AN3398 datasheet(6/11 Pages) STMICROELECTRONICS

6 / 11 page

Calibration flow chart

AN3398

6/11

Doc ID 018812 Rev 1

Cv,i = 8192 * Kv,i - 7168

Indicating, with AvI and AvV, the average values read from the device and with XI and XV the

ideal values of RMS current and voltage readings, the following can be reported:

PM = 0:

Equation 5

XV = (Kv * AvV) / 0.875

Equation 6

XI = (Ki * AvI) / 0.875

PM = 1:

Equation 7

XV = (Kv * AvV) / 0.9375

Equation 8

XI = (Ki * AvI) / 0.9375

1.1.2

Algorithm choice

It is possible to use two different algorithms to calculate the parameters to be used during

calibration:

R1 and R2 constant in order to carry out the sensor sensitivity KS

Current sensor sensitivity and R2 constant in order to carry out R1.

The methods are the same and the choice is left to the designer.

According to the chosen algorithm, the next calibration step produces the value of sensor KS

or resistor R1 to be mounted on the measurement board to achieve calibration.

Algorithm formulas are reported below for both current transformer/Shunt and Rogowski coil

current sensors.

1.1.3

Offline parameter calculations

First of all, it is necessary to determine the target power sensitivity (from the LED pin) to be

achieved with the calibration process, for example:

C = 128000 pulses/kWh

The calibration procedure outputs Cv and Ci values that allow the above power sensitivity of

the meter.

This sensitivity is used to calculate target frequency at the LED pin for nominal voltage and

current values:

Equation 9

XF = f * 64

with:

Equation 10

AN3398 Datasheet(PDF) 6 Page - STMicroelectronics