STPM32 datasheet(54/89 Pages) STMICROELECTRONICS

54 / 89 page

Application design and calibration

The choice of external components in the transduction section of the application is a crucial point in the

application design, affecting the precision and the resolution of the whole system. A compromise has to be found

among the following needs:

Maximizing signal-to-noise ratio in the voltage and current channel

Choosing current-to-voltage conversion ratio kS and the voltage divider ratio in a way that calibration can be

achieved for a given constant pulse CP

Choosing kS to take advantage of the whole current dynamic range according to desired maximum current

and resolution

In this section, the rules for a good application design are described. After the design phase, any tolerance of the

real components from these values or device internal parameter drift can be compensated through calibration.

Please refer to Section 8.4.6 and Section 8.4.7 for device basic calculations.

9.1

Application design

To reach CP target output constant pulse at default LPW value, the analog front end component choice has to

depend on:

•

value of R1 voltage divider resistor, given R2 and kS current sensor sensitivity

•

kS given R1 and R2 voltage divider resistors Calculations for these two methods are developed below:

First method: constant kS

Given R2 (smaller voltage divider resistor), kS (current sensor sensitivity) and the target meter constant pulse CP

(pulses/kWh) as input of the calculations, the value of the voltage divider resistor R1 comes from the following

formula:

Equation 23

R1=R2∙ 1800∙kS∙kint∙AV∙AI∙calV∙calI∙DClk

Vref2∙CP

−1 Ω

(23)

Second method: constant R1

Given R1, R2 (voltage divider resistors) and CP target meter constant pulse (pulses/kWh) as input of the

calculations, the value of kS current sensor comes from the following formula:

Equation 24

kS= Vref2∙CP∙1+R1R2

1800∙AV∙AI∙kint∙calV∙calI∙DClk mV/A

(24)

Note:

The resistor (the former) or the current channel sensor sensitivity (the latter) must be chosen as closer as

possible to the target; small tolerance is compensated by the calibration, to reach the target constant pulse CP.

With the above external components, the maximum measurable values of RMS voltage and current are:

Equation 25

VMAX=12∙ VrefAV∙2∙R1+R2R2 V

(25)

Equation 26

IMAX=12∙ VrefAI∙2∙ 1kS∙kint A

(26)

These values are calculated leaving some available room for the input range with the peak value and minimizing

modulator distortions.

The current resolution value is equal to 4 times LSBIRMS:

Equation 27

IMIN= Vref

AI∙calI∙kint∙kS∙215 A

(27)

Example: current transformer case

STPM32, STPM33, STPM34

Application design and calibration

DS10272 - Rev 13

page 54/89

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