ADE7759ARS datasheet(25/32 Pages) AD

25 / 32 page

REV. 0

ADE7759

–25–

For 255 half cycles this would give a total integration time of 2.125

seconds. This would mean the energy register was updated

2.125/1.1175

µs (4/CLKIN) times. The average output value of

LPF2 is given as:

Contents of LENERGY

at the end

Number of times LENERGY

was updated

[: ]

39 0

Or, equivalently, in terms of contents of various ADE7759

registers and CLKIN and line frequencies (fl):

Average Word LPF

LENERGY

LINECYC

CLKIN

()

[: ]

39 0

13 0

××

(16)

where fl is the line frequency.

Calibrating the Frequency at CF

Once the average Active Power signal is calculated it can be used

to determine the frequency at CF before calibration. When the

frequency before calibration is known, the pair of CF Frequency

Divider registers (CFNUM and CFDEN) can be adjusted so as

to produce the required frequency on CF. In this example a

meter constant of 3200 imp/kWh is chosen as an appropriate

constant. This means that under a steady load of 1 kW, the

output frequency on CF would be:

Frequency CF

imp kWh

()

3200

3600

0 8888

min

sec

Assuming the meter is set up with a test current (basic current) of

20 A and a line voltage of 220 V for calibration, the load is cal-

culated as 220 V

× 20 A = 4.4 kW. Therefore, the expected output

frequency on CF under this steady load condition would be

4.4

× 0.8888 Hz = 3.9111 Hz. Under these load conditions the

transducers on Channel 1 and Channel 2 should be selected such

that the signal on the voltage channel should see approximately

half scale and the signal on the current channel about 1/8 of full

scale (assuming a maximum current of 80 A). The average value

from LPF2 is calculated as 3,276.81 decimal using the calibration

mode as described above. Then using Equation 8 (Energy to Fre-

quency Conversion), the frequency under this load is calculated as:

Frequency CF

MHz

()

3276 81

3 579545

349 566

This is the frequency with the contents of the CFNUM and CFDEN

registers equal to 000h. The desired frequency out is 3.9111 Hz.

Therefore, the CF frequency must be divided by 349.566/3.9111 Hz

or 89.3779 decimal. This is achieved by loading the pair of CF

Divider registers with the closest rational number. In this case

the closest rational number is found to be 25/2234 (or 19h/8BAh).

Therefore, 18h and 8B9h should be written to the CFNUM and

CFDEN registers respectively. Note that the CF frequency is

divided by the contents of (CFNUM + 1)/(CFDEN + 1). With

the CF Divide registers contents equal to 18h/8B9h, the output

frequency is given as 349.566 Hz / 89.36 = 3.91188 Hz. Note

that this setting has an error of +0.02%.

Calibrating CF is made easy by using the Line Cycle Energy

Accumulation mode on the ADE7759 provided that the line

frequency is accurately known during calibration. Using Line

Cycle Energy Accumulation mode, the calibration time can be

reduced by synchronizing energy accumulation to the zero cross-

ing of the voltage channel. See Line Cycle Energy Accumulation

Mode section. However, this requires the line frequency to be

precisely known. As shown in Equation 16, the average value of

LPF2 is directly proportional to the line frequency. Any deviation

from the nominal frequency will directly affect the calibration

result. The line frequency could be measured using the ZX output

of the ADE7759. Alternatively, the average value of LPF2 can

be calculated from the output frequency from CF—see Energy

to Frequency Conversion section.

Note that besides CFNUM and CFDEN registers, changing

APGAIN[11:0] register will also affect the output frequency from

CF. The APGAIN register has a resolution of 0.0244%/LSB.

Energy Meter Display

Besides the pulse output, which is used to verify calibration, a

solid state energy meter will very often require some form of

display. The display should show the amount of energy consumed

in kWh (Killowatt Hours). One convenient and simple way to

interface the ADE7759 to a display or energy register (e.g., MCU

with nonvolatile memory) is to use CF. For example the CF

frequency could be calibrated to 1,000 imp/kWhr. The MCU

would count pulses from CF. Every pulse would be equivalent

to 1 watt-hour. If more resolution is required, the CF frequency

could be set to, say, 10,000 imp/kWh.

If more flexibility is required when monitoring energy usage, the

Active Energy register (AENERGY) can be used to calculate

energy. A full description of this register can be found in the

Energy Calculation section. The AENERGY register gives the

user both sign and magnitude information regarding energy

consumption. On completion of the CF frequency output cali-

bration, i.e., after the Active Power Gain (APGAIN) register has

been adjusted, a second calibration sequence can be initiated.

The purpose of this second calibration routine is to determine a

kWh/LSB coefficient for the AENERGY register. Once the

coefficient has been calculated, the MCU can determine the

energy consumption at any time by reading the AENERGY

contents and multiplying by the coefficient to calculate kWh.

CLKIN FREQUENCY

In this data sheet, the characteristics of the ADE7759 are shown

with the CLKIN frequency equal to 3.579545 MHz. However,

the ADE7759 is designed to have the same accuracy at any

CLKIN frequency within the specified range. If the CLKIN

frequency is not 3.579545 MHz, various timing and filter charac-

teristics will need to be redefined with the new CLKIN frequency.

For example, the cutoff frequencies of all digital filters (LPF1,

LPF2, HPF1, etc.) will shift in proportion to the change in CLKIN

frequency according to the following equation:

New Frequency

Original Frequency

CLKIN Frequency

MHz

=×

3 579545

(17)

The change of CLKIN frequency does not affect the timing

characteristics of the serial interface because the data transfer is

synchronized with serial clock signal (SCLK). But one needs to

observe the read/write timing of the serial data transfer—see

Timing Characteristics. Table III lists various timing changes

that are affected by CLKIN frequency.

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