MPC5643LF0MLQ8 datasheet(74/98 Pages) FREESCALE

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MPC5643L Microcontroller Data Sheet, Rev. 3

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

Figure 23. Transient Behavior during Sampling Phase

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

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:

VA1

VA2

VCS

Voltage Transient on CS

ΔV < 0.5 LSB

1 < (RSW + RAD) CS << TS

2 = RL (CS + CP1 + CP2)

()

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()

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()

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()

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()

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MPC5643LF0MLQ8 Datasheet(PDF) 74 Page - Freescale Semiconductor, Inc

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