EVAL-ADE7763EB datasheet(39/56 Pages) AD

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ADE7763

Rev. A | Page 39 of 56

Calibrating Watt Offset with an Accurate Source Example

Figure 74 is the flowchart for watt offset calibration with an

accurate source.

SET HALF LINE CYCLES FOR ACCUMULATION

IN LINECYC REGISTER ADDR. 0x1C

SET ITEST = IMIN, VTEST = VNOM, PF = 1

CALCULATE APOS. SEE EQUATION 45.

SET MODE FOR LINE CYCLE

ACCUMULATION ADDR. 0x09 = 0x0080

ENABLE LINE CYCLE ACCUMULATION

INTERRUPT ADDR. 0x0A = 0x04

READ LINE ACCUMULATION ENERGY

ADDR. 0x04

RESET THE INTERRUPT STATUS

READ REGISTER ADDR. 0x0C

INTERRUPT?

YES

RESET THE INTERRUPT STATUS

READ REGISTER ADDR. 0x0C

INTERRUPT?

WRITE APOS VALUE TO THE APOS

Figure 74. Calibrating Watt Offset with an Accurate Source

For this example:

Meter Constant:

MeterConstant(imp/Wh) = 3.2

Line Voltage:

Vnominal = 220 V

Line Frequency:

fl = 50 Hz

CF Numerator:

CFNUM = 0

CF Denominator:

CFDEN = 489

Base Current:

Ib = 10 A

Half Line Cycles Used at Base Current:

LINECYC(IB) = 2000

Period Register Reading:

PERIOD = 8959

Clock Frequency:

CLKIN = 3.579545 MHz

Expected LAENERGY Register Value at Base Current

(from the Watt Gain section):

LAENERGYIB(expected) = 19186

Minimum Current:

IMIN = 40 mA

Number of Half Line Cycles used at Minimum Current:

LINECYC(IMIN) = 35700

Active Energy Reading at Minimum Current:

LAENERGYIMIN(nominal) = 1395

The LAENERGYexpected at IMIN is 1255 using Equation 49.

LAENERGYIMIN(expected) =

INT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

MIN

expected

MIN

LINECYC

LINECYCI

LAENERGY

)

(

(49)

LAENERGYIMIN(expected) =

INT

1370

)

1369

(

2000

35700

19186

⎟

⎠

⎞

⎜

⎝

⎛

INT

where:

LAENERGYIB(expected) is the expected LAENERGY reading at Ib

from the watt gain calibration.

LINECYCIMIN is the number of half line cycles that energy is

accumulated over when measuring at IMIN.

More line cycles could be required at the minimum current to

minimize the effect of quantization error on the offset calibration.

For example, if a test current of 40 mA results in an active energy

accumulation of 113 after 2000 half line cycles, one LSB variation

in this reading represents a 0.8% error. This measurement does

not provide enough resolution to calibrate a <1% offset error.

However, if the active energy is accumulated over 37,500 half

line cycles, one LSB variation results in 0.05% error, reducing the

quantization error.

APOS is −672 using Equations 55 and 49.

LAENERGY Absolute Error =

LAENERGYIMIN(nominal) − LAENERGYIMIN(expected)

LAENERGY Absolute Error = 1395 − 1370 = 25

(50)

AENERGY Error Rate (LSB/s) =

PERIOD

CLKIN

LINECYC

Error

Absolute

LAENERGY

(51)

AENERGY Error Rate (LSB/s) =

069948771

8959

579545

35700

CLKIN

Rate

Error

AENERGY

APOS = −

672

579545

069948771

−

APOS = −

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