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ST10F271Z1 Datasheet, PDF (148/185 Pages) STMicroelectronics – 16-bit MCU with 128 Kbyte Flash memory and 12 Kbyte RAM
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Electrical characteristics
ST10F271Z1
RL sizing is obtained:
10 ⋅ τ2 = 10 ⋅ RL ⋅ (CS + CP1 + CP2) ≤ TS
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):
VA2 ⋅(CS + CP1 + CP2 + CF) = VA⋅CF + VA1⋅(CP1 + CP2 + CS)
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 45).
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 45. Anti-aliasing filter and conversion rate
Analog Source Bandwidth (VA)
Noise
TC ≤ 2 RFCF (Conversion Rate vs. Filter Pole)
fF = f0 (Anti-aliasing Filtering Condition)
2 f0 ≤ fC (Nyquist)
f0
f
Anti-Aliasing Filter (fF = RC Filter pole) Sampled Signal Spectrum (fC = conversion Rate)
fF
f
f0
fC
f
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
voltage on CS:
--V-----A---- = --------C-----P----1-----+-----C----P-----2-----+-----C----F----------
VA2 CP1 + CP2 + CF + CS
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