Datasheet

ADE7753
Rev. C | Page 46 of 60
Apparent Energy
Voltage rms compensation is done after the LPF3 filter (see
Figure 56).
Apparent energy gain calibration is provided for both meter-to-
meter gain adjustment and for setting the VAh/LSB constant.
VRMS = VRMS0 + VRMSOS (64)
VAENERGY =
⎟
⎠
⎞
⎜
⎝
⎛
+××
12
2
1
1 VAGAIN
VADIV
VAENERGY
initial
(68)
where:
VRMS0 is the rms measurement without offset correction.
VRMS is linear from full-scale to full-scale/20.
VADI V is similar to the CFDEN for the watt hour calibration. It
should be the same across all meters and determines the VAh/LSB
constant. VAGA IN is used to calibrate individual meters.
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 the smallest linear VRMS
reading is at V
nominal
/10.
VRMSOS =
12
1221
VV
VRMSVVRMSV
−
×−×
(65)
Apparent energy gain calibration should be performed before
rms offset correction to make 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 at the same time as the watt-hour gain calibration using
line cycle accumulation. In this case, LAENERGY and
LVA E NE RG Y , the line cycle accumulation apparent energy
register, are both read following the line cycle accumulation
interrupt. Figure 87 shows a flowchart for calibrating active and
apparent energy simultaneously.
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, the voltage channel offset register, CH2OS, can be
used to correct the VRMS offset.
Current rms compensation is performed before the square root:
IRMS
2
= IRMS0
2
+ 32768 × IRMSOS (66)
VAG AI N = INT
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
×
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−
12
)(
)(
21
nominalIB
expectedIB
LVAENERGY
LVAENERGY
(69)
where IRMS0 is the rms measurement without offset correction.
The current rms calculation is linear from full-scale to full-
scale/100.
LVA E NE RG Y
IB(expected)
=
INT
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
×
×
×
(s)
s/h3600
timeonAccumulati
constant
LSB
VAh
IV
B
nominal
(70)
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 the smallest linear IRMS reading is at
I
MAX
/50.
IRMSOS =
2
1
2
2
2
1
2
2
2
2
2
1
II
IRMSIIRMSI
−
×
−
×
×
32768
1
(67)
The accumulation time is determined from Equation 37 and the
line period can be determined from the PERIOD register accord-
ing to Equation 38. The VAh represented by the VAENERGY
register is
where IRMS
1
and IRMS
2
are rms register values without offset
correction for input I
1
and I
2
, respectively.
VAh = VAENERGY × VAh /LSB constant (71)
The VAh /LSB constant can be verified using this equation:
LVAENERG
Y
timeonAccumulati
VA
constant
LSB
VAh
3600
(s)
×
=
(72)