STPM10 datasheet(40/47 Pages) STMICROELECTRONICS

40 / 47 page

Theory of operation

STPM10

40/47

DocID17728 Rev 5



The device computes different kinds of energies: active, reactive and apparent. The

active energy is produced without the 2nd harmonic of the line frequency. It also

computes RMS values of measured voltage and current



The device produces an energy output pulse signal, but information can also be read

through the serial port interface (SPI) and communication channel



The device has an embedded memory of 56 bits, used for configuration and

calibration purposes. The values of these bits can be read, or they can be changed

temporarily through the SPI communication channel

Let's consider the basic information needed to start the calibration procedure:

Table 13: Working point settings

Parameter

Value

Line RMS voltage

230 V

Line RMS current

5 A

Power sensitivity

LED: P=128000 pulses/kWh

Shunt sensor

0.42 mv/A

The typical STPM10 parameters and constants are also known (see table below).

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

± 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

As shown in the table above, analog parameters only 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 the

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 bit 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:

STPM10 数据表(PDF) 40 Page - STMicroelectronics

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