AN3322 datasheet(9/38 Pages) STMICROELECTRONICS

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AN3322

Watt-hour meter using the STM32

1.3.2

There are two current ranges in the meter

Low gain range (LG): for 100 A range a resolution and accuracy with the

12-bit ADC are:

Imax = 100 x 2 x 1.414 = 300 Apeak-peak

Resolution in A for maximum value: 300A x 0.02% = 60 mA

Resolution in A for RMS value: 100 ARMS x 0.02% = 20 mARMS

Minimal error of measurement of current about:



Starting current:

Ist: 20 mARMS / 20 mARMS = 100% (no precision prescribed)



Minimum current:

Imin: 20 mARMS / 250 mARMS = 8% (1,5% prescribed)



Transition current: Itr: 20 mARMS / 500 mARMS = 4% (1% prescribed)



Reference current: Iref: 20 mARMS / 5 ARMS = 0.4% (1% prescribed)



Maximum current: Imax: 20 mARMS / 100 ARMS = 0.02% (1% prescribed).

Error values are lower than prescribed standards in case of Iref and Imax. Lower currents Imin

and Itr cannot be measured within this LG range.

High gain range (HG): for 10 A range a resolution and accuracy with the

12-bit ADC are:

Imax = 10 x 2 x 1.414 = 28 Apeak-peak

Resolution in A for maximum value: 28 A x 0.02% = 5.8 mA

Resolution in A for RMS value: 10 ARMSx 0.02% = 2.4 mARMS

Minimal error of measurement of current about:



Starting current:

Ist: 2.4 mARMS / 20 mARMS = 12% (no precision prescribed)



Minimum current:

Imin: 2.4 mARMS / 0.5 ARMS = 0.48% (1,5% prescribed)



Transition current: Itr: 2.4 mARMS / 1 ARMS= 0.24% (1% prescribed)



Reference current: Iref: 2.4 mARMS / 5 ARMS = 0.048% (1% prescribed)



Maximum current: Imax: 2.4 mARMS / 10 ARMS = 0.024% (1% prescribed).

Error values are lower than prescribed standards in the case of Ist, Imin, Itr, and Iref. Higher

current Imax cannot be measured within this HG range due to ADC saturation.

From the analysis performed in this section it can be seen that a system using one range for

voltage and two ranges for current measurement can fulfill requirements for the prescribed

precision of the system.

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