STPM10 datasheet(41/51 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

Doc ID 17728 Rev 3

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The DSP then performs the integration of the computed powers into energies. These

integrators are implemented as up/down counters and they can roll over. 20-bit output buses

of the counters are assigned as the most significant part of the energy data records. It is the

responsibility of an application to read the counters at least every second, to avoid missing

any rollover.

7.24

STPM10 calibration

Energy meters based on the STPM10 device can be calibrated in a fast and simple way. The

calibration is essentially based on the single calibration of the voltage and current channel

considering their RMS values rather than on the frequency of the output pulse signal. When

the two channels are calibrated, all the other measurements are calibrated too. This allows

the calibration to be performed in only one point, thereby shortening the production time of

the meter.

This procedure is possible due to the following key factors:

●

The device comprises two independent meter channels for line voltage and current,

respectively. Each channel includes its own digital calibrator, to adjust the RMS in the

range of ±12.5% in 256 steps, and a digital filter, to remove any signal DC component.

None of the final results are subject to the calibration procedure because they are

achieved from such corrected signals by mathematical modules implemented by hard-

wired DSP.

●

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:

The typical STPM10 parameters and constants are also known (see Table 14).

⋅

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

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