AN2834 datasheet(38/45 Pages) STMICROELECTRONICS

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How to get the best ADC accuracy

AN2834

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DocID15067 Rev 2

Workaround for extra high impedance sources

This workaround consists of 2 parts: a hardware change and a software change.

Hardware change:

The hardware change consists of adding a large external capacitor (Cext) to the input pin.

The capacity size connected to the input pin must reach the value that causes the

discharging of the internal sampling capacity Csh to the external capacitor Cext without

increasing the voltage on Cext to more than 0.5 lsb. 

Example:



If the internal capacitor (Csh = 16 pF) is charged to full scale (Umax, which corresponds to

4096 lsb), then the external capacitor Cext must be charged at maximum 0.5 lsb voltage

level (Ulsb) after discharging Csh to it. The capacity of Cext will then be:

The closest larger standard value chosen here is: Cext = 150 nF.

If the internal sampling capacitor Csh is not charged to full voltage range (4096 level) before

sampling, you can calculate the Cext value replacing “4096” in the formula above.

Calculating with 4096 level gives precise measurement results also in the case of ADC input

channels switching (Csh was charged from different ADC input in the previous

measurement).

A side effect of this hardware workaround is the cyclical charging of Cext which must be

taken into account. Each ADC conversion transfers charge from Csh to Cext. One transfer

charges the Cext below 0.5 lsb, as described above, but more transfers can charge Cext to

larger values if it is not discharged between two conversions. Figure 36 shows an example

of this scenario where the ADC measurement is performed faster.

Figure 36. Charging the external capacitor with too short time between conversions

Cext Csh

Umax

Ulsb

--------------





16pF

4096

0.5

-------------

131nF





Too short

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