AN3322 datasheet(10/38 Pages) STMICROELECTRONICS

10 / 38 page

Watt-hour meter using the STM32

AN3322

10/38

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1.4

Error sources on the STM32 meter

There are a lot of sources of error when measuring current and voltage. They can be

compensated in mostly all cases.

ADC linearity: it limits total accuracy compensated by ADC symmetry, see Section 1.5.

Temperature: external voltage reference keeps ADC conversion stable.

Op amp and ADC offsets: they are removed by AC coupling, calibration and digital

filtering.

ADC gain: it is compensated by calibration of the meter.

Thermal noise present on the STM32 meter PCB board: see Section 1.6.

Crosstalk between subsequent channel measurement: compensation is discussed in

Section 1.7.

Linearity of op amps: it is present, not compensated. Selection of a good op amp is

made.

Phase shift between channels; the compensation is shown in Section 2.7.

1.5

Linearity of STM32 ADC and its INL reduction

Increasing the ADC resolution does not result in higher accuracy since the techniques for

obtaining higher resolution can reduce DNL (differential non-linearity) but not INL (integral

non-linearity), see Section 1.9: ADC enhancement by oversampling technique.

The linearity can be enhanced (INL reduction) from 0.7 to 0.17 LSB by the application of

a simple technique to the HW and SW design. Good knowledge of the ADC is assumed for

this technique, described in a separate document supplied on request.

1.6

Thermal noise present on STM32 meter platform

The number of effective bits of a given ADC depends on the presence of noise in the

system, namely on the PCB and microcontroller itself. If the noise is white Gaussian noise

(or similar), the number of effective bits can be recovered by oversampling and decimation

mentioned in Section 1.9. Figure 3 brings a direct noise measurement of the system. The

single voltage level was connected to an input of the ADC on the STM32 power meter board

and a thousand samples were collected. Figure 3 shows a histogram of the values taken.

AN3322 Datasheet(PDF) 10 Page - STMicroelectronics