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PXS20 Datasheet, PDF (90/119 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller
Electrical characteristics
VCS
VA
VA2
1
VA1
Voltage Transient on CS
2
V <0.5 LSB
1 < (RSW + RAD) CS << TS
2 = RL (CS + CP1 + CP2)
TS t
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
1
=
RSW
+
RAD

-C----P---------C----S--
CP + CS
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:
1  RSW + RAD  CS « TS
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:
VA1  CS + CP1 + CP2 = VA  CP1 + CP2
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:
2  RL  CS + CP1 + CP2
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:
10  2 = 10  RL  CS + CP1 + CP2  TS
Eqn. 9
PXS20 Microcontroller Data Sheet, Rev. 1
90
Freescale Semiconductor
Preliminary—Subject to Change Without Notice