ST10F276E datasheet(194/235 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F276E

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Doc ID 12303 Rev 3

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

RL sizing is obtained:

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

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

definitely 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, cannot provide the extra charge to compensate for 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 49).

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, meaning 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 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

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10 R

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ST10F276E Datasheet(PDF) 194 Page - STMicroelectronics