STPM32 datasheet(78/121 Pages) STMICROELECTRONICS

78 / 121 page

Application design and calibration

STPM32, STPM33, STPM34

78/121

DS10272 Rev 8

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 on page 48 and Section 8.4.7 on page 50 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



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

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.

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