mpc5604e Freescale Semiconductor, Inc, mpc5604e Datasheet - Page 38

• Manufacturer Part Number: mpc5604e
• Description: Mpc5604e Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

Electrical characteristics
be measured. The filter at the input pins must be designed taking into account the dynamic characteristics of the input signal
(bandwidth) and the equivalent input impedance of the ADC itself.
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 4
and R
A second aspect involving the capacitance network shall be considered. Assuming the three capacitances C
initially charged at the source voltage V
is installed when the sampling phase is started (A/D switch close).
38
AD
) can be neglected with respect to external resistances.
S
generates a constraint for external network design, in particular on resistive path. Internal switch resistances (R
), where fc represents the conversion rate at the considered channel). To minimize the error induced by the voltage
V
A
Source
R
R
C
R
R
R
C
C
R
S
F
F
L
SW1
AD
P
S
S
Equation
EXTERNAL CIRCUIT
Source Impedance
Filter Resistance
Filter Capacitance
Current Limiter Resistance
Channel Selection Switch Impedance
Sampling Switch Impedance
Pin Capacitance (two contributions, C
Sampling Capacitance
Preliminary—Subject to Change Without Notice
MPC5604E Microcontroller Data Sheet, Rev. 2
V A
A
4:
(refer to the equivalent circuit reported in
R
Filter
Figure 10. Input equivalent circuit

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
+
P1
S
P2
R AD
equal to 3 pF, a resistance of 330 k is obtained (R
)

V
INTERNAL CIRCUIT SCHEME
S
DD
1
-- - LSB
2
+ R
Figure
F
Selection
Channel
+ R
R
SW1
C
L
P2
10): A charge sharing phenomenon
+ R
SW
Sampling
R
+ R
AD
Freescale Semiconductor
AD
, the external circuit
F
, C
S
P1
being
and C
Eqn. 4
P2
are
SW
EQ