Datasheet

Data Sheet ADE7758
Rev. E | Page 45 of 72
Step 4: Set APCFNUM (0x45) and APCFDEN (0x46) to the
calculated value to perform a coarse adjustment on the
imp/ kWh r atio. For VAR/VA cal ibration, set VARCFNUM
(0x47) and VARCFDEN (0x48) to the calculated value.
The pulse output frequency with one phase at full-scale inputs
is approximately 16 kHz. A sample set of meters could be tested
to find a more exact value of the pulse output at full scale in the
user application.
To calculate the values for APCFNUM/APCFDEN and
VARCFNUM/VARCFDEN, use the following f or mulas:
FULLSCALE
TEST
FULLSCALE
NOM
NOMINAL
I
I
V
V
APCF
××= kHz16
(45)
()
θ×
×
××
= cos
36001000
NOM
TEST
EXPECTED
VIMC
APCF
(46)
⎟
⎠
⎞
⎜
⎝
⎛
=
EXPECTED
NOMINAL
APCF
APCF
INTAPCFDEN (47)
where:
MC is the meter constant.
I
TEST
is the test current.
V
NOM
is the nominal voltage at which the meter is tested.
V
FULLSCALE
and I
FULLSCALE
are the values of current and voltage,
which correspond to the full-scale ADC inputs of the ADE7758.
θ is the angle between the current and the voltage channel.
APCF
EXPECTED
is equivalent to the reference meter output under
the test conditions.
APCFNUM is written to 0 or 1.
The equations for calculating the VARCFNUM and
VARCFDEN during VAR cal ibr ation are si milar:
()
θ×
×
××
= sin
36001000
NOM
TEST
EXPECTED
VIMC
VARCF
(48)
Because the APCFDEN and VARCFDEN values can be
calculated from the meter design, these values can be written
to the part automatically during production calibration.
Step 5: Set the test system for I
TEST
, V
NOM
, and the unity power
factor. For VAR calibration, the power factor should be set to 0
inductive in this step. For watt and VA, the unity power factor
should be used. VAGAIN can be calibrated at the same time as
WGAIN because VAGAIN can be calibrated at the unity power
factor, and both pulse outputs can be measured simultaneously.
However, when calibrating VAGAIN at the same time as WGAIN,
the rms offsets should be calibrated first (see the Calibration of
IRMS and VRMS Offset section).
Step 6: Measure the percent error in the pulse output, APCF
and/or VARCF, from the reference meter:
%100
–
% ×=
REF
REF
CF
CFAPCF
Error (49)
where CF
REF
= APCF
EXPECTED
= the pulse output of the reference
meter.
Step 7: Calculate xWG adjustment. One LSB change in xWG
(12 bits) changes the WATTHR register by 0.0244% and
therefore APCF by 0.0244%. The same relationship holds true
for VARCF.
[]
[]
[]
⎟
⎠
⎞
⎜
⎝
⎛
+××
=
12
2
0:11
1
0:11
0:11 xWG
APCFDEN
APCFNUM
APCF
APCF
NOMINAL
EXPECTED
(50)
%0244.0
%
–
Error
xWG
=
(51)
When APCF is calibrated, the xWATTHR registers have the
same Wh/LSB from meter to meter if the meter constant and
the APCFNUM/APCFDEN ratio remain the same. The
Wh/LSB constant is
WDIVAPCFNU
M
APCFDENMC
LSB
Wh
1
1000
4
1
×××
= (52)
Return to Step 2 to calibrate Phase B and Phase C gain.
Example: Watt Gain Calibration of Phase A Using Pulse
Output
For this example, I
TEST
= 10 A, V
NOM
= 220 V, V
FULLSCALE
= 500 V,
I
FULLSCALE
= 130 A, MC = 3200 impulses/kWh, Power Factor = 1,
and Frequency = 50 Hz.
Clear APCFNUM (0x45) and write the calculated value to
APCFDEN (0x46) to perform a coarse adjustment on the
imp/kWh ratio, using Equation 45 through Equation 47.
kHz542.0
130
10
500
220
kHz16 =××=
NOMINAL
APCF
()
Hz9556.10cos
36001000
220103200
=×
×
××
=
EXPECTED
APCF
277
Hz9556.1
Hz542
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
= INTAPCFDEN
With Phase A contributing to CF, at I
TEST
, V
NOM
, and the unity
power factor, the example ADE7758 meter shows 2.058 Hz on
the pulse output. This is equivalent to a 5.26% error from the
reference meter value using Equation 49.
%26.5%100
Hz9556.1
Hz9556.1–Hz058.2
=×=%Error
The AWG value is calculated to be −216 d using Equation 51,
which means the value 0xF28 should be written to AWG.
2802165.215
%0244.0
%26.5
– xFAWG
=−=−==