ADE7768

Rev. A | Page 14 of 20

CF

TIME

±10%

FREQUENCY

RIPPLE

AVERAGE

FREQUENCY

ADE7768

COUNTER

TIMER

MCU

CF

Figure 27. Interfacing the ADE7768 to an MCU

As shown in Figure 27, the frequency output CF is connected

to an MCU counter or port. This counts the number of pulses

in a given integration time, which is determined by an MCU

internal timer. The average power proportional to the average

frequency is given by

Time

Counter

Power

Average

Frequency

Average

=

=

(8)

The energy consumed during an integration period is given by

)

9

(

Counter

Time

Time

Counter

Time

Power

Average

Energy

=

×

=

×

=

For the purpose of calibration, this integration time could be

10 seconds to 20 seconds, to accumulate enough pulses to

ensure correct averaging of the frequency. In normal operation,

the integration time could 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, which depends on the integration period used

in the MCU to determine average power and also on the load.

For example, at light loads, the output frequency may 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 ADE7768 output frequency is running asynchro-

nously to the MCU timer. This results in a 1-in-20 or 5% error

in the power measurement. When REVP is logic high, the

ADE7768 does not generate any pulse on F1, F2, and CF.

INTERNAL OSCILLATOR (OSC)

The nominal internal oscillator frequency is 450 kHz when

used with RCLKIN, with a nominal value of 6.2 kΩ. The

frequency outputs are directly proportional to the oscillator

frequency, thus RCLKIN must have low tolerance and low

temperature drift to ensure stability and linearity of the chip.

The oscillator frequency is inversely proportional to the

RCLKIN, as shown in Figure 28. Although the internal

oscillator operates when used with RCLKIN values between

5.5 kΩ and 20 kΩ, choosing a value within the range of the

nominal value, as shown in Figure 28, is recommended.

RESISTANCE (k

Ω)

5.8

5.9

6.1

6.3

6.7

420

430

440

450

460

480

470

490

6.0

6.2

6.4

6.5

6.6

410

400

Figure 28. Effect of RCLKIN on Internal Oscillator Frequency (OSC)

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7768 calculates the product of two voltage signals

(on Channel V1 and Channel V2) and then low-pass filters this

product to extract positive-only real power information. This

positive-only real power information is then converted to a

frequency. The frequency information is output on F1 and F2

in the form of active low pulses. The pulse rate at these outputs

is relatively low—for example, 0.175 Hz maximum for ac signals

with S0 = S1 = 0 (see Table 6). This means that the frequency at

these outputs is generated from positive-only real power

information accumulated over a relatively long period of time.

The result is an output frequency that is proportional to the

average positive-only real power. The averaging of the positive-

only real power signal is implicit to the digital-to-frequency

conversion. The output frequency or pulse rate is related to the

input voltage signals by the following equation:

2

75

.

494

REF

4

1

rms

rms

V

F

V2

V1

Freq

−

×

×

×

=

(10)

where:

Freq is the output frequency on F1 and F2 (Hz).

S0 and S1 logic inputs (see Table 5).