Datasheet

STPM10 Theory of operation
Doc ID 17728 Rev 4 45/53
To obtain the greatest correction dynamic, initially calibrators are set in the middle of the
range, thus obtaining a calibration range of 12.5% per voltage or current channel:
Calibrator value
K
v
= K
i
= 0.875
C
i
= C
v
= 128
In this way, it is possible to tune K
v
and K
i
to obtain a precise measurement: for example C
v
= 0 generates a correction factor of -12.5% (K
v
= 0.75) and C
v
= 255 determines a
correction factor of +12.5% (K
v
= 1), and so on.
Based on the above, the following formulas relating to K
v
,i and C
v
,i are obtained:
K
v,i
= (C
v,i
/ 128) * 0.125 + 0.75
C
v,i
= 1024 * K
v,i
- 768
The calibration procedure outputs C
v
and C
i
values, which allow the above power sensitivity
of the meter.
This sensitivity is used to calculate target frequency at the LED pin for nominal voltage and
current values:
X
F
= f * 64
with:
f = PM * I
n
* V
n
/ 3600000
From the values above and for both chosen amplification factor A
I
= 32 and initial calibration
data, the following target values can be calculated:
Target RMS reading for a given I
n
:
X
I
= In * K
S
* A
I
* K
i
* G
INT
* G
DF
* G
DIF
* B
I
/ (V
BG
* 1000) = 1573
Target RMS reading for a given V
n
:
X
V
= f * B
V
* B
I
* D
UD
/ (f
M
* X
I
) = 852
The output of the voltage divider is then:
V
DIV
= (X
V
* V
BG
)/ (2 * G
DIF
* A
V
* K
v
* G
DF
* G
INT
* B
V
)= 145.6 mV
Choosing R
2
= 500 Ω (connected between V
I
and V
SS
), the R
1
resistor (connected between
V
LINE
and V
IP
) value is obtained:
R
1
= R
2
* (V
n
- V
DIV
) / V
DIV
= 789.3 Ω
Indicating, with I
A
and V
A
, the real readings on the STPM10 RMS voltage and current
registers, and with X
I
and X
V
ideal values of RMS current and voltage readings already
calculated, the final values for calibrators can be calculated as:
X
V
= (K
v
* V
A
) / 0.875
X
I
= (K
i
* I
A
) / 0.875
If the computed final calibration data would fall out of the calibration data range, the energy
meter should be recognized as bad, or the given presumptions and calculations above
should be checked. Otherwise, if the final data of the calibrators would be written into the
energy meter, the RMS readings should be very close to the target values I and V, and the
frequency of the LED output should be very close to the target value f.