Datasheet

ADE7763 Data Sheet
Rev. C | Page 40 of 56
Calibrating Watt Offset with an Accurate Source Example
Figure 74 is the flowchart for watt offset calibration with an
accurate source.
SET HALF LINE CYCLES FOR ACCUMULATION
IN LINECYC REGISTER ADDR. 0x1C
SET I
TEST
= I
MIN
, V
TEST
= V
NOM
, PF = 1
CALCULATE APOS. SEE EQUATION 45.
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?
NO
NO
YES
YES
RESET THE INTERRUPT STATUS
READ REGISTER ADDR. 0x0C
INTERRUPT?
WRITE APOS VALUE TO THE APOS
REGISTER: ADDR. 0x11
04481-A-088
Figure 74. Calibrating Watt Offset with an Accurate Source
For this example:
Meter Constant: MeterConstant(imp/Wh) = 3.2
Line Voltage: V
nominal
= 220 V
Line Frequency: f
l
= 50 Hz
CF Numerator: CFNUM = 0
CF Denominator: CFDEN = 489
Base Current: I
b
= 10 A
Half Line Cycles Used at Base Current:
LINECYC
(IB)
= 2000
Period Register Reading: PERIOD = 8959
Clock Frequency: CLKIN = 3.579545 MHz
Expected LAENERGY Register Value at Base Current
(from the Watt Gain section):
LAENERGY
IB(expected)
= 19186
Minimum Current: I
MIN
= 40 mA
Number of Half Line Cycles used at Minimum Current:
LINECYC
(IMIN)
= 35700
Active Energy Reading at Minimum Current:
LAENERGY
IMIN(nominal)
= 1395
The LAENERGY
expected
at I
MIN
is 1255 using Equation 49.
LAENERGY
IMIN(expected)
=
INT








×
×
IB
MIN
expected
IB
B
MIN
LINECYC
LINECYCI
LAENERGY
I
I
)
(
(49)
LAENERGY
IMIN(expected)
=
INT
1370
)80
.
1369(
2000
35700
19186
10
04.
0
=
=






×
×
INT
where:
LAENERGY
IB(expected)
is the expected LAENERGY reading at I
b
from the watt gain calibration.
LINECYC
IMIN
is the number of half line cycles that energy is
accumulated over when measuring at I
MIN
.
More line cycles could be required at the minimum current to
minimize the effect of quantization error on the offset calibration.
For example, if a test current of 40 mA results in an active energy
accumulation of 113 after 2000 half line cycles, one LSB variation
in this reading represents a 0.8% error. This measurement does
not provide enough resolution to calibrate a <1% offset error.
However, if the active energy is accumulated over 37,500 half
line cycles, one LSB variation results in 0.05% error, reducing the
quantization error.
APOS is −672 using Equations 55 and 49.
LAENERGY Absolute Error =
LAENERGY
IMIN(nominal)
− LAENERGY
IMIN(expected)
LAENERGY Absolute Error = 1395 − 1370 = 25 (50)
AENERGY Error Rate (LSB/s) =
PERIOD
CLKIN
LINECYC
ErrorAbsoluteLAENERGY
×
×
82/
(51)
AENERGY Error Rate (LSB/s) =
069948771.0
89598
10579545.3
2/35700
25
6
=
×
×
×
APOS = −
CLKIN
RateErrorAENERGY
35
2×
APOS = −
672
10579545.3
2
069948771.0
6
35
−=
×
×