SPC560B64L5 datasheet(102/134 Pages) STMICROELECTRONICS

102 / 134 page

Electrical characteristics

SPC560B54/6x

102/134

Doc ID 15131 Rev 6

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

Equation 5:

<Cross Refs>Equation 5 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:

Equation 6:

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

voltage VA1 on the capacitance according to <Cross Refs>Equation 7:

Equation 7:

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:

Equation 8:

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:

Equation 9:

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. <Cross Refs>Equation 10 must

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

Equation 10:

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 antialiasing.

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