ADE7755

Rev. A | Page 16 of 20

F1

F2

DIGITAL-TO-

FREQUENCY

CF

DIGITAL-TO-

FREQUENCY

MULTIPLIER

F1

TIME

TIME

V

I

0

FREQUENCY (RAD/s)

ω

2ω

cos(2ωt)

ATTENUATED BY LPF

V×I

2

LPF TO EXTRACT

REAL POWER

(DC TERM)

INSTANTANEOUS ACTIVE POWER SIGNAL

(FREQUENCY DOMAIN)

LPF

Figure 32. Active Power-to-Frequency Conversion

As can be seen in Figure 32, the frequency output CF varies over

time, even under steady load conditions. This frequency variation

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

active 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

active power signal over a much shorter time while converting

it to a frequency. This shorter accumulation period means less

averaging of the cos(2 ωt) component. Consequently, some of

this instantaneous power signal passes through the digital-to-

frequency conversion, which is not a problem in the

application. When CF is used for calibration purposes, the

frequency should be averaged by the frequency counter. This

averaging operation removes any ripple. If CF is measuring

energy, for example, 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 active power signal is

carried out. The result is a greatly attenuated sinusoidal content

and a virtually ripple-free frequency output.

INTERFACING THE ADE7755 TO A

MICROCONTROLLER FOR ENERGY MEASUREMENT

The easiest way to interface the ADE7755 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 8). With full-scale ac signals on the analog

inputs, the output frequency on CF is approximately 5.5 kHz.

Figure 33 illustrates one scheme that can be used to digitize the

output frequency and carry out the necessary averaging described

in the Digital-to-Frequency Conversion section.

CF

TIME

±10%

FREQUENCY

RIPPLE

AVERAGE

FREQUENCY

ADE7755

DIRECTION OF ENERGY FLOW IS NEEDED

COUNTER

UP/DOWN

TIMER

MCU

CF

Figure 33. Interfacing the ADE7755 to an MCU

As shown in Figure 33, the frequency output CF is connected to

an MCU counter or port, which counts the number of pulses in

a given integration time that is determined by an MCU internal

timer. The average power proportional to the average frequency

is given by

Timer

Counter

Power

Active

Average

Frequency

Average

=

=

The energy consumed during an integration period is given by

Counter

Time

Time

Counter

Time

Power

Average

Energy

=

×

=

×

=

For the purpose of calibration, this integration time can be

10 seconds to 20 seconds to accumulate enough pulses to ensure

correct averaging of the frequency. In normal operation, the

integration time can be reduced to 1 second or 2 seconds

depending, for example, on the required update rate of a display.

With shorter integration times on the MCU, the amount of

energy in each update may still have some small amount of

ripple, even under steady load conditions. However, over a

minute or more, the measured energy has no ripple.

POWER MEASUREMENT CONSIDERATIONS

Calculating and displaying power information always has some

associated ripple that depends on the integration period used in

the MCU to determine average power and also the load. For

example, at light loads, the output frequency can be 10 Hz. With

an integration period of 2 seconds, only about 20 pulses are

counted. The possibility of missing one pulse always exists because

the ADE7755 output frequency is running asynchronously to the

MCU timer. This possibility results in a 1-in-20 (or 5%) error in

the power measurement.