AN3398

Calibration flow chart

Doc ID 018812 Rev 1

5/11

Only analog parameters are objects of calibration because they introduce a certain error.

Voltage ADC amplification Av is constant, while Ai is chosen according to the used sensors.

measurements, compensate small tolerances of analog components that affect energy

calculations.

representation of the corresponding configuration bytes CVR, CVS, CVT and CIR, CIS, CIT,

CIN, the values of those bits are obtained at the end of calibration.

In the following procedure CVR, CVS, and CVT are indicated as Cv; CIR, CIS, CIT, and CIN

indicated as Ki.

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).

If PM=1, two bits are appended to each Cv and Ci (see the STPMC1 datasheet for details),

and the corresponding tunings vary from 0.875 to 1 in 1024 steps.

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

range, therefore obtaining a calibration range of ± 12.5% (± 6.25% when PM is set) per

voltage or current channel:

In this way it is possible to tune Kv and Ki having a precise measurement: for example, with

PM=0, 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 the information above, the following formulas, which relate Kv,i and Cv,i to each

other are obtained:

Equation 1

Kv,i = (Cv,i/128) * 0.125 + 0.75

Equation 2

Cv,i = 1024 * Kv,i - 768

or when PM = 1

Equation 3

Kv,i = (Cv,i/512) * 0.0625 + 0.875

Equation 4

Table 3.

Calibrator value according to PM

PM

Calibrator value

0

Kv = Ki = 0.875

Ci = Cv = 128

1

Kv = Ki = 0.9375

Ci = Cv = 512