AN2834 datasheet(24/45 Pages) STMICROELECTRONICS

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How to get the best ADC accuracy

AN2834

24/45

DocID15067 Rev 2

Note:

The voltage on VREF+ may range between 2.4 V and VDDA.

Figure 23. Selecting the reference voltage

Using a preamplifier

If the measured signal is too small (in comparison with the ADC range) then it is good to use

an external preamplifier. This method can be implemented with STM32x devices delivered

in all packages, and more specifically so in packages that do not have the VREF+ input.

For example, if the measured signal varies between 0 V to 1 V and VDDA is 3 V, the signal

can be amplified so that its peak-to-peak amplitude is similar to the VDDA value. The gain

will then be equal to 3. Figure 24 illustrates this example.

This amplifier can adapt the input signal range to the ADC range. It can also insert offsets

between the input signal and the ADC input. Care must be taken in the preamplifier design

not to generate additional errors (for example additional offset, amplifier gain stability,

linearity, frequency response, etc.).

Figure 24. Preamplification

Digital

Output

4095

2.5

------------- V

Digital

Output

4095

3.3

------------- V

2.5 V

(3102h)

VREF+ = 3.3 V

(4095h)

unused ADC

transition range

VREF+ = 2.5 V

(4095h)

Change VREF+

value from

3.3 V to 2.5 V

ai15600

Digital

Output

4095

Digital

Output

4095

------------- V

1 V

(1365h)

VDDA = 3 V

(4095h)

unused ADC

transition range

Input signal max

value is 3 V

(4095h)

G = 3

ai15601

Before amplification

After amplification

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