mpc5645s Freescale Semiconductor, Inc, mpc5645s Datasheet - Page 95

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

Calling f
according to the Nyquist theorem the conversion rate f
than or at least equal to twice the conversion period (T
which is just a portion of it, even when fixed channel continuous conversion mode is selected (fastest conversion rate at a
specific channel): in conclusion it is evident that the time constant of the filter R
sampling time T
sampling switch is closed.
The considerations above lead to impose new constraints on the external circuit, to reduce the accuracy error due to the voltage
drop on C
voltage on C
From this formula, in the worst case (when V
half a count, a constraint is evident on C
Freescale Semiconductor
0
S
the bandwidth of the source signal (and as a consequence the cut-off frequency of the anti-aliasing filter, f
; from the two charge balance equations above, it is simple to derive
S
:
S
, so the charge level on C
Anti-Aliasing Filter (f
Analog Source Bandwidth (V
Figure 16. Spectral Representation of Input Signal
f
f
0
F
MPC5645S Microcontroller Data Sheet, Rev. 6
F
F
value:
= RC Filter pole)
S
Noise
cannot be modified by the analog signal source during the time in which the
A
----------- -
V A2
V A
is maximum, that is for instance 5V), assuming to accept a maximum error of
f
f
A
)
=
C
C
C F 2048 C S
must be at least 2f
). Again the conversion period T
------------------------------------------------------- -
C P1
C P1

+
Sampled Signal Spectrum (f
C P2
T
f
2 f
F
+
C
 f
0
2 R
C P2

f
0
(Anti-aliasing Filtering Condition)
+
C
C F
f
(Nyquist
0
+
F
C
0
C F
F
; it means that the constant time of the filter is greater
+
(Conversion Rate vs. Filter Pole)
C S
Equation 12
F
C
C
F
= conversion Rate)
is definitively much higher than the
f
C
C
is longer than the sampling time T
between the ideal and real sampled
Electrical characteristics
f
F
Eqn. 12
Eqn. 13
),
95
S
,