PXD10 datasheet(98/130 Pages) FREESCALE

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Electrical characteristics

PXD10 Microcontroller Data Sheet, Rev. 1

Freescale Semiconductor

(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

Eqn. 8

Equation 8 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:

Eqn. 9

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 10:

Eqn. 10

•

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:

Eqn. 11

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:

Eqn. 12

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 13 must be respected (charge balance assuming now CS already charged at

VA1):

Eqn. 13

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

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PXD10 Datasheet(PDF) 98 Page - Freescale Semiconductor, Inc

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