DocID026176 Rev 1

9/20

AN4470

System calibration

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System calibration

The calibration procedure is a key feature among main meter requirements. In fact, it

impacts directly on accuracy, cost, manufacturing and reliability of the meter. After the final

assembly phase, an energy meter requires a calibration procedure due to unknown

tolerances respect to nominal values of the following analog blocks:

•

Voltage and current sensors

•

Oscillator frequency

•

Internal or external reference voltage source

•

Analog-to-digital converter gain

The STPM3x device is composed of independent channels for line voltage and current

respectively. Each channel includes its own 12-bit digital calibrator to adjust the signal

amplitude, digital filter to remove any signal DC component; moreover the device embeds

phase calibration registers for each line and power offset compensation registers.

Calibration is carried out in three steps:

•

Amplitude calibration is mandatory for class accuracy higher than Class 2

•

Phase-shift calibration is mandatory for CT-based meters

•

Power offset calibration (optional for class accuracy higher than 0.2)

To calibrate, the following equipment has to be interfaced:

•

Precision current and voltage source (Gen)

•

Meter under calibration (MUC)

•

Higher class precision energy meter (HPM) (optional)

•

Calibration process controller (CPC)

•

UART/SPI interface to the STPM3x device

Please, see Figure 3:

Figure 3. Meter calibration setup

HPM*

MUC

Calibration

process

controller

M2*

M1*

Interface

SPI / UART

pulses

error

(*)

optional equipment

optional signal

Gen

GIPG0304141302LM