STPM10 datasheet(17/47 Pages) STMICROELECTRONICS

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STPM10

Theory of operation

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Theory of operation

7.1

General operation description

The STPM10 is capable of performing measurements of active, reactive and apparent

energy, RMS and instantaneous voltage and current values, and line frequency

information. Most of the functions are fully programmable using internal configuration bits

accessible through the SPI interface. The STPM10 works as a peripheral in

microcontroller-based metering systems. The ZCR and WDG pins are used to provide

zero-crossing and watchdog information, and the SPI pins are used to communicate with

the microcontroller. The STPM10 includes volatile internal registers that hold the useful

information about the metering system. Two kinds of active energy are available: wide-

band active energy (AW) which includes all harmonic content (also called type 0) and

fundamental active energy (AF), limited to the 1st harmonic (also called type 1). This latter

energy value is obtained by filtering type 0 active energy. Both of the two active energies

are stored in up-down counting accumulator registers with a 20-bit length. Reactive and

apparent energies are also available with a 20-bit accumulation. The STPM10 also

provides the RMS values for voltage and current. Due to the modest dynamic variation of

the voltage, the RMS value is stored with a resolution of 11 bits, while the RMS current

value has a resolution of 16 bits. The instantaneous (momentary) sampled value of voltage

and current are also available with a resolution of 11 and 16 bits, respectively. The line

frequency value is stored with a resolution of 14 bits. Due to the proprietary energy

computation algorithm, the STPM10 calibration is quick and simple, allowing calibration at

only one point over the entire current range. The configuration and calibration parameters

must be downloaded in the internal non-volatile memory of the STPM10 at power-up.

7.2

Analog inputs

The STPM10 has one fully differential voltage input channel and two fully differential

current input channels. The voltage channel consists of a differential amplifier with a gain of

4. The maximum differential input voltage for the voltage channel is ± 0.3 V. The two

current channels are multiplexed (see Section 7.9: "Tamper detection module" for details)

to provide a single input to a preamplifier with a gain of 4. The output of this preamplifier is

connected to the input of a programmable gain amplifier (PGA) with possible gain

selections of 2 and 8. The total gain of the current channels are then 8 and 32. The gain

selections are made by writing to the gain register, and they can be different for the two

current channels. If the tamper function is not used, the secondary current can be disabled.

The maximum differential input voltage is dependent on the selected gain, in accordance

with the table below.

Table 6: Gain of voltage and current channels

Voltage channels

Current channels

Gain

Max. input voltage (V)

Gain

Max. input voltage (V)

±0.30

±0.15

32X

±0.35

The gain register is included in the device configuration register with the address name

PST. The table below shows the gain configuration according to the register values:

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