ADE7753ARSZ datasheet(42/60 Pages) AD

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ADE7753

Rev. C | Page 42 of 60

LAENERGYIB(expected) =

))

(

)

(

CFDEN

CLKIN

PERIOD

LINECYC

INT

expected

The calculated Wh/LSB ratio for the active energy register,

using Equation 39 is 6.378 × 10−4:

378

imp/Wh

200

)

489

(

LSB

−

Watt Offset

Offset calibration allows outstanding performance over a wide

dynamic range, for example, 1000:1. To do this calibration two

measurements are needed at unity power factor, one at Ib and

the other at the lowest current to be corrected. Either

calibration frequency or line cycle accumulation measurements

can be used to determine the energy offset. Gain calibration

should be performed prior to offset calibration.

Offset calibration is performed by determining the active

energy error rate. Once the active energy error rate has been

determined, the value to write to the APOS register to correct

the offset is calculated.

APOS = −

CLKIN

Rate

Error

AENERGY

(49)

The AENERGY registers update at a rate of CLKIN/4. The twos

complement APOS register provides a fine adjustment to the

active power calculation. It represents a fixed amount of power

offset to be adjusted every CLKIN/4. The 8 LSBs of the APOS

1/256 of the least significant bit of the internal active energy

of the AENERGY[23:0] active energy register.

The steps involved in determining the active energy error rate

for both line accumulation and reference meter calibration

options are shown in the following sections.

Calibrating Watt Offset Using a Reference Meter Example

Figure 82 shows the steps involved in calibrating watt offset

with a reference meter.

WRITE APOS VALUE TO THE APOS

MEASURE THE % ERROR BETWEEN THE

CF OUTPUT AND THE REFERENCE METER

OUTPUT, AND THE LOAD IN WATTS

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

02875-A-008

CALCULATE APOS. SEE EQUATION 49.

Figure 82. Calibrating Watt Offset Using a Reference Meter

For this example:

Meter Constant:

MeterConstant(imp/Wh) = 3.2

Minimum Current:

IMIN = 40 mA

Load at Minimum Current:

WIMIN = 9.6 W

CF Error at Minimum Current:

%ERRORCF(IMIN) = 1.3%

CF Numerator:

CFNUM = 0

CF Denominator:

CFDEN = 489

Clock Frequency:

CLKIN = 3.579545 MHz

Using Equation 49, APOS is calculated to be −522 for this

example.

CF Absolute Error = CFIMIN(nominal) − CFIMIN(expected)

(50)

CF Absolute Error =

(%ERRORCF(IMIN)) × WIMIN ×

3600

(imp/Wh)

ant

MeterConst

(51)

CF Absolute Error =

000110933

3600

200

100

⎟

⎠

⎞

⎜

⎝

⎛

Then,

AENERGY Error Rate (LSB/s) =

CF Absolute Error ×

CFNUM

CFDEN

(52)

AENERGY Error Rate (LSB/s) =

0.000110933 ×

05436

490 =

Using Equation 49, APOS is −522.

APOS

= −

522

579545

05436

−

APOS

can be represented as follows with CFNUM and WDIV

set at 0:

APOS

−

CLKIN

CFDEN

ant

MeterConst

ERROR

IMIN

)

(

)

(

3600

(imp/Wh)

)

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