ADE7758_15 datasheet(45/72 Pages) AD

45 / 72 page

Data Sheet

ADE7758

Rev. E | Page 45 of 72

Step 4: Set APCFNUM (0x45) and APCFDEN (0x46) to the

calculated value to perform a coarse adjustment on the

imp/kWh ratio. For VAR/VA calibration, set VARCFNUM

(0x47) and VARCFDEN (0x48) to the calculated value.

The pulse output frequency with one phase at full-scale inputs

is approximately 16 kHz. A sample set of meters could be tested

to find a more exact value of the pulse output at full scale in the

user application.

To calculate the values for APCFNUM/APCFDEN and

VARCFNUM/VARCFDEN, use the following formulas:

FULLSCALE

TEST

FULLSCALE

NOM

NOMINAL

APCF

kHz

(45)

()

cos

3600

1000

NOM

TEST

EXPECTED

APCF

(46)

⎟

⎠

⎞

⎜

⎝

⎛

EXPECTED

NOMINAL

APCF

INT

APCFDEN

(47)

where:

MC is the meter constant.

ITEST is the test current.

VNOM is the nominal voltage at which the meter is tested.

VFULLSCALE and IFULLSCALE are the values of current and voltage,

which correspond to the full-scale ADC inputs of the ADE7758.

θ is the angle between the current and the voltage channel.

APCFEXPECTED is equivalent to the reference meter output under

the test conditions.

APCFNUM is written to 0 or 1.

The equations for calculating the VARCFNUM and

VARCFDEN during VAR calibration are similar:

()

sin

3600

1000

NOM

TEST

EXPECTED

VARCF

(48)

Because the APCFDEN and VARCFDEN values can be

calculated from the meter design, these values can be written

to the part automatically during production calibration.

Step 5: Set the test system for ITEST, VNOM, and the unity power

factor. For VAR calibration, the power factor should be set to 0

inductive in this step. For watt and VA, the unity power factor

should be used. VAGAIN can be calibrated at the same time as

WGAIN because VAGAIN can be calibrated at the unity power

factor, and both pulse outputs can be measured simultaneously.

However, when calibrating VAGAIN at the same time as WGAIN,

the rms offsets should be calibrated first (see the Calibration of

IRMS and VRMS Offset section).

Step 6: Measure the percent error in the pulse output, APCF

and/or VARCF, from the reference meter:

100

–

REF

APCF

Error

(49)

where CFREF = APCFEXPECTED = the pulse output of the reference

meter.

Step 7: Calculate xWG adjustment. One LSB change in xWG

(12 bits) changes the WATTHR register by 0.0244% and

therefore APCF by 0.0244%. The same relationship holds true

for VARCF.

[]

⎟

⎠

⎞

⎜

⎝

⎛ +

xWG

APCFDEN

APCFNUM

APCF

NOMINAL

EXPECTED

(50)

0244

–

Error

xWG =

(51)

When APCF is calibrated, the xWATTHR registers have the

same Wh/LSB from meter to meter if the meter constant and

the APCFNUM/APCFDEN ratio remain the same. The

Wh/LSB constant is

WDIV

APCFNUM

APCFDEN

LSB

1000

(52)

Return to Step 2 to calibrate Phase B and Phase C gain.

Example: Watt Gain Calibration of Phase A Using Pulse

Output

For this example, ITEST = 10 A, VNOM = 220 V, VFULLSCALE = 500 V,

IFULLSCALE = 130 A, MC = 3200 impulses/kWh, Power Factor = 1,

and Frequency = 50 Hz.

Clear APCFNUM (0x45) and write the calculated value to

APCFDEN (0x46) to perform a coarse adjustment on the

imp/kWh ratio, using Equation 45 through Equation 47.

kHz

542

130

500

220

kHz

NOMINAL

APCF

()

9556

cos

3600

1000

220

3200

EXPECTED

APCF

277

9556

542

⎟

⎠

⎞

⎜

⎝

⎛

= INT

APCFDEN

With Phase A contributing to CF, at ITEST, VNOM, and the unity

power factor, the example ADE7758 meter shows 2.058 Hz on

the pulse output. This is equivalent to a 5.26% error from the

reference meter value using Equation 49.

100

9556

–

058

%Error

The AWG value is calculated to be −216 d using Equation 51,

which means the value 0xF28 should be written to AWG.

216

215

0244

–

AWG

−

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