AN3398

Calibration flow chart

Doc ID 018812 Rev 1

9/11

1.1.4

Online procedure

According to the used current sensor and the chosen algorithm, a component (resistor or

current sensor) of the value calculated through the formulas above must be mounted on the

board.

To start the online calibration procedure, the following must be verified:

●

EM is connected to the calibration system and is properly configured according to the

chosen application

●

EM calibrator parameters are preset to initial data

●

Target values of line signals are stable.

A 3-phase voltage signal must be provided to all phases and current signal only to the phase

under calibration.

When the system is connected and powered on, a certain number of readings of the RMS

values must be performed.

Due to the fact that 0.4% of ripple is present in the measured RMS values, more than ten

readings of these values should be gathered each cycle (20 ms at 50 Hz) and the average

values of RMS current and voltage readings AvI and AvV should be computed.

Consequently, having the average values AvI and AvV, a pair of final 8-bit (or 10-bit if PM =

1) calibration data can be calculated as shown below:

Equation 27

Equation 28

1.1.5

Coherency check

We can assume 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% or

±6.25% if PM =1 per voltage or current channel.

If, after the calibration, calculated values for Cv or Ci are out of their 8 or 10-bit range, it may

mean that the application cannot reach the target value of power sensitivity. In this case,

steps 3 and 4 must 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 may mean that the energy meter

board is not good enough to perform such measurements, possibly because the tolerance

of the components is too big, or no care has been taken in the layout phase, so the

application must be re-designed.

Otherwise, if the final calibrator data is written into STPMC1, the average RMS readings are

the target value f.