ADE7756 datasheet(25/32 Pages) AD

25 / 32 page

REV. 0

ADE7756

–25–

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

calculated 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 80A). 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 Frequency Conver-

sion), the frequency under this load is calculated as:

Frequency CF

MHz

()

3276 81 3 579545

349 566

However, this is the frequency with the contents of the CFDIV

and APGAIN 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 89.378 decimal. This is achieved by

loading the CF Divide register with 88 (or 58h)—Note the CF

frequency is divided by the contents of CFDIV + 1.

The fine adjustment of the output frequency can be made using

the Active Power Gain register. This register has a fine gain

adjustment of 0.0244%/LSB. With the CF Divide register con-

tents equal to 58h, the output frequency is given as 349.556 Hz/

89 = 3.9276 Hz. This setting has an error of 0.42%. This

error can be further reduced by writing –(0.21/0.0244) or –17 to

APGAIN[11:0] i.e., FEFh.

Calibrating CF is made easy by using the Calibration mode on

the ADE7756. The only critical part of the setup is that the line

frequency be exactly known. If this is not possible, it could be

measured by using the ZX output of the ADE7756.

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 display the amount of energy con-

sumed in kWh (Kilowatt Hours). One convenient and simple

way to interface the ADE7756 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/kWh. 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 ADE7756 are shown

with CLKIN frequency equal to 3.579545 MHz. However, the

ADE7756 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 characteristics will

need to be redefined with the new CLKIN frequency. For

example, the cut-off 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.

APPLICATION INFORMATION

Application note AN-564 contains detail information on how

to design a ANSI Class 100 Watt-Hour meter based on the

ADE7756. It is available from the ADE7756 product home page

under the Application Note link. Figure 35 shows the block dia-

gram of the ADE7756 reference meter implemented in AN-564.

RS-232

220V

LOAD

ADE7756

16 2 LCD DISPLAY

EEPROM

SPI BUS

PIC16C62B

Figure 35. Block Diagram of the ADE7756 Reference

Meter Described in AN-564

Table III. Frequency Dependencies of the

ADE7756 Parameters

CLKIN

Parameter

Dependency

Nyquist Frequency for CH 1 and 2 ADCs

CLKIN/8

PHCAL Resolution (Seconds per LSB)

16/CLKIN

Active Energy Register Update Rate (Hz)

CLKIN/4

Waveform Sampling Rate

(Number of Samples per Second)

WAVSEL 1, 0 = 0

CLKIN/128

CLKIN/256

CLKIN/512

CLKIN/1024

Maximum ZXTOUT Period

524,288/CLKIN

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