ST10F272 datasheet(145/179 Pages) STMICROELECTRONICS

145 / 179 page

ST10F272

Electrical characteristics

145/179

Supposing to design the filter with the pole exactly at the maximum frequency of the

signal, the time constant of the filter is:

Using the relation between CF and CS and taking some margin (4000 instead of 2048),

it is possible to define CF:

As a consequence of step 1 and 2, RC can be chosen:

Considering the current injection limitation and supposing that the source can go up to

12V, the total series resistance can be defined as:

from which is now simple to define the value of RL:

Now the three element of the external circuit RF, CF and RL are defined. Some

conditions discussed in the previous paragraphs have been used to size the

component, the other must now be verified. The relation which allow to minimize the

accuracy error introduced by the switched capacitance equivalent resistance is in this

case:

So the error due to the voltage partitioning between the real resistive path and CS is

less then half a count (considering the worst case when VA = 5V):

The other conditions to be verified is the time constants of the transients are really and

significantly shorter than the sampling period duration TS:

For complete set of parameters characterizing the ST10F272 A/D Converter equivalent

circuit, refer to Section 24.7: A/D converter characteristics on page 136.

πf

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

15.9

µs

4000 C

⋅

16nF

πf

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

995

Ω 1kΩ

≅

INJ

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

Ω

INJ

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

2.9k

Ω

–

---------------10M

Ω

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

2.35mV

⋅

---LSB

()

2.8ns

⋅

<< T

S = 1µs

⋅

10 R

⋅

() 290ns

⋅

< T

S = 1µs

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