Datasheet

Data Sheet ADE7758
Rev. E | Page 51 of 72
To set APCFNUM (0x45) and APCFDEN (0x46) to the
calculated value to perform a coarse adjustment on the
imp/kW-hr ratio, use Equation 45 to Equation 47.
kHz5415.0
130
10
500
220
kH16
=××= zAPCF
NOMINAL
()
Hz1.956cos
36001000
220103200
=θ×
×
××
=
EXPECTED
APCF
277
Hz956.1
Hz5.541
INT =
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=APCFDEN
Under the test conditions above, the AWATTHR register value
is 15559d after the LENERGY interrupt. Using Equation 60 and
Equation 61, the value to be written to AWG is −199d, 0xF39.
[]
SelectedPhasesofNo.
FREQ
:LINECYC
AccumTime
×
××
×
=
−6
106.9]0:11[
1
2
015
6.832128s
3
106.92085
1
2
8000
6
=
×
××
×
=
−
x
AccumTime
148041
1
277
36001000
832.612201032004
=××
×
×××××
=
EXPECTED
WATTHR
0xF39–199–198.8764021
15559
14804
12
===×
⎟
⎠
⎞
⎜
⎝
⎛
−=xWG
Using Equation 66, the Wh/LSB constant is
00.000282
148043600
832.622010
=
×
××
=
LSB
Wh
Phase Calibration Using Line Accumulation
The ADE7758 includes a phase calibration register on each
phase to compensate for small phase errors. Large phase errors
should be compensated by adjusting the antialiasing filters. The
ADE7758 phase calibration is a time delay with different weights
in the positive and negative direction (see the Phase
Compensation section). Because a current transformer is a
source of phase error, a fixed nominal value can be decided on to
load into the xPHCAL (0x3F to 0x41) registers at power-up.
During calibration, this value can be adjusted for CT-to-CT
error. Figure 83 shows the steps involved in calibrating the
phase using the line accumulation mode.
STEP 1
SET LCYCMODE,
LINECYC AND MASK
REGISTERS
STEP 2
SET UP SYSTEM FOR
I
TEST
,V
NOM
,PF=0.5,
INDUCTIVE
STEP 3
RESET STATUS
REGISTER
STEP 4
READ ALL xWATTHR
REGISTERS AFTER
LENERGY
INTERRUPT
STEP 5
CALCULATE PHASE
ERROR IN DEGREES
FOR ALL PHASES
STEP 6
CALCULATE AND
WRITE TO ALL
xPHCAL REGISTERS
04443-082
Figure 83. Phase Calibration Using Line Accumulation
Step 1: If the values were changed after gain calibration, Step 1,
Step 3, and Step 4 from the gain calibration should be repeated
to configure the LCYCMODE and LINECYC registers.
Step 2: Set the test system for I
TEST
, V
NOM
, and 0.5 power factor
inductive.
Step 3: Reset the interrupt status register by reading RSTATUS
(0x1A).
Step 4: The xWATTHR registers should be read after the
LENERGY interrupt. Measure the percent error in the energy
register readings (AWATTHR, BWATTHR, and CWATTHR)
compared to the energy register readings at unity power factor
(after gain calibration) using Equation 69. The readings at unity
power factor should have been repeated after the gain calibration
and stored for use in the phase calibration routine.
2
2
–
1PF
1PF
5PF
=
=
=
=
xWATTHR
xWATTHR
xWATTHR
Error
(69)
Step 5: Calculate the Phase Error in degrees using the equation
()
⎟
⎠
⎞
⎜
⎝
⎛
=°
3
–
Error
ArcsinErrorPhase
(70)
Step 6: Calculate xPHCAL and write to the xPHCAL registers
(0x3F to 0x41).
°
×××
=
360
1
)(
1
__
1
sPeriodLineWeightLSBPHCAL
ErrorPhase
xPHCAL
(71)
where PHCAL_LSB_Weight is 1.2 s if the %Error is negative
or 2.4 s if the %Error is positive (see the Phase Compensation
section).