REV. A

–12–

ADE7757

Interfacing the ADE7757 to a Microcontroller for Energy

Measurement

The easiest way to interface the ADE7757 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 III). With full-scale ac signals on

the analog inputs, the output frequency on CF will be approxi-

mately 2.867 kHz. Figure 14 illustrates one scheme that could

be used to digitize the output frequency and carry out the neces-

sary averaging mentioned in the previous section.

CF

TIME

10%

FREQUENCY

RIPPLE

AVERAGE

FREQUENCY

ADE7757

COUNTER

TIMER

MCU

CF

Figure 14. Interfacing the ADE7757 to an MCU

As shown, the frequency output CF is connected to an MCU

counter or port. This will count 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

Time

==

The energy consumed during an integration period is given by

Energy

Average Power

Time

Counter

Time

Time

Counter

=×

=

×

=

For the purpose of calibration, this integration time could be

10 seconds to 20 seconds in order to accumulate enough pulses to

ensure correct averaging of the frequency. In normal opera-

tion, the integration time could be reduced to one or two seconds,

depending, for example, on the required update rate of a dis-

play. 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 will have no ripple.

Power Measurement Considerations

Calculating and displaying power information will always have

some associated ripple that will depend 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 two seconds, only about

20 pulses will be counted. The possibility of missing one pulse

always exists as the ADE7757 output frequency is running

asynchronously to the MCU timer. This would result in a one-

in-twenty 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 k

Ω. 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 15. Although the internal oscillator operates when used

with RCLKIN values between 5.5 k

Ω and 20 kΩ, choosing a

value within the range of the nominal value, as shown in Figure 15,

is recommended.

RESISTANCE – k

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 15. Effect of RCLKIN on Internal Oscillator

Frequency (OSC)

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7757 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, e.g.,

0.175 Hz maximum for ac signals with S0 = S1 = 0 (see Table II).

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 propor-

tional 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

VV

F

V

rms

rms

REF

=

××

×

515 84

1

2

14

2

.

–

where

Freq =Output frequency on F1 and F2 (Hz).

± 8%) (V).

=One of four possible frequencies selected by using the

logic inputs S0 and S1—see Table I.