ADE7768

Rev. A | Page 13 of 20

FREQUENCY (Hz)

0.30

0.25

0.20

0.15

0.10

0.05

0

40

45

50

55

60

65

70

Figure 25. Phase Error Between Channels (40 Hz to 70 Hz)

Digital-to-Frequency Conversion

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

after multiplication contains the positive-only real power

information. However, because this LPF is not an ideal brick

wall filter implementation, the output signal also contains

attenuated components at the line frequency and its harmonics,

that is, cos(hωt) where h = 1, 2, 3, ... and so on.

The magnitude response of the filter is given by

()

2

2

45

.

4

1

1

f

f

H

+

=

(7)

For a line frequency of 50 Hz, this gives an attenuation of

the 2ω (100 Hz) component of approximately 22 dB. The

dominating harmonic is twice the line frequency (2ω) due to

the instantaneous power calculation.

Figure 26 shows the instantaneous positive-only real power

signal at the output of the LPF that still contains a significant

amount of instantaneous power information, that is, cos(2ωt).

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

where it is compared to 0 and only positive real power is inte-

grated (accumulated) over time to produce an output frequency.

The accumulation of the signal suppresses or averages out any

non-dc components in the instantaneous positive-only real

power signal. The average value of a sinusoidal signal is 0. Thus,

the frequency generated by the ADE7768 is proportional to the

average positive-only real power. Figure 26 shows the digital-to-

frequency conversion for steady load conditions, that is, the

constant voltage and current.

F1

F2

DIGITAL-TO-

FREQUENCY

CF

DIGITAL-TO-

FREQUENCY

MULTIPLIER

F1

TIME

CF

TIME

V

I

0

FREQUENCY (RAD/s)

ω

2

ω

COS (2

ω)

ATTENUATED BY LPF

V

× I

2

LPF TO EXTRACT

REAL POWER

(DC TERM)

INSTANTANEOUS REAL POWER SIGNAL

(FREQUENCY DOMAIN)

LPF

≥0

Figure 26. Positive-Only, Real Power-to-Frequency Conversion

In Figure 26, the frequency output CF varies over time, even

under steady load conditions. This frequency variation is

primarily due to the cos(2ωt) component in the instantaneous

positive-only 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 frequency is generated by accumulating

the instantaneous positive-only real power signal over a much

shorter time while converting it to a frequency. This shorter

accumulation period means less averaging of the cos(2ωt)

component. Consequently, some of this instantaneous power

signal passes through the digital-to-frequency conversion. This

is not a problem in the application. Where CF is used for

calibration purposes, the frequency should be averaged by the

frequency counter, which removes any ripple. If CF is used to

measure energy, such as in a microprocessor-based application,

the CF output should also be averaged to calculate power.

Because the F1 and F2 outputs operate at a much lower

frequency, much more averaging of the instantaneous positive-

only real power signal is carried out. The result is a greatly

attenuated sinusoidal content and a virtually ripple-free

frequency output.

Connecting to a Microcontroller for Energy

Measurement

The easiest way to interface the ADE7768 to a microcontroller

is to use the CF high frequency output with the output

frequency scaling set to 2048 × F1, F2. This is done by setting

SCF = 0 and S0 = S1 = 1 (see Table 7). With full-scale ac

signals on the analog inputs, the output frequency on CF is

approximately 2.867 kHz. Figure 27 illustrates one scheme that

could be used to digitize the output frequency and carry out the

necessary averaging mentioned in the previous section.