Datasheet

Data Sheet ADE7763
Rev. C | Page 41 of 56
Phase Calibration
The PHCAL register is provided to remove small phase errors.
The ADE7763 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 I
B
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 =
2
2
)
(
)(
5
.0
,
expectedIB
expectedIB
PFIB
LAENERGY
LAENERGY
LAENERGY −
=
(52)
If watt gain and offset calibration have been performed, there
should be 0% error in CF at unity power factor, and then
Error = %ERROR
CF(IB,PF = 0.5)
/100 (53)
The phase error is
Phase Error (°) = −Arcsin








3
Error
(54)
The relationship between phase error and the PHCAL phase
correction register is
PHCAL = INT
( )
+






°
×°
360
PERIOD
ErrorPhase
0x0D (55)
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 (56)
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
(57)
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 percent 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
REGISTER: ADDR. 0x10
MEASURE THE % ERROR BETWEEN
THE CF OUTPUT AND THE
REFERENCE METER OUTPUT
SET I
TEST
= I
b
, V
TEST
= V
NOM
, PF = 0.5
04481-A-089
CALCULATE PHCAL. SEE EQUATION 55.
Figure 75. Calibrating Phase Using a Reference Meter
For this example:
CF %ERROR at PF = 0.5 Inductive: %ERROR
CF(IB,PF = 0.5)
= 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
°−=








07.0
3
00215.0
PHCAL = INT






°
×°−
360
8959
07.0
+0x0D = −2 + 13 = 11
PHCAL can be expressed as follows:
PHCAL =
INT








π
×








×
−
2
3
100%
Arcsin
PERIOD
ERROR
+ 0x0D (58)
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 (°) = −
°=
°
×− 08.0
8960
360
)0x0D11(