MPC5744P datasheet(82/114 Pages) FREESCALE

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MPC5744P Data Sheet, Rev. 0.3

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

Figure 30. 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. 10

Equation 10 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. 11

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 12:

Eqn. 12

•

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. 13

VA1

VA2

VCS

Voltage Transient on CS

ΔV < 0.5 LSB

τ1 < (RSW + RAD) CS << TS

τ2 = RL (CS + CP1 + CP2)

()

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()

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()

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()

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()

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