AN3322 datasheet(17/38 Pages) STMICROELECTRONICS

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AN3322

Watt-hour meter using the STM32

1.10.1

RMS voltage

The voltage analog circuitry is described in detail in this section. Main phase voltage is

reduced by a voltage divider directly connected to the phase voltage. The voltage divider

consists of a parallel and serial connection of these components:

L1 + R1 + C1||C2 + 12 x R2 and R14

The output voltage from the voltage divider is given by the VOLTDIVIDER that is

computed from:

Equation 7

Rall = R1 + 12 x R2 + R14 = 100 R + 12 x 47 k + 1.5 k = 565.6 k

Equation 8

Voltage division is: VOLTDIVIDER =

[-]

VOLTDIVIDER constant means that the input voltage is divided by number 377.

Voltage represented in volts is computed by:

Equation 9

ULSB in LSB - actual output of ADC channel

UV in volts - output from METER

UV = CU x ULSB

The calibration constant for the voltage circuitry consists of:

Equation 10

CU = VOLTDIVIDER x LSB12-BIT x V_IMPRECISION

where LSB12-BIT is defined in Section 1.1: STM32 ADC parameters, V_IMPRECISION is

the imprecision and tolerance of the components used in analog circuitry, mainly resistors

and soldering.

Since we want the smallest value to be 1 µV, we must multiply everything by 1.000.000

additionally. This also helps to maintain the preservation of the precision.

In order to save the microcontroller computational power and to keep precision as high as

possible, it is beneficial to avoid division operation during all the computations during

a period and do it as the last step after every period computation.

According to Section 1.9: ADC enhancement by oversampling technique, during

oversampling, the sum of 18 samples from ADC results as the VSUM value representing

measured voltage with higher precision.

Computation of the voltage RMS value takes 44 VSUM values. They may only be squared,

then summed, and finally they can be divided by common decimation and RMS constant.

R14

Rall

---------

1,5k



565,6k



-----------------------

0,002652

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