Converting readings into measured values

AN2159

20/23

Doc ID 11400 Rev 3

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:

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

2^15

Momentary Current Register Length

len_u_mom

2^10

Momentary Voltage Register Length

len_f

2^14

Frequency Register Length

Kint_comp

1,004

Gain of decimation filter

π

3.14159

Dint

2^16

Dint_p

2^15

Fm

2^23

if oscillator frequency is 4.194 or 8.388 MHz

8 * 10^6

if oscillator frequency is 4.000 or 8.000 MHz

Kut

2

for CT/Shunt

1

for Rogowski coil

Vref

1.23

Internal voltage reference

Kdif

0,6135

Gain of differentiator @ line frequency = 50 Hz

0.7359

Gain of differentiator @ line frequency = 60 Hz

Kint

0,815

Gain of integrator @ line frequency = 50 Hz

0.679

Gain of integrator @ line frequency = 60 Hz