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ADE7755ARSRL

Energy Metering IC with Pulse Output

Analog Devices 

PRELIMINARY TECHNICAL DATA

ADE7755

0.30

0.25

0.20

0.15

0.10

0.05

0

–0.05

–0.10

0

100 200 300 400 500 600 700 800 900 1000

FREQUENCY – Hz

Figure 10. Phase Error between Channels (0 Hz to 1 kHz)

0.30

0.25

0.20

0.15

0.10

0.05

0

–0.05

–0.10

40

45

50

55

60

65

70

FREQUENCY – Hz

Figure 11. 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. How-

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

tation, the output signal also contains attenuated components

at the line frequency and its harmonics, i.e., cos(h ωt) where

h = 1, 2, 3, and so on.

The magnitude response of the filter is given by:

|H( f )|=

1

1 + ( f /8.9 Hz)

(5)

For a line frequency of 50 Hz this would give an attenuation of

the 2w (100 Hz) component of approximately –22 dBs. The

dominating harmonic will be at twice the line frequency, i.e.,

cos (2 ωt), and this is due to the instantaneous power signal.

Figure 12 shows the instantaneous real power signal at the

output of the CPF, 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 to produce an output frequency.

This 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

generated by the ADE7755 is proportional to the average real

power. Figure 12 shows the digital-to-frequency conversion for

steady load conditions, i.e., constant voltage and current.

F1

V

LPF

MULTIPLIER

I

DIGITAL-TO-

FREQUENCY

⌺

F1

F2

DIGITAL-TO-

FREQUENCY

TIME

FOUT

⌺

CF

V؋I

2

LPF TO EXTRACT

REAL POWER

(DC TERM)

TIME

cos(2␻t)

ATTENUATED BY LPF

0

␻

2␻

FREQUENCY – RAD/S

INSTANTANEOUS REAL POWER SIGNAL

(FREQUENCY DOMAIN)

Figure 12. 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 frequency is generated by accumu-

lating the instantaneous real power signal over a much shorter

time while converting it to a frequency. This shorter accumula-

tion period means less averaging of the cos (2 ωt) component.

As a consequence, some of this instantaneous power signal passes

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

problem in the application. When CF is used for calibration

purposes, the frequency should be averaged by the frequency

counter. This will remove any ripple. If CF is measuring energy,

e.g., in a microprocessor-based application, the CF output

should also be averaged to calculate power. Because the outputs

F1 and F2 operate at a much lower frequency, 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.

REV. PrA

–13–

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