mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 92

• Manufacturer Part Number: mpc5645s
• Description: Mpc5645s Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

Electrical characteristics
In fact a current sink contributor is represented by the charge sharing effects with the sampling capacitance: C
substantially a switched capacitance, with a frequency equal to the conversion rate of the ADC, it can be seen as a resistive path
to ground. For instance, assuming a conversion rate of 1 MHz, with C
1 / (fc*C
partitioning between this resistance (sampled voltage on C
must be designed to respect the
Equation 5
and R
92
AD
) can be neglected with respect to external resistances.
S
), where fc represents the conversion rate at the considered channel). To minimize the error induced by the voltage
generates a constraint for external network design, in particular on resistive path. Internal switch resistances (R
V
A
Source
R
R
C
R
R
R
C
C
R
S
F
F
L
SW1
AD
P
S
Figure 13. Input Equivalent Circuit (Precise Channels)
S
Equation
EXTERNAL CIRCUIT
Source Impedance
Filter Resistance
Filter Capacitance
Current Limiter Resistance
Sampling Switch Impedance
Pin Capacitance (two contributions, C
Sampling Capacitance
Channel Selection Switch Impedance
MPC5645S Microcontroller Data Sheet, Rev. 6
V A
5:
R
Filter

F
C
R S
-------------------------------------------------------------------------- -
F
+
R F
Current Limiter
+
R L
R
R EQ
S
L
) and the sum of R
+
R SW
P1
and C
C
+
S
P1
P2
equal to 3 pF, a resistance of 330k is obtained (R
R AD
)

V
INTERNAL CIRCUIT SCHEME
S
DD
1
-- - LSB
2
+ R
F
Selection
Channel
+ R
R
SW1
C
L
P2
+ R
SW
Sampling
R
+ R
AD
Freescale Semiconductor
C
AD
S
, the external circuit
S
being
Eqn. 5
EQ
SW
=