ST10F296 datasheet(309/346 Pages) STMICROELECTRONICS

309 / 346 page

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Transient periods one and two are not influenced by the voltage source that cannot provide

the extra charge to compensate for the voltage drop on CS with respect to the ideal source

VA (due to the presence of the RFCF filter). The time constant RFCF of the filter is very high

with respect to the sampling time (TS). The filter is typically designed to be anti-aliasing (see

Figure 101).

If f0 is the bandwidth of the source signal (and consequently is also the cut-off frequency of

the anti-aliasing filter, fF), then according to Nyquist’s theorem, the conversion rate, fC, must

be at least 2f0. This means that the constant time of the filter is greater than or equal to twice

the conversion period (TC). The conversion period, TC, is longer than the sampling time, TS,

even when fixed channel continuous conversion mode is selected (the fastest conversion

rate at a specific channel). In conclusion, the time constant of the filter RFCF is 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 101. Anti-aliasing filter and conversion rate

≤2 RFCF (conversion rate vs. filter pole).

fF = f0 (anti-aliasing filtering condition).

2 f0

≤fC (Nyquist’s theorem).

The considerations above impose new constraints on the external circuit. Accuracy error,

due to the voltage drop on CS, must be reduced. Based on Equation 30 and Equation 31

above, Equation 32 is derived to explain the relationship between the ideal and real sampled

voltage on CS.

Equation 32

In the worst case scenario (VA = 5 V), Equation 32 assumes a maximum error of half a count

(~2.44 mV) which leads to a constraint on the CF value as shown in Equation 33.

Equation 33

Analog source bandwidth (VA)

Sampled signal spectrum (fC = conversion rate)

Anti-aliasing filter (fF = RC filter pole)

Noise

⁄

()

⁄

2048

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