AN3322 datasheet(6/38 Pages) STMICROELECTRONICS

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Watt-hour meter using the STM32

AN3322

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Watt-hour meter using the STM32

The following sections describe the necessary feasibility study, theory and computation for

the STM32 power meter implementation. The system design, STM32 parameter evaluation,

ADC feature enhancement, and international standards are also covered here.

1.1

STM32 ADC parameters

The most important part of the power meter and its microcontroller used in metering

applications is the embedded ADC. Its parameters are the key to deciding if the

requirements for precision of the implemented meter can be fulfilled (international standards

mentioned in these sections). The first step is to evaluate the parameters given in the ADC

specifications:

For the given STM32 ADC, the specifications and tests show:

Equation 1

Integral non-linearity error is

Equation 2

The resolution of the 12-bit ADC is

= 0.02% of the full scale ADC range.

Equation 3

, then the accuracy = resolution of given ADC.

Equation 4

The accuracy of the 12-bit ADC is

= 0.02% of the full scale ADC range.

1.2

Requirements for meter in Class B

The European standard defining the requirements for electricity metering equipment should

be the source of constraints and limits for the design presented in this application note.

EN 50470-1:2006 and EN 50470-3:2006 are referenced in this case. All the computation is

directed to the precision of Class B.

Table 1. STM32 ADC parameter specifications

Symbol

Parameter

Test conditions

Typ.

Max.

Unit

Total unadjusted error

fPCLK2 = 24 MHz,

fADC = 12 MHz, RAIN < 10 k,

VDDA = 3 to 3.6 V

TA = 25 °C

Measurements made after ADC

calibration

VREF+ = VDDA

±1.3

±2

LSB

Offset error

±1

±1.5

Gain error

±0.5

±1.5

Differential linearity error

±0.7

±1

Integral linearity error

±0.8

±1.5

EINL

12 bit

–

0,8LSB12 bit

–

--------

EINL

12 bit

–

1LSB



--------

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