ADE7753ARS datasheet(45/60 Pages) AD

45 / 60 page

ADE7753

Rev. C | Page 45 of 60

Calibrating Phase with an Accurate Source Example

With an accurate source, line cycle accumulation is a good

method of calibrating phase error. The value of LAENERGY

must be obtained at two power factors, PF = 1 and PF = 0.5

inductive.

SET HALF LINE CYCLES FOR ACCUMULATION

IN LINECYC REGISTER ADDR. 0x1C

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

CALCULATE PHCAL. SEE EQUATION 59.

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 PHCAL VALUE TO THE PHCAL

02875-A-011

Figure 85. Calibrating Phase 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:

LINECYCIB = 2000

PERIOD Register:

PERIOD = 8959

Expected Line Accumulation at Unity Power Factor (from Watt

Gain Section:

LAENERGYIB(expected) =

19186

Active Energy Reading at PF = .5 inductive:

LAENERGYIB, PF

= .5

= 9613

The error using Equation 56 is

Error =

0021

19186

9613

−

Phase Error (°) = −Arcsin

−

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

0021

Using Equation 59, PHCAL is calculated to be 11.

PHCAL = INT

0x0D

360

8959

−

⎟

⎠

⎞

⎜

⎝

⎛

−

Note that PHCAL is a signed twos complement register.

The phase lead is corrected by 0.08° when the PHCAL register

is set to 11:

Phase Correction (°) =

PERIOD

PHCAL

−

360

)

0x0D

(

Phase Correction (°) =

−

8960

360

)

0x0D

(

VRMS and IRMS Calibration

VRMS and IRMS are calculated by squaring the input in a

digital multiplier.

)

cos(

)

sin(

)

sin(

)

(

−

(63)

The square of the rms value is extracted from v2(t) by a low-pass

filter. The square root of the output of this low-pass filter gives

the rms value. An offset correction is provided to cancel noise

and offset contributions from the input.

There is ripple noise from the 2ω term because the low-pass

filter does not completely attenuate the signal. This noise can be

minimized by synchronizing the rms register readings with the

zero crossing of the voltage signal. The IRQ output can be

configured to indicate the zero crossing of the voltage signal.

This flowchart demonstrates how VRMS and IRMS readings

are synchronized to the zero crossings of the voltage input.

SET INTERRUPT ENABLE FOR ZERO

CROSSING ADDR. 0x0A = 0x0010

RESET THE INTERRUPT STATUS

READ REGISTER ADDR. 0x0C

INTERRUPT?

YES

02875-A-003

READ VRMS OR IRMS

ADDR. 0x17; 0x16

RESET THE INTERRUPT STATUS

READ REGISTER ADDR. 0x0C

Figure 86. Synchronizing VRMS and IRMS Readings with Zero Crossings

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