ST10F276 datasheet(192/229 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F276

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time constant of the filter RFCF is definitely much higher than the sampling time TS, so the

charge level on CS cannot be modified by the analog signal source during the time in which

the sampling switch is closed.

Figure 49.

Anti-aliasing filter and conversion rate

The considerations above lead to impose new constraints to the external circuit, to reduce

the accuracy error due to the voltage drop on CS; from the two charge balance equations

above, it is simple to derive the following relation between the ideal and real sampled volt-

age on CS:

From this formula, in the worst case (when VA is maximum, that is for instance 5V), assum-

ing to accept a maximum error of half a count (~2.44mV), it is immediately evident that a

constraint is on CF value:

The next section provides an example of how to design the external network, based on

some reasonable values for the internal parameters and on a hypothesis on the characteris-

tics of the analog signal to be sampled.

Analog source bandwidth (VA)

Sampled signal spectrum (fC = conversion Rate)

Anti-aliasing filter (fF = RC Filter pole)

2 f0

≤ fC (Nyquist)

fF = f0 (Anti-aliasing Filtering Condition)

≤ 2 RFCF (Conversion rate vs. filter pole)

Noise

VA2

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

CP1 CP2

+CF

CP1 CP2

+CF CS

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

CF 2048 CS

⋅

ST10F276 Datasheet(PDF) 192 Page - STMicroelectronics