AN3322 datasheet(18/38 Pages) STMICROELECTRONICS

18 / 38 page

Watt-hour meter using the STM32

AN3322

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DocID18304 Rev 1

A standard computation of the decimation and RMS value computation

Equation 11

V (i) = 1/18 x SUM [U (i)]

URMS = 1/44 sqrt [V (i)]2

Computation with delayed division saving the computational power and the precision

Equation 12

VSUM = SUM [U (i)]

URMS = 1/÷44 x 1/÷18 x sqrt (SUM {SUM [U (i)] (y)}2)

The final value must be divided by 28.1424 (or 28142 if the values are in millivolts) or

multiplied by 0.035533 (or 0.000035533 if the values are in millivolts).

Figure 9. Voltage circuitry - calibration CU constant computation

1.10.2

RMS current - high gained

Current analog circuitry is described in detail in this section. Main phase current is

measured as a voltage on small resistance - shunt. The shunt (R142) converting current to

the voltage to be measured has a value of:

200 µ



400 µ



The voltage from the shunt is amplified by the first stage of the current amplification with

a gain of 50. For current that needs further gain, an additional gain of 10 is added.

If a path with the additional multiplication 10 is used, all the variables have index HG (high

gain). The complete gain is 50 x 10 = 500 times. The value obtained after the amplification

must be multiplied by OPAMPGAINHG in order to get the original value.

Equation 13

OPAMPGAINHG = 0.002

IAHG = CIHG x ILSBHG

where ILSB is the value in LSB from ADC and IA is the current in amperes.

CIHG = SHUNT400 x OPAMPGAINHG x LSB12-BIT x HG_IMPRECISION [A / LSB]

Where SHUNT400 and LSB12-BIT were defined in Section 1.1: STM32 ADC parameters.

HG_IMPRECISION is the imprecision and tolerance of the components used in the HG path

of analog circuitry; mainly resistors and soldering.

AN3322 Datasheet(PDF) 18 Page - STMicroelectronics