ADE7763ARS datasheet(36/56 Pages) AD

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ADE7763

Rev. A | Page 36 of 56

CFnominal(Hz) =

MAX

kHz

(39)

CFIB(nominal)(Hz) =

958

kHz

The nominal CF on a sample set of meters should be measured

using the default CFDEN, CFNUM, and WDIV to ensure that

the best CFDEN is chosen for the design.

With the CFNUM register set to 0, CFDEN is calculated to be

489 for the example meter:

CFDEN =

)

(

)

(

−

⎟

⎠

⎞

⎜

⎝

⎛

expected

nominal

INT

(40)

CFDEN =

489

)

490

(

9556

958

−

⎟

⎠

⎞

⎜

⎝

⎛

INT

This value for CFDEN should be loaded into each meter before

calibration. The WGAIN register can then be used to finely

calibrate the CF output. The following sections explain how to

calibrate a meter based on ADE7763 when using a reference

meter or an accurate source.

Calibrating Watt Gain Using a Reference Meter Example

The CFDEN and CFNUM values for the design should be

written to their respective registers before beginning the

calibration steps shown in Figure 71. When using a reference

meter, the percent error in CF is measured by comparing the CF

output of the ADE7763 meter with the pulse output of the

reference meter, using the same test conditions for both meters.

Equation 41 defines the percent error with respect to the pulse

outputs of both meters (using the base current, Ib):

%ERRORCF(IB) =

100

)

(

)

(

−

ref

(41)

CALCULATE CFDEN VALUE FOR DESIGN

WRITE CFDEN VALUE TO CFDEN REGISTER

ADDR. 0x15 = CFDEN

WRITE WGAIN VALUE TO THE WGAIN

MEASURE THE % ERROR BETWEEN

THE CF OUTPUT AND THE

REFERENCE METER OUTPUT

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

CALCULATE WGAIN. SEE EQUATION 42.

Figure 71. Calibrating Watt Gain Using a Reference Meter

For this example:

Meter Constant:

MeterConstant(imp/Wh) = 3.2

CF Numerator:

CFNUM = 0

CF Denominator:

CFDEN = 489

%ERROR Measured at Base Current:

%ERRORCF(IB) = −3.07%

One LSB change in WGAIN changes the active energy registers

and CF by 0.0244%. WGAIN is a signed, twos complement

error, WGAIN is 126:

WGAIN = INT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

0244

)

(IB

ERROR

(42)

WGAIN = INT

126

0244

⎟

⎠

⎞

⎜

⎝

⎛ −

−

When CF is calibrated, the AENERGY register has the same

Wh/LSB constant from meter to meter if the meter constant,

WDIV, and the CFNUM/CFDEN ratio remain the same. The

Wh/LSB ratio for this meter is 6.378 × 10−4 using Equation 35

with WDIV at the default value.

(imp/Wh)

)

(

)

(

LSB

ant

MeterConst

WDIV

CFDEN

CFNUM

378

490

imp/Wh

200

)

490

(

LSB

−

Calibrating Watt Gain Using an Accurate Source Example

The CFDEN value calculated using Equation 40 should be

written to the CFDEN register before beginning calibration and

zero should be written to the CFNUM register. Enable the line

accumulation mode and the line accumulation interrupt. Then,

write the number of half line cycles for the energy accumulation

to the LINECYC register to set the accumulation time. Reset the

interrupt status register and wait for the line cycle accumulation

interrupt. The first line cycle accumulation results might not

use the accumulation time set by the LINECYC register and,

therefore, should be discarded. After resetting the interrupt

status register, the following line cycle readings will be valid.

When LINECYC half line cycles have elapsed, the IRQ pin goes

active low and the nominal LAENERGY with the test current

applied can be read. This LAENERGY value is compared to the

expected LAENERGY value to determine the WGAIN value. If

apparent energy gain calibration is performed at the same time,

LVAENERGY can be read directly after LAENERGY. Both

registers should be read before the next interrupt is issued on

the IRQ pin. Figure 72 details steps to calibrate the watt gain

using an accurate source.

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