71M6521 Demo Board User’s Manual
Page: 52 of 111
© 2005-2009 TERIDIAN Semiconductor Corporation
Revision 2.18
12.
+
+
−
=
−
)
2
)(
60
tan(
)
(
tan
180
0
300
60
1
E
E
E
E
S
φ
Now that we know the A
XV
, A
XI
, and
φ
S
errors, we calculate the new calibration voltage gain coefficient from
the previous ones:
XV
NEW
A
V
CAL
V
CAL
_
_
=
We calculate PHADJ from
φ
S
, the desired phase lag:
[
]
[
]
−
−
−
−
−
−
−
+
=
−
−
−
−
)
2
cos(
)
2
1
(
1
)
tan(
)
2
sin(
)
2
1
(
)
2
cos(
)
2
1
(
2
)
2
1
(
1
)
tan(
2
0
9
0
9
0
9
2
9
20
T
f
T
f
T
f
PHADJ
S
S
π
φ
π
π
φ
And we calculate the new calibration current gain coefficient, including compensation for a slight gain
increase in the phase calibration circuit.
2
9
0
9
0
9
20
20
)
2
1
(
)
2
cos(
)
2
1
(
2
1
))
2
cos(
)
2
1
(
2
2
2
(
2
1
1
_
_
−
−
−
−
−
−
+
−
−
−
−
+
+
=
T
f
T
f
PHADJ
PHADJ
A
I
CAL
I
CAL
XI
NEW
π
π
2.1.3 FAST CALIBRATION
A very fast calibration process can be implemented by offering a known voltage and current at exactly zero
degree phase angle to the meter. If the meter is exposed to the current and voltage source for a known
integer number of accumulation intervals, the accumulated real (Wh) and reactive (VARh) energy values
along with the accumulated voltage (V
2
h) can be used to determine the calibration factors quickly.
Another advantage of this method is that no pulse counts are necessary, since the energy accumulation
values are directly accessible via their associated registers in the Demo Code.
The phase angle can be directly calculated from the inverse tangens of the ratio of reactive and real energy:
=
−
P
Q
S
1
tan
φ
The energy quantities are directly available as the values read from the accumulation registers (n
Q
,
n
P
)
multiplied with the LSB value, which is the product of the LSB of the CE and the VMAX and IMAX design
constants:
Q = n
Q
* LSB = n
Q
*
IMAX
*
VMAX
* LSB
CE
P = n
P
* LSB = n
P
*
IMAX
*
VMAX
* LSB
CE
