Fast digital calibration procedure

AN2299

14/26

Rev2

Through hardwired formulas, Kv and Ki tune measured values varying from 0,75 to 1 in 256

steps, according to the value of Cv and Ci (from 0 to 255).

To obtain the greatest correction dynamic, calibrators are initially set in the middle of the

range, thus obtaining a calibration range of ±12.5% per voltage or current channel:

●

Calibrators' value

–

Kv = Ki = 0.875

–

Ci = Cv = 128

In this way it is possible to tune Kv and Ki having a precise measurement: for example Cv=0

generates a correction factor of -12.5% (Kv=0.75) and Cv=255 determines a correction

factor of +12.5% (Kv=1), and so on.

According to what pointed out above, the following formulas, which relate Kv,i and Cv,i are

obtained:

and voltage readings (as calculated in the next steps), from what explained the following

can be written:

3.3.2

Algorithm choice

It is possible to use two different algorithms to calculate the parameters to be used during

the calibration:

1. R1 and R2 constants in order to carry out the sensor sensitivity KS;

2. Current sensor sensitivity and R2 constants in order to carry out R1.

The methods are equivalent. The choice is left to the designer.

According to the algorithm chosen, the next calibration step will produce the value of sensor

Algorithms formulas are reported below for both current transformer/shunt and Rogowski

coil current sensors.

Kv i

,

Cv i

,

128

------------

0.125

0.75

+

•

=

Eq.7

Cv i

1024

Kv i

,

•

768

–

=

,

Eq.8

X

V

K

v

V

A

•

0.875

-------------------

=

Eq.9

Eq.10

X

I

Ki

I

A

•

0.875

----------------

=