
MPC5604B/C Microcontroller Data Sheet, Rev. 9
Electrical characteristics
Freescale Semiconductor
70
Figure 21. Input equivalent circuit (extended channels)
A second aspect involving the capacitance network shall be considered. Assuming the three capacitances CF, CP1 and CP2 are
initially charged at the source voltage VA (refer to the equivalent circuit in Figure 20): A charge sharing phenomenon is installed when the sampling phase is started (A/D switch close).
Figure 22. Transient behavior during sampling phase
In particular two different transient periods can be distinguished:
RF
CF
RS
RL
RSW1
CP3
CS
VDD
Sampling
Source
Filter
Current Limiter
EXTERNAL CIRCUIT
INTERNAL CIRCUIT SCHEME
CP1
RAD
Channel
Selection
VA
CP2
Extended
RSW2
Switch
RS: Source impedance
RF: Filter resistance
CF: Filter capacitance
RL: Current limiter resistance
RSW1: Channel selection switch impedance (two contributions, RSW1 and RSW2)
RAD: Sampling switch impedance
CP: Pin capacitance (two contributions, CP1, CP2 and CP3)
CS: Sampling capacitance
VA
VA1
VA2
t
ts
VCS
Voltage transient on CS
V <0.5 LSB
1
2
1 < (RSW + RAD) CS << ts
2 = RL (CS + CP1 + CP2)