ST10F271 datasheet(138/173 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F271

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and CS were in parallel to CP1 (since the time constant in reality would be faster), the

time constant is:

In this case, the time constant depends on the external circuit: in particular imposing

that the transient is completed well before the end of sampling time TS, a constraints on

RL sizing is obtained:

Of course, RL shall be sized also according to the current limitation constraints, in

combination with RS (source impedance) and RF (filter resistance). Being CF

definitively bigger than CP1, CP2 and CS, then the final voltage VA2 (at the end of the

charge transfer transient) will be much higher than VA1. The following equation must be

respected (charge balance assuming now CS already charged at VA1):

The two transients above are not influenced by the voltage source that, due to the presence

of the RFCF filter, is not able to provide the extra charge to compensate the voltage drop on

CS with respect to the ideal source VA; the time constant RFCF of the filter is very high with

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

Figure 42).

Calling f0 the bandwidth of the source signal (and as a consequence the cut-off frequency of

the anti-aliasing filter, fF), according to Nyquist theorem the conversion rate fC must be at

least 2f0; it means that the constant time of the filter is greater than or at least equal to twice

the conversion period (TC). Again the conversion period TC is longer than the sampling time

TS, which is just a portion of it, even when fixed channel continuous conversion mode is

selected (fastest conversion rate at a specific channel): in conclusion it is evident that the

time constant of the filter RFCF is definitively 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 42.

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

()

⋅

10 R

⋅

()

≤

⋅

+++

()

⋅

()

⋅

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)

TC ≤ 2 RFCF (Conversion Rate vs. Filter Pole)

Noise

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