SPC56AP54x datasheet(78/105 Pages) STMICROELECTRONICS

78 / 105 page

Electrical characteristics

SPC56xP54x, SPC56xP60x

78/105

DocID18340 Rev 6

In particular two different transient periods can be distinguished:



A first and quick charge transfer from the internal capacitance 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

Equation 5

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 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 constraint 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. 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



RSW RAD



CP CS



CP CS

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





RSW RAD

 C





VA1

CS CP1 CP2





CP1 CP2







CS CP1 CP2







8.5





8.5 RL

CS CP1 CP2







VA2 CS CP1 CP2 CF





VA CF



VA1 CP1 CP2





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