AN3137 datasheet(30/42 Pages) STMICROELECTRONICS

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Methods for precision improvement

AN3137

30/42

Doc ID 16983 Rev 3

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 17 shows an example

of this scenario where the ADC measurement is performed faster.

Figure 17.

Charging the external capacitor with too short time between conversions

Software change:

The side effect mentioned above can be solved by software. The objective is to create a

delay in order to let Cext discharge through Rin (not measure so often) giving a “discharge

time” between ADC conversions. The “discharge time” (tC) is equal to the transferred charge

from Csh to Cext (charging) and from Cext to Rin (discharging). The assumption is that Cext

>> Csh.

where Umax ..... 4096 lsb voltage level (worst case)

ing

arg

max

⋅

disch

ing

arg

lsb

-----------

in Cext

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

–

∫

⋅

AN3137 Fiches technique(PDF) 30 Page - STMicroelectronics