AN2299

Fast digital calibration procedure

Rev2

13/26

3.3

Calibration procedure

3.3.1

Working point setting

According to what pointed out previously, the device can be calibrated in a single point.

Thus, voltage and current nominal values must be defined before running the calibration

procedure, for example:

●

Line RMS voltage:

–

Vn=230V

●

Line RMS current:

–

In=5A

The following other parameters and constants of STPM01 (and relative tolerances) are also

known:

As shown in Table 3., only analog parameters are object of calibration because they

introduce a certain error. Voltage ADC amplification Av is constant, while Ai is chosen

according to used sensors.

The calibration procedure has as final result the correction parameters, called Kv and Ki,

which applied to STPM01 voltage and current measures compensate small tolerances of

analog components that affect energy calculation.

Since Kv and Ki calibration parameters are the decimal representation of the corresponding

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) bits values are obtained.

In the following procedure CHV, CHP and CHS will be indicated as Cv and Ci.

Note:

For more info see paragraph "Two current channels calibration".

Table 3.

Metering constants

Parameter

Value

Tolerance

Internal reference voltage

1.23V

±2%

Internal Calculation Frequency

±50 ppm

Amplification of voltage ADC

4

±1%

Amplification of current ADC

8, 16, 24, 32

±2%

Gain of differentiator

0,6135

Gain of integrator

0,815

Gain of decimation filter

1.004

RMS Voltage register length

RMS Current register length

Constant