PXS30 Datasheet, PDF (83/139 Pages) Freescale Semiconductor, Inc – PXS30 Microcontroller

Electrical characteristics
A second aspect involving the capacitance network shall be considered. Assuming the three capacitances
C_F, C_P1, and C_P2 are initially charged at the source voltage V_A (please see the equivalent circuit in
Figure 10): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch is
closed).
V_CS
V_A
V_A2
1
V_A1
Voltage Transient on C_S
2
V <0.5 LSB
₁ < (R_SW + R_AD) C_S << T_S
₂ = R_L (C_S + C_P1 + C_P2)
T_S t
Figure 11. Transient behavior during sampling phase
In particular two different transient periods can be distinguished:
• A first and quick charge transfer from the internal capacitance C_P1 and C_P2 to the sampling
capacitance C_S occurs (C_S is supposed initially completely discharged): considering a worst case
(since the time constant in reality would be faster) in which C_P2 is reported in parallel to C_P1 (call
C_P = C_P1 + C_P2), the two capacitances C_P and C_S are in series, and the time constant is:
₁
=
RSW
+
RAD

-C----P-----^----C----S--
CP + CS
Eqn. 12
Equation 12 can again be simplified considering only C_S 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 T_S is always much longer than the internal time constant:
₁  RSW + RAD  CS « TS
Eqn. 13
The charge of C_P1 and C_P2 is redistributed also on C_S, determining a new value of the voltage V_A1
on the capacitance according to Equation 14:
VA1  CS + CP1 + CP2 = VA  CP1 + CP2
Eqn. 14
• A second charge transfer involves also C_F (that is typically bigger than the on-chip capacitance)
through the resistance R_L: again considering the worst case in which C_P2 and C_S were in parallel
to C_P1 (since the time constant in reality would be faster), the time constant is:
₂  RL  CS + CP1 + CP2
Eqn. 15
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 T_S, a constraints on R_L sizing is
obtained:
PXS30 Microcontroller Data Sheet, Rev. 1
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
83