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ADE7769

Energy Metering IC with Integrated Oscillator and No-Load Indication

Analog Devices 

ADE7769

CF

AVERAGE

FREQUENCY

FREQUENCY

RIPPLE

±10%

ADE7769

CF

TIME

MCU

COUNTER

TIMER

Figure 26. Interfacing the ADE7769 to an MCU

As shown in Figure 26, 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

Average Frequency = Average Power = Counter

(8)

Time

The energy consumed during an integration period is given by

Energy = Average Power ×Time = Counter × Time = Counter (9)

Time

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 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 ADE7769 output frequency is running asynchro-

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

in the power measurement.

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 27. 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 27, is recommended.

490

480

470

460

450

440

430

420

410

400

5.8 5.9 6.0 6.1 6.2 6.3 6.4 6.5 6.6 6.7

RESISTANCE (kΩ)

Figure 27. Effect of RCLKIN on Internal Oscillator Frequency (OSC)

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7769 calculates the product of two voltage signals

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

product to extract real power information. This 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 real power information accumulated over a

relatively long period of time. The result is an output frequency

that is proportional to the average real power. The averaging of

the 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:

Freq

=

494.75×V1rms ×V2rms

VREF 2

×

F1−4

(10)

where:

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

V1rms is the differential rms voltage signal on Channel V1 (V).

V2rms is the differential rms voltage signal on Channel V2 (V).

VREF = is the reference voltage (2.45 V ± 200 mV) (V).

F1–4 = are one of four possible frequencies selected by using

the S0 and S1logic inputs (see Table 5).

Rev. A | Page 14 of 20

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