ADE7758 datasheet(44/68 Pages) AD

44 / 68 page

ADE7758

Rev. A | Page 44 of 68

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.

To calculated the values for APCFNUM/APCFDEN and

VARCFNUM/VARCFDEN use the following formulas

FULLSCALE

TEST

FULLSCALE

NOM

NOMINAL

kHz

APCF

= 16

(27)

( )

cos

600

000

NOM

TEST

EXPECTED

APCF

(28)

⎟

⎠

⎞

⎜

⎝

⎛

EXPECTED

NOMINAL

APCF

INT

APCFDEN

(29)

where MC is the meter constant, ITEST is the test current, VNOM is

the nominal voltage that the meter is tested at, and 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, and the

APCFEXPECTED value is equivalent to the reference meter output

under the test conditions.

The equations for calculating the VARCFNUM and

VARCFDEN during VAR calibration are similar, with one

exception

( )

sin

600

000

NOM

TEST

EXPECTED

VARCF

(30)

Because the CFNUM and CFDEN 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

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

(31)

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.

EXPECTED

APCF

[]

[

]

⎟

⎠

⎞

⎜

⎝

⎛ +

xWG

APCFDEN

APCFNUM

APCFNOMINAL

(32)

0244

–

Error

xWG =

(33)

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

000

(34)

Step 8: 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.

Set APCFNUM(0x45) and APCFDEN(0x46) to the calculated

value to perform a coarse adjustment on the imp/kWh ratio.

Using Equations 27 through 29.

kHz

542

130

500

220

kHz

APCFNOMINAL

()

cos

600

000

220

200

EXPECTED

APCF

277

542

⎟

⎠

⎞

⎜

⎝

⎛

= INT

APCFDEN

With ITEST, VNOM, and the unity power factor, the example

ADE7758 meter shows 1.92 Hz on the pulse output. This is

equivalent to a 2.04% error from the reference meter value

using Equation 31.

100

–

Error

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