ADE7758 datasheet(49/68 Pages) AD

49 / 68 page

ADE7758

Rev. A | Page 49 of 68

Step 8: Read all six xWATTHR (0x01 to 0x03) and xVAHR

(0x07 to 0x09) energy registers after the LENERGY interrupt

and store the values.

Step 8a: Calculate the values to be written to xWG registers

according to the following equation.

()

600

000

cos

TEST

xWG

[]

WDIV

xWATTHR

AccumTime

(42)

where Accumulation Time is

[]

Selected

Phases

No.

Frequency

Line

LINECYC

AccumTime

(43)

MC is the meter constant, θ is the angle between the current

and voltage, Line Frequency is read from the FREQ register or is

known, and the No. of Phases Selected are the number of ZXSEL

bits set to Logic 1 in LCYCMODE (0x17).

Step 8b: Calculate the values to be written to the xVAG registers

according to the following equation.

()

600

000

cos

TEST

xVAG

[]

VADIV

xVAHR

AccumTime ×

(44)

Step 9: Write to xWG and xVAG.

Step 10: Set the test system for ITEST, VNOM, and zero power factor

(calibrate VAR gain).

Step 11: Repeat Step 7.

Step 12: Read the xVARHR (0x04 to 0x06) after the LENERGY

interrupt and store the values.

Step 13: Calculate the values to be written to the xVARG

registers (to adjust VARCF to the expected value).

()

600

000

sin

TEST

xVAG

[]

VADIV

xVAHR

AccumTime ×

(45)

Step 14: Write to xVARG.

Step 15: Calculate the Wh/LSB, VARh/LSB, and VAh/LSB

constants.

xWATTHR

AccumTime

LSB

NOM

TEST

600

(46)

xVAHR

AccumTime

LSB

VAh

NOM

TEST

600

(47)

xVARHR

AccumTime

LSB

VARh

NOM

TEST

600

(48)

Example—Watt Gain Calibration Using Line Accumulation

This example only shows Phase A watt calibration. The steps

outlined in the Gain Calibration Using Line Accumulation

section show how to calibrate watt, VA, and VAR. All three

phases can be calibrated simultaneously because there are nine

energy registers.

For this example, ITEST = 10 A, VNOM = 220 V, Power Factor = 1,

Frequency = 50 Hz, LINECYC (0x1C) is set to 1FF, and

MC = 3200 imp/kWhr.

To set APCFNUM (0x45) and APCFDEN (0x46) to the

calculated value to perform a coarse adjustment on the

imp/kW-hr ratio, use Equation 27 to Equation 29:

130

500

220

APCFNOMINAL

()

cos

600

000

220

200

EXPECTED

APCF

227

541

= INT

APCFDEN

Under the test conditions above, the AWATTHR register value

is 24008d after the LENERGY interrupt. Using Equation 42 and

Equation 43, the value to be written to AWG is 02d.

AccumTime

268

008

600

000

220

200

xWG

Using Equation 46, the Wh/LSB constant is

–

008

600

220

LSB

Phase Calibration Using Line Accumulation

The ADE7758 includes a phase calibration register on each

phase to compensate for small phase errors. Large phase errors

should be compensated by adjusting the antialiasing filters. The

ADE7758’s phase calibration is a time delay with different

weights in the positive and negative direction (see the Phase

Compensation section). Since a current transformer is a source

of phase error, a fixed nominal value may be decided on to load

into the xPHCAL (0x3F to 0x41) registers at power-up. During

calibration, this value can be adjusted for CT-to-CT error.

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