Datasheet

Theory of operation STPM10
44/53 Doc ID 17728 Rev 4
Let's consider the basic information needed to start the calibration procedure:
The typical STPM10 parameters and constants are also known (see Table 14).
As shown in Tabl e 14 , only analog parameters are the object of calibration because they
introduce a certain error. Voltage ADC amplification A
v
is constant, while A
i
is chosen
according to the sensors used.
The calibration algorithm first calculates the voltage divider ratio and, as a final result, the
correction parameters, called K
v
and K
i
, which applied to STPM10 voltage and current
measures compensate the small tolerances of the analog components that affect energy
calculation.
Since K
v
and K
i
calibration parameters are the decimal representation of the corresponding
configuration bytes CHV and CHP or CHS (respectively, the voltage channel, primary
current channel and secondary current channel calibration bytes), at the end of calibration,
CHV and CHP or CHS (according to the current channel under calibration, primary or
secondary, respectively) the bits' values are obtained.
In the following procedure CHV, CHP and CHS are indicated as C
v
and C
i
.
Through hard-wired formulas, K
v
and K
i
tune measured values varying from 0.75 to 1, in 256
steps, according to the value of C
v
and C
i
(from 0 to 255).
Table 13. Working point settings
Parameter Value
Line RMS voltage V
n
230 V
Line RMS current I
n
5 A
Power sensitivity P LED: P=128000 pulses/kWh
Shunt sensor K
S
0.42 mv/A
Table 14. Device constants
Parameter Value Tolerance
Internal reference voltage V
BG
1.23 V ± 2%
Internal calculation frequency f
M
2
23
Hz ± 50 ppm
Amplification of voltage ADC A
V
4± 1%
Amplification of current ADC A
I
8, 16, 24, 32 ± 2%
Gain of differentiator G
DIF
0.6135
Gain of integrator G
INT
0.815
Gain of decimation filter G
DF
1.004
RMS voltage record length B
V
2
11
RMS current record length B
I
2
16
Constant D
UD
2
17