PXS20 Datasheet, PDF (90/119 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Electrical characteristics
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 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 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. 5
Equation 5 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. 6
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 7:
VA1  CS + CP1 + CP2 = VA  CP1 + CP2
Eqn. 7
• 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. 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 T_S, a constraints on R_L 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