Datasheet

STPM10 Theory of operation
Doc ID 17728 Rev 4 43/53
For the apparent power, another value is produced:
Equation 21
Multiplying Equation 18 and Equation 21, the apparent power is produced:
Equation 22
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.
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