Datasheet

Data Sheet ADE7763
Rev. C | Page 43 of 56
Voltage rms compensation is done after the square root.
VRMS = VRMS0 + VRMSOS (60)
where:
VRMS0 is the rms measurement without offset correction.
VRMS is linear from full-scale to full-scale/20.
To calibrate the offset, two VRMS measurements are required,
for example, at V
nominal
and V
nominal
/10. V
nominal
is set at half of the
full-scale analog input range so that the smallest linear VRMS
reading is at V
nominal
/10.
VRMSOS =
12
221
VV
VRMSVVRMSV
−
×−×
1
(61)
where VRMS
1
and VRMS
2
are rms register values without offset
correction for input V
1
and V
2
, respectively.
If the range of the 12-bit, twos complement VRMSOS register is
not enough, use the voltage channel offset register, CH2OS, to
correct the VRMS offset.
Current rms compensation is performed before the square root:
IRMS
2
= IRMS0
2
+ 32768 × IRMSOS (62)
where IRMS0 is the rms measurement without offset correction.
The current rms calculation is linear from full scale to full
scale/100.
To calibrate this offset, two IRMS measurements are required,
for example, at I
b
and I
MAX
/50. I
MAX
is set at half of the full-scale
analog input range so that the smallest linear IRMS reading is at
I
MAX
/50.
IRMSOS =
2
1
2
2
2
1
2
2
2
2
2
1
I
I
IRMSIIRMSI
−
×
−×
×
32768
1
(63)
where IRMS
1
and IRMS
2
are rms register values without offset
correction for input I
1
and I
2
, respectively.
Apparent Energy
Apparent energy gain calibration is provided for both meter-to-
meter gain adjustment and for setting the VAh/LSB constant.
VAENERGY =






+××
12
2
1
1 VAGAIN
VADIV
VAENERGY
initial
(64)
VADIV is similar to the CFDEN for the watt-hour calibration. It
should be the same across all meters and determines the VAh/LSB
constant. VAG AIN is used to calibrate individual meters.
Apparent energy gain calibration should be performed before
rms offset correction to make the most efficient use of the current
test points. Apparent energy gain and watt gain compensation
require testing at I
b
, while rms and watt offset correction require
a lower test current. Apparent energy gain calibration can be
done simultaneously with the watt-hour gain calibration
using line cycle accumulation. In this case, LAENERGY and
LVA EN E RG Y , the line cycle accumulation apparent energy
registers, are both read following the line cycle accumulation
interrupt. Figure 78 shows a flowchart for calibrating active and
apparent energy simultaneously.
VAGAIN = INT








×








−
12
)
(
)
(
2
1
nominal
IB
expected
IB
LVAENERGY
LVAENERGY
(65)
LVA EN E RG Y
IB(expected)
=
INT












×
×
×
)
(
s/h3600
stime
onAccumulati
constant
LSB
VAh
I
V
B
nominal
(66)
The accumulation time is determined from Equation 33, and
the line period can be determined from the period register
according to Equation 34. The VAh represented by the
VAENERGY register is
VAh = VAENERGY × VAh/LSB constant (67)
The VAh/LSB constant can be verified using this equation:
LVAENERGY
stimeonAccumulati
VA
constant
LSB
VAh
3600
)(
×
=
(68)