AN2159 Datasheet, PDF (20/23 Pages) STMicroelectronics – SPI protocol for STPM01/STPM10 metering devices

Converting readings into measured values
AN2159
Table 3. STPMxx internal parameters value
Parameter
Value
Meaning
Ku
0,875
Voltage Calibrator Ideal Value
Ki
0,875
Current Calibrator Ideal Value
Au
4
Voltage Channel Gain
len_i
2^16
RMS Current Register Length
len_u
2^11
RMS Voltage Register Length
len_i_mom
len_u_mom
2^15
2^10
Momentary Current Register Length
Momentary Voltage Register Length
len_f
2^14
Frequency Register Length
Kint_comp
π
1,004
3.14159
Gain of decimation filter
Dint
Dint_p
2^16
2^15
2^23
if oscillator frequency is 4.194 or 8.388 MHz
Fm
8 * 10^6
if oscillator frequency is 4.000 or 8.000 MHz
2
for CT/Shunt
Kut
1
for Rogowski coil
Vref
1.23
Internal voltage reference
Line frequency is calculated as follows:
frequency = 2 * Fm * x_f /( π * len_f * Dint)
kf = frequency / 50
The differentiator and integrator gains are calculated from the frequency result as follows:
Kdif = (2* π * frequency * Dint_p) / (2 * Fm)
Kint = Fm * 2 / (2 * π * frequency * Dint)
Typical values are:
0,6135
Gain of differentiator @ line frequency = 50 Hz
Kdif
0.7359
Gain of differentiator @ line frequency = 60 Hz
0,815
Gain of integrator @ line frequency = 50 Hz
Kint
0.679
Gain of integrator @ line frequency = 60 Hz
20/23
Doc ID 11400 Rev 3