PXS20 datasheet(90/119 Pages) FREESCALE

90 / 119 page

PXS20 Microcontroller Data Sheet, Rev. 1

Preliminary—Subject to Change Without Notice

Electrical characteristics

Freescale Semiconductor

Figure 10. 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. 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

•

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:

Eqn. 9

VA1

VA2

VCS

Voltage Transient on CS

V <0.5 LSB





1 < (RSW + RAD) CS << TS



2 = RL (CS + CP1 + CP2)







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



























10 R



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