AN2834 datasheet(39/45 Pages) STMICROELECTRONICS

39 / 45 page

DocID15067 Rev 2

39/45

AN2834

How to get the best ADC accuracy

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 enough

“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:



Ulsb ....... 0.5 lsb voltage level

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

Qcharging = Qdischarging

Simplification of the above formula gives the final formula for the required waiting time

between conversions:

This final formula shows dependency between the external capacitor Cext and the required

waiting time between two conversions if the precision Ulsb is needed.

From the same formula you can see that the argument in logarithm must be positive and

therefore there is a condition for the minimal value of Cext:

Choosing a larger Cext decreases more the time between conversions (tC).

An extra large Cext (Cext>>

) enables sampling more often.

However, increasing Cext limits the frequency bandwidth of measurement signal (increasing

the “external” timing constant Rin . Cext).

Qch ing

arg

Qsh

Csh Umax



Qdisch ing

arg

Ulsb

Rin

-----------

RinCext

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

–





Csh Umax



Ulsb

Rin

-----------

RinCext

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





Rin Cext





–

In 1

Csh

Cext

-----------

Umax

Ulsb

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

–



Csh

Cext

-----------

Umax

Ulsb

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

–



Csh

Cext

-----------

Umax

Ulsb

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



Cext Csh

Umax

Ulsb

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





Csh

Umax

Ulsb

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



AN2834 bảng dữ liệu(PDF) 39 Page - STMicroelectronics

AN2834 bảng dữ liệu(HTML) 39 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45

bảng dữ liệu tải về

Link URL