AN3137 datasheet(32/42 Pages) STMICROELECTRONICS

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Methods for precision improvement

AN3137

32/42

Doc ID 16983 Rev 3

After simplification we obtain the final formula for optimal Cext:

and the corresponding waiting time between conversions:

Practically the firmware must not program the ADC in continuous mode but only in single

mode and must ensure that there will be a time gap between conversions with duration

equal to tC. This waiting time is the software change which must be applied together with the

hardware change (adding an external capacitor Cext).

Without implementation of tC waiting time in software (for instance, running a conversion just

after the first one) the external capacitor Cext will cyclically charge from the Csh capacitor.

After a lot of cycles the voltage on Cext will reach a quite high error value (as previously

shown in Figure 17).

A practical example of implementation for STM8L/STM8AL ADC is shown below:

Csh = 16 pF

..... ADC property

Rin = 150 k

..... signal source property

Umax = 4096 lsb

..... ADC property

Ulsb = 0.5 lsb

..... required precision

–

In 1

ext

-----------

max

lsb

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

–

ext

-----------

max

lsb

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

–

ext

max

lsb

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

–

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

158

max

lsb

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

⋅

≈

ext

⋅

()

–

In 1

158

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

–

⋅

ext

⋅

()

≈≈

ext

158

max

lsb

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

⋅

158

16pF

4096

0.5

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

⋅⋅

207nF

220nF

⇒

≈

⋅

()

–

In 1

ext

-----------

max

lsb

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

–

⋅

150k

Ω 220nF

⋅

()

–

In 1

16pF

220nF

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

4096

0.5

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

–

⋅

29891

μs30ms

⇒

≈

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