filter imple-

mentation, the output signal also contains attenuated

components at the line frequency and its harmonics, i.e.,

cos(h

ωt) where h = 1, 2, 3, . . . etc.

The magnitude response of the filter is given by:

2

2

9

8

1

1

.

)

(

f

f

H

+

=

(5)

For a line frequency of 50 Hz this would give an attenua-

tion of the 2

ω (100 Hz) component of approximately –

22 dB. The dominating harmonic will be at twice the line

frequency (2

ω) due to the instantaneous power calculation.

Figure 21 shows the instantaneous real power signal at the

output of the LPF which still contains a significant amount

of instantaneous power information, i.e., cos (2

ωt). This

signal is then passed to the digital-to-frequency converter

where it is integrated (accumulated) over time in order to

produce an output frequency. The accumulation of the

signal will suppress or average out any non-dc components

in the instantaneous real power signal. The average value

of a sinusoidal signal is zero. Hence the frequency gener-

ated by the ADE7757 is proportional to the average real

power. Figure 21 shows the digital-to-frequency conver-

sion for steady load conditions, i.e., constant voltage and

current.

ω

FREQUENCY (RAD/S)

LPF

DIGITAL-TO-

FREQUENCY

F1

F2

DIGITAL-TO-

FREQUENCY

CF

INSTANTANEOUS REAL POWER SIGNAL

(FREQUENCY DOMAIN)

MULTIPLIER

TIME

F1

CF

TIME

V

I

0

LPF TO EXTRACT

REAL POWER

(DC TERM)

2

I

V

×

∑

∑

)

t

2

(

cos

ω

LPF

BY

ATTENUATED

ω

2

Figure 21. Real Power-to-Frequency Conversion

As can be seen in the diagram, the frequency output CF is

seen to vary over time, even under steady load conditions.

This frequency variation is primarily due to the cos (2

ωt)

component in the instantaneous real power signal. The

output frequency on CF can be up to 2048 times higher

than the frequency on F1 and F2. This higher output fre-

quency is generated by accumulating the instantaneous

real power signal over a much shorter time while convert-

ing it to a frequency. This shorter accumulation period

means less averaging of the cos (2

ωt) component. Conse-

quently, some of this instantaneous power signal passes

through the digital-to-frequency conversion. This will not

be a problem in the application. Where CF is used for

calibration purposes, the frequency should be averaged by

the frequency counter which will remove any ripple. If CF

is being used to measure energy; for example, in a micro-

processor-based application, the CF output should also be

averaged to calculate power.

Because the outputs F1 and F2 operate at a much lower

frequency, a lot more averaging of the instantaneous real

power signal is carried out. The result is a greatly attenu-

ated sinusoidal content and a virtually ripple-free fre-

quency output.

Interfacing the ADE7757 to a Microcontroller for Energy

Measurement

The easiest way to interface the ADE7757 to a

microcontroller is to use the CF high frequency output

with the output frequency scaling set to 2048 x F1, F2.

This is done by setting SCF = 0 and S0 = S1 = 1, see

Table III. With full-scale ac signals on the analog inputs,

the output frequency on CF will be approximately

2.867 kHz. Figure 22 illustrates one scheme which could

be used to digitize the output frequency and carry out the

necessary averaging mentioned in the previous section.

TIME

AVERAGE

FREQUENCY

CF

FREQUENCY

RIPPLE

MCU

COUNTER

TIMER

CF

ADE7757

Figure 22. Interfacing the ADE7757 to an MCU

As shown, the frequency output CF is connected to an

MCU counter or port. This will count the number of

pulses in a given integration time which is determined by

an MCU internal timer. The average power is propor-

tional to the average frequency is given by:

Time

Counter

Power

Average

Frequency

Average

=

=

The energy consumed during an integration period is

given by:

Counter

Time

Time

Counter

Time

Power

Average

Energy

=

×

=

×

=