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ADE7755ARSRL

Energy Metering IC with Pulse Output

Analog Devices 

ADE7755

PRELIMINARY TECHNICAL DATA

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 IV). With full-scale ac signals on the

analog inputs, the output frequency on CF will be approximately

5.5 kHz. Figure 13 illustrates one scheme that could be used to

digitize the output frequency and carry out the necessary

averaging mentioned in the previous section.

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 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,

since the ADE7755 output frequency is running asynchronously

to the MCU timer. This would result in a one-in-twenty (or

5%) error in the power measurement.

CF

AVERAGE

FREQUENCY

FREQUENCY

RIPPLE

؎10%

ADE7755

CF

REVP*

TIME

MCU

COUNTER

UP/ DOWN

TIMER

*REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR

DIRECTION OF ENERGY FLOW IS NEEDED

Figure 13. Interfacing the ADE7755 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 that is determined by an MCU internal timer.

The average power proportional to the average frequency is

given by:

Average Frequency = Average Real Power = Counter

Timer

The energy consumed during an integration period is given by:

Energy = Average Power × Time = Counter × Time = Counter

Time

For the purpose of calibration, this integration time can be 10 to

20 seconds to accumulate enough pulses to ensure correct aver-

aging of the frequency. In normal operation, the integration time

can be reduced to one or two seconds depending, for example,

on the required undate rate of a display. With shorter integra-

tion 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.

TRANSFER FUNCTION

Frequency Outputs F1 and F2

The ADE7755 calculates the product of two voltage signals (on

Channel 1 and Channel 2) 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., 0.34 Hz maximum

for ac signals with S0 = S1 = 0 (see Table III). 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 = 8.06 × V1 × V 2 × Gain × F1−4

VREF 2

where:

Freq = Output frequency on F1 and F2 (Hz)

V1 = Differential rms voltage signal on Channel 1 (Volts)

V2 = Differential rms voltage signal on Channel 2 (Volts)

Gain = 1, 2, 8, or 16, depending on the PGA gain selection

made using logic inputs G0 and G1

VREF = The reference voltage (2.5 V ± 8%) (Volts)

F1–4 = One of four possible frequencies selected by using the

logic inputs S0 and S1—see Table II

Table II. F1–4 Frequency Selection

S1

S0

0

0

0

1

1

0

1

1

F1–4 (Hz)

1.7

3.4

6.8

13.6

XTAL/CLKIN*

3.579 MHz/221

3.579 MHz/220

3.579 MHz/219

3.579 MHz/218

NOTE

*F1–4 is a binary fraction of the master clock and therefore will vary if the speci-

fied CLKIN frequency is altered.

–14–

REV. PrA

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