ADE7769

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 24. 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 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 25 shows the instantaneous 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 integrated (accumulated) over time to produce an

output frequency. The accumulation of the signal suppresses or

averages out any non-dc components in the instantaneous real

power signal. The average value of a sinusoidal signal is zero.

Thus, the frequency generated by the ADE7769 is proportional

to the average real power. Figure 25 shows the digital-to-

frequency conversion for steady load conditions, that is,

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

Figure 25. Real Power-to-Frequency Conversion

In Figure 25, 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

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 instan-

taneous 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 being used to measure energy, for

example 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 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 ADE7769 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 26 shows one scheme that

could be used to digitize the output frequency and carry out

the necessary averaging mentioned in the previous section.