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AD7755

Energy Metering IC with Pulse Output

Analog Devices 

AD7755

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 28. 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 29. 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(hωt) where

h = 1, 2, 3, . . . etc.

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

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

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

Figure 30 shows the instantaneous real power signal at the output

of the CPF which still contains a significant amount of instanta-

neous power information, i.e., cos (2ωt). This signal is then

passed to the digital-to-frequency converter where it is integrated

(accumulated) over time in order to produce an output fre-

quency. 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 AD7755 is proportional to the

average real power. Figure 30 shows the digital-to-frequency

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

current.

F1

V

MULTIPLIER

I

V؋I

2

LPF

LPF TO EXTRACT

REAL POWER

(DC TERM)

DIGITAL-TO-

FREQUENCY

⌺

F1

F2

DIGITAL-TO-

FREQUENCY

⌺

CF

TIME

FOUT

TIME

cos(2␻t)

ATTENUATED BY LPF

0

␻

2␻

FREQUENCY – RAD/S

INSTANTANEOUS REAL POWER SIGNAL

(FREQUENCY DOMAIN)

Figure 30. 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 fre-

quency 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. Where CF is used for calibration

purposes, the frequency should be averaged by the frequency

counter. This will remove any ripple. If CF is being used to

measure 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, a lot

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. B

–13–

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