Fast digital calibration procedure

AN2299

18/26

Rev2

Eq. 8, Eq. 9 and Eq. 10 a pair of final 8-bits calibration data can be calculated as shown

below:

3.3.5

Coherency check

One assumes that the EM works correctly and that built-in voltage and current sensors allow

the target power sensitivity constant to be achieved, because the correction parameters Ki

and Kv can tune measured values within the calibration range of

±12.5% per voltage or

current channel.

If after the calibration, calculated values for Cv or Ci are out of range (less than 0 or more

than 255), it could mean that the application cannot reach the target value of power

sensitivity. In this case, steps 3 and 4 have to be repeated choosing a smaller power

sensitivity value. If the values of Cv or Ci are out of range even for small values of PM it

could mean that the energy meter board is not good enough to perform such

measurements, maybe because the tolerance of the components is too big, or no care has

been taken in the layout phase, so the application has to be re-designed.

Otherwise, if calibrators values are written into STPM01, the average RMS readings will be

target value f.

3.4

Two current channels calibration

If the meter uses two current sensors, calibration procedure must be repeated twice, one for

each current channel calibration.

Steps 1 to 3 of fast digital calibration procedure (working point setting, algorithm choice and

calculation of offline parameters) must be followed as explained before and they are

common for both channels.

Before running the online procedure, configuration bit 7 must be cleared (PST2 = 0) to

disable tamper function, then calibration is split for the two channels:

●

Primary channel calibration: mode signal CSEL must be cleared to select primary

channel, then step 4 and 5 (online measure and coherency check) must be executed. If

Cv and Ci are valid values they can be written in bytes CHV and CHP.

●

Secondary channel calibration: mode signal CSEL must be set to select secondary

channel, then step 4 and 5 (online measure and coherency check) must be executed.

This time only Ci, if valid, should be written in byte CHS.

For more info see STPM01 datasheet, "Configuration bytes" paragraph.

Ci

896

Xi

I

A

-----

•

768

–

=

Eq. 29

Cv

896

Xv

V

A

------

•

768

–

=

Eq. 30