AN2834 datasheet(34/45 Pages) STMICROELECTRONICS

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How to get the best ADC accuracy

AN2834

34/45

DocID15067 Rev 2

3.3.4

ADC calibration

This method requires knowledge of internal ADC structure and how the ADC converter is

implemented inside the microcontroller. This knowledge is necessary in order to design a

physical/mathematical model of the ADC implementation.

A proper physical model (which is usually a schematic diagram) is used as the base for

describing it mathematically. From the mathematical model, each element in the model can

be obtained by a set of equations (for example, resistor/capacitor values which represent bit

weights). To solve these equations, it is necessary to perform a set of practical

measurements and obtain a set of solvable equations.

From the measured values and mathematical computation of the model, all known values of

model elements (resistors, voltages, capacitors,...) can be put into the schematic diagram.

So instead of the ADC schematic with the designed values, you obtain an ADC schematic

with the real values for a given microcontroller.

Computed model parameters are stored in the microcontroller memory after calibration and

used in post-processing to correct ADC values.

3.3.5

Minimizing internal CPU noise

When the CPU operates, it generates a lot of internal and external signal changes which are

transferred into the ADC peripheral through capacitive coupling. This disturbance influences

ADC precision (unpredictable noise due to different microcontroller operations).

To minimize influences of the CPU (and of other peripherals) on ADC, it is necessary to

minimize digital signal changes during sampling and conversion time (digital silence). This is

done using one of the following methods (applied during sampling and conversion time):



minimizing I/O pin changes



minimizing internal CPU changes (CPU stop, wait mode)



stopping clock for unnecessary peripherals (timers, communications...)

AN2834 Datasheet(PDF) 34 Page - STMicroelectronics