STPM10 datasheet(42/51 Pages) STMICROELECTRONICS

42 / 51 page

Theory of operation

STPM10

42/51

Doc ID 17728 Rev 3

As shown in Table 14, only analog parameters are the object of calibration because they

introduce a certain error. Voltage ADC amplification Av is constant, while Ai is chosen

according to the sensors used.

The calibration algorithm first calculates the voltage divider ratio and, as a final result, the

correction parameters, called Kv and Ki, which applied to STPM10 voltage and current

measures compensate the small tolerances of the 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, the 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) the bits' values are obtained.

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

Through hard-wired 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 value

Kv = Ki = 0.875

Ci = Cv = 128

In this way, it is possible to tune Kv and Ki to obtain 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.

Based on the above, the following formulas relating to Kv,i and Cv,i are obtained:

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

Cv,i = 1024 * Kv,i - 768

Table 14.

Device constants

Parameter

Value

Tolerance

Internal reference voltage

VBG

1.23 V

± 2%

Internal calculation frequency

223 Hz

± 50 ppm

Amplification of voltage ADC

4± 1%

Amplification of current ADC

8, 16, 24, 32

± 2%

Gain of differentiator

GDIF

0.6135

Gain of integrator

GINT

0.815

Gain of decimation filter

GDF

1.004

RMS voltage record length

211

RMS current record length

216

Constant

DUD

217

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