ST10F276 datasheet(191/229 Pages) STMICROELECTRONICS

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ST10F276

Electrical characteristics

191/229

A first and quick charge transfer from the internal capacitances CP1 and CP2 to the

sampling capacitance CS occurs (CS is supposed initially completely discharged):

Considering a worst case (since the time constant in reality would be faster) in which

CP2 is reported in parallel to CP1 (call CP = CP1 + CP2), the two capacitances CP and

CS are in series and the time constant is:

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 also be robust 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 also redistributed on CS, determining a new value of the

voltage VA1 on the capacitance according to the following equation:

A second charge transfer also involves 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 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

τ1 RSW RAD

()

CP CS

⋅

CP CS

-----------------------

⋅

τ1 RSW RAD

()

⋅

<< TS

VA1 CS CP1 CP2

()

⋅

VA CP1 CP2

()

⋅

τ2 RL

CS CP1 CP2

()

⋅

τ2

⋅

10 R

⋅ L

CS CP1 CP2

()

≤

⋅

VA2 CS CP1 CP2 CF

+++

()

⋅

VACF

⋅

VA1

CP1 CP2

+CS

()

⋅

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