AN2834 datasheet(40/45 Pages) STMICROELECTRONICS

40 / 45 page

How to get the best ADC accuracy

AN2834

40/45

DocID15067 Rev 2

The formulas below show how to choose the optimal Cext value: signal bandwidth in

correlation with sample time. Signal bandwidth is characterized by an “external” timing

constant, so optimal solution is to charge Cext during tC:

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 adding of 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 be cyclically charged 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 36).

A practical example of implementation for STM32L1xx 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

Rin Cext





Rin Cext





Rin Cext





–

In 1

Csh

Cext

-----------

Umax

Ulsb

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

–



–

In 1

Csh

Cext

-----------

Umax

Ulsb

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

–

Csh

Cext

-----------

Umax

Ulsb

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

–

Cext

Csh

Umax

Ulsb

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

–

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

158



Csh

Umax

Ulsb

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





Rin Cext





–

In 1

158



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

–



Rin Cext









Cext

158



Csh

Umax

Ulsb

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



158



16pF

4096

0.5

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



207nF

220nF





Rin Csh





–

In 1

Csh

Cext

-----------

Umax

Ulsb

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

–



150k

 220nF





–

In 1

16pF

220nF

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

4096

0.5

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

–



29891

s30ms





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