AN3322 datasheet(19/38 Pages) STMICROELECTRONICS

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AN3322

Watt-hour meter using the STM32

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

additionally.

No decimation, shifts or division are performed during computation in order to preserve

precision and reduce microcontroller computation. The correct multiplication or division is

done at the end of the whole computational chain.

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

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

Figure 10. Current circuitry - calibration CI constant computation

1.10.3

RMS current - low gained

The basic gain of the current input is 50. There is an additional gain of 1. If the path with the

additional multiplication 1 is used, all the variables have index LG (low gain). The complete

gain is:

Equation 14

OPAMPGAINLG = 0.02

IALG = CILG x ILSBLG [A] = [A/LSB] x [LSB]

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

Equation 15

CILG = SHUNT400 x OPAMPGAINLG x LSB12-BIT x LG_IMPRECISION

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

LG_IMPRECISION is the imprecision and tolerance of the components used in the LG path

of analog circuitry; mainly resistors and soldering.

The final computation is described in Section 1.10.4: Power, Section 1.10.5: Energy, and

Section 1.10.6: Pulse output.

1.10.4

Power

Instant power is computed by:

Equation 16

p = i (t) x u (t)

PW = (CI x CU) x IIN x UIN

PW = CP x IIN x UIN

AN3322 Datasheet(PDF) 19 Page - STMicroelectronics