Datasheet

Data Sheet ADE7758
Rev. E | Page 53 of 72
Power Offset Calibration Using Line Accumulation
Power offset calibration should be used for outstanding
performance over a wide dynamic range (1000:1). Calibration
of the power offset is done at or close to the minimum current.
The ADE7758 has power offset registers for watts and VAR,
xWATTOS (0x39 to 0x3B) and xVAROS (0x3C to 0x3E). Offsets in
the VA measurement are compensated by adjusting the rms offset
registers (see the Calibration of IRMS and VRMS Offset section).
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 current of 40 mA results in an
active energy accumulation of 113 after 2000 half line cycles,
one LSB variation in this reading represents an 0.8% error. This
measurement does not provide enough resolution to calibrate
out 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.
Figure 84 shows the steps to calibrate the power offsets using
the line accumulation mode.
Step 1: If the values change after gain calibration, Step 1, Step 3,
and Step 4 from the gain calibration should be repeated to
configure the LCYCMODE, LINECYC, and MASK registers.
Select Phase A, Phase B, or Phase C for a line period measure-
ment with the FREQSEL[1:0] bits in the MMODE register (0x14).
For example, clearing Bit 1 and Bit 0 selects Phase A for line
period measurement.
Step 2: Set the test system for I
MIN
, V
NOM
, and unity power factor.
Step 3: Reset the interrupt status register by reading RSTATUS
(0x1A).
Step 4: Read all xWATTHR energy registers (0x01 to 0x03) after
the LENERGY interrupt and store the values.
Step 4a: If it is not known, the line period is available in the
ADE7758 frequency register, FREQ (0x10). To configure line
period measurement, select the phase for period measurement
in the MMODE[1:0] and set LCYCMODE[7].
Step 5: Calculate the value to be written to the xWATTOS
registers according to the following equations:
TEST
MIN
MIN
ITEST
IMIN
I
TEST
I
II
I
LINECYC
LINECYC
xWATTHRIxWATTHR
Offset
TEST
MIN
–
– ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××
=
(73)
[]
29
2
4
0:11 ×
×
×
=
CLKINAccumTime
Offset
xWATTOS
(74)
where:
AccumTime is defined in Equation 61.
is the value in the energy register at I
TEST
.
is the value in the energy register at I
MIN
.
LINECYC
IMIN
is the number of line cycles accumulated at I
MIN
.
LINECYC
IMAX
is the number of line cycles accumulated at I
MAX
.
TEST
I
xWATTHR
MIN
I
xWATTHR
Step 6: Write to all xWATTOS registers (0x39 to 0x3B).
Step 7: Set the test system for I
MIN
, V
NOM
, and zero power factor
inductive to calibrate VAR gain.
Step 8: Repeat Steps 3, 4, and 5.
Step 9: Calculate the value written to the xVAROS registers
according to the following equations:
TEST
MIN
MIN
ITEST
IMIN
I
TEST
I
II
I
LINECYC
LINECYC
xVARHRIxVARHR
Offset
TEST
MIN
–
– ×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
××
=
(75)
26
2
202
]0:11[
4
0]:[11 ××
×
×
=
FREQ
CLKINAccumTime
Offset
xVAROS (76)
where the FREQ[11:0] register is configured for line period
readings.
Example: Power Offset Calibration Using Line Accumulation
This example only shows Phase A of the phase active power
offset calibration. Both active and reactive power offset for
all phases can be calibrated simultaneously using the method
explained in the Power Offset Calibration Using Line
Accumulation section.
For this example, I
MIN
= 50 mA, I
TEST
= 10 A, V
NOM
= 220 V,
V
FULLSCALE
= 500 V, I
FULLSCALE
= 130 A, MC = 3200 impulses/kWh,
Power Factor = 1, Frequency = 50 Hz, and CLKIN = 10 MHz.
Also, LINECYC
ITEST
= 0x800 and LINECYC
IMIN
= 0x4000.
After accumulating over 0x800 line cycles for gain calibration at
I
TEST
, the example ADE7758 meter shows 14804d in the
AWATTHR (0x01) register. At I
MIN
, the meter shows 592d in the
AWATTHR register. By using Equation 73, this is equivalent to
0.161 LSBs of offset; therefore, using Equation 61 and Equation 74,
the value written to AWATTOS is 0d.
0.16
10–0.05
0.05
0x800
0x4000
14804–10592
=
×
⎟
⎠
⎞
⎜
⎝
⎛
××
=
Offset
s64.45
3
106.92085
1
2
40000
6
=
×
××
×
×
=
−
AccumTime