STPM01 datasheet(48/56 Pages) STMICROELECTRONICS

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STPM01 Calibration

STPM01

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As shown in Tab. 19, only analog parameter are object of calibration because introduce a

certain error. Voltage ADC amplification Av is constant, while Ai is chosen according to used

sensors.

The calibration algorithm will firstly calculate the voltage divider ratio and, as final result, the

correction parameters, called Kv and Ki, which applied to STPM01 voltage and current

measures compensate small tolerances of analog components that affect energy

calculation.

Since Kv and Ki calibration parameters are the decimal representation of the corresponding

configuration bytes CHV and CHP or CHS (respectively voltage channel, primary current

channel and secondary current channel calibration bytes), at the end of calibration CHV and

CHP or CHS (according to the current channel under calibration, primary or secondary

respectively) bits' values are obtained.

In the following procedure CHV, CHP and CHS will be indicated as Cv and Ci.

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 initially calibrators are set in the middle of the

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

Calibrator’s value

Kv = Ki = 0.875

Ci = Cv = 128

In this way it is possible to tune Kv and Ki having a precise measured: 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:

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

Cv,i = 1024 * Kv,i - 768.

The calibration procedure will output Cv and Ci values that will allow the above power

sensitivity of the meter.

This sensitivity is used to calculate target frequency at LED pin for nominal voltage and

current values:

XF = f * 64;

with:

f = PM * In * Vn / 3600000;

From values above and for both chosen amplification factor AI=32 and initial calibration data,

the following target values can be calculated:

Target RMS reading for given In:

XI = In * KS * AI * Ki * GINT * GDF * GDIF * BI / (VBG * 1000)=1573

Target RMS reading for given Vn:

XV = f * BV * BI * DUD / (fM * XI)=852

The output of the voltage divider is then:

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