ST10F272Z2_08 datasheet(152/189 Pages) STMICROELECTRONICS

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Electrical characteristics

ST10F272Z2

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This relation can again be simplified considering only CS as an additional worst

condition. In reality, the transient is faster, but the A/D Converter circuitry has been

designed to be robust also in the very worst case: the sampling time TS is always much

longer than the internal time constant:

The charge of CP1 and CP2 is redistributed also on CS, determining a new value of the

voltage VA1 on the capacitance according to the following equation:

●

A second charge transfer involves also CF (that is typically bigger than the on-chip

capacitance) through the resistance RL: again considering the worst case in which CP2

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 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.

τ1 RSW RAD

()

⋅

VA1 CS CP1 CP2

()

⋅

VA CP1 CP2

()

⋅

τ2 RL

CS CP1 CP2

()

⋅

τ2

⋅

10 R

⋅ L

CS CP1 CP2

()

≤

⋅

VA2 CS CP1 CP2 CF

+++

()

⋅

VA CF

⋅

VA1

CP1 CP2

+CS

()

⋅

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