MPC5646C datasheet(75/114 Pages) FREESCALE

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MPC5646C Microcontroller Data Sheet, Rev. 3

Preliminary—Subject to Change Without Notice

Freescale Semiconductor

A second aspect involving the capacitance network shall be considered. Assuming the three capacitances CF, CP1 and CP2 are

initially charged at the source voltage VA (refer to the equivalent circuit reported in Figure 16): A charge sharing phenomenon

is installed when the sampling phase is started (A/D switch close).

Figure 18. Transient behavior during sampling phase

In particular two different transient periods can be distinguished:

1. 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

Eqn. 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:

Eqn. 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:

Eqn. 7

2. 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. 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:

VA1

VA2

VCS

Voltage Transient on CS

V <0.5 LSB





1 < (RSW + RAD) CS << TS



2 = RL (CS + CP1 + CP2)

1 RSW RAD



CP CS



CP CS

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



1 RSW RAD





CS TS



VA1 CS CP1 CP2





VA CP1 CP2





2 RL



CS CP1 CP2





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