ADE7753ARSZ datasheet(44/60 Pages) AD

44 / 60 page

ADE7753

Rev. C | Page 44 of 60

Phase Calibration

The PHCAL register is provided to remove small phase errors.

The ADE7753 compensates for phase error by inserting a small

time delay or advance on the voltage channel input. Phase leads

up to 1.84° and phase lags up to 0.72° at 50 Hz can be corrected.

The error is determined by measuring the active energy at IB

and two power factors, PF = 1 and PF =0.5 inductive.

Some CTs may introduce large phase errors that are beyond the

range of the phase calibration register. In this case, coarse phase

compensation has to be done externally with an analog filter.

The phase error can be obtained from either CF or LAENERGY

measurements:

Error

)

(

)

(

expected

LAENERGY

−

(56)

If watt gain and offset calibration have been performed, there

should be 0% error in CF at unity power factor and then:

Error

= %ERRORCF(IB,PF = .5) /100

(57)

The phase error is

Phase Error

(°) = −Arcsin

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

Error

(58)

The relationship between phase error and the PHCAL phase

correction register is

PHCAL

INT

()

⎟

⎠

⎞

⎜

⎝

⎛

360

PERIOD

Error

Phase

0x0D

(59)

The expression for PHCAL can be simplified using the

assumption that at small x:

Arcsin(x) ≈ x

The delay introduced in the voltage channel by PHCAL is

Delay

= (PHCAL − 0x0D) × 8/CLKIN

(60)

The delay associated with the PHCAL register is a time delay if

(PHCAL − 0x0D) is positive but represents a time advance if

this quantity is negative. There is no time delay if PHCAL =

0x0D.

The phase correction is in the opposite direction of the phase

error.

Phase Correction

(°) = −(PHCAL − 0x0D)

PERIOD

× 360

(61)

Calibrating Phase Using a Reference Meter Example

A power factor of 0.5 inductive can be assumed if the pulse

output rate of the reference meter is half of its PF = 1 rate. Then

the %ERROR between CF and the pulse output of the reference

meter can be used to perform the preceding calculations.

WRITE PHCAL VALUE TO THE PHCAL

MEASURE THE % ERROR BETWEEN

THE CF OUTPUT AND THE

REFERENCE METER OUTPUT

SET ITEST = Ib, VTEST = VNOM, PF = 0.5

02875-A-010

CALCULATE PHCAL. SEE EQUATION 59.

Figure 84. Calibrating Phase Using a Reference Meter

For this example:

CF % Error at PF = .5 Inductive:

%ERRORCF(IB,PF = .5) =

0.215%

PERIOD Register Reading:

PERIOD = 8959

Then PHCAL is 11 using Equations 57 through 59:

Error = 0.215% / 100 = 0.00215

Phase Error (°) = −Arcsin

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

00215

PHCAL = INT

⎟

⎠

⎞

⎜

⎝

⎛

−

360

8959

+0x0D = −2 + 13 = 11

PHCAL can be expressed as follows:

PHCAL =

INT

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

Arcsin

PERIOD

Error

+ 0x0D

(62)

Note that PHCAL is a signed twos complement register.

Setting the PHCAL register to 11 provides a phase correction

of 0.08° to correct the phase lead:

Phase Correction (°) =

PERIOD

PHCAL

−

360

)

0x0D

(

Phase Correction (°) =

−

8960

360

)

0x0D

(

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