English
Language : 

PXD20 Datasheet, PDF (93/130 Pages) Freescale Semiconductor, Inc – PXD20 Microcontroller
◁
Electrical characteristics
EXTERNAL CIRCUIT
Source
RS
VA
Filter
RF
Current Limiter
RL
CF
CP1
INTERNAL CIRCUIT SCHEME
VDD
Channel Extended
Selection
Switch
Sampling
RSW1
RSW2
RAD
CP3
CP2
CS
RS Source Impedance
RF Filter Resistance
CF Filter Capacitance
RL Current Limiter Resistance
RSW Channel Selection Switch Impedance (two contributions RSW1 and RSW2)
RAD Sampling Switch Impedance
CP Pin Capacitance (three contributions, CP1, CP2 and CP3)
CS Sampling Capacitance
Figure 15. 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 reported in Figure 14): A charge sharing phenomenon
is installed when the sampling phase is started (A/D switch close).
VCS
VA
VA2
1
Voltage Transient on CS
2
VA1
V <0.5 LSB
1 < (RSW + RAD) CS << TS
2 = RL (CS + CP1 + CP2)
TS t
Figure 16. 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. 6
Equation 6 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:
PXD20 Microcontroller Data Sheet, Rev. 2
Freescale Semiconductor
Preliminary—Subject to Change Without Notice
93
 ▷