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Signal Connection Considerations
• The external capacitor is perfect (no leakage, no significant dielectric absorption
characteristics, etc.).
• All parasitic capacitance associated with the input signal is included in the value of
the external capacitor.
• Inductance is ignored.
• The "on" resistance of the internal switches is 0 ohms and the "off" resistance is
infinite.
27.9.7.1 Settling Time for the External Circuit
The values for R
SRC
required to charge C
Figure 27-53 while S1 is open, disconnecting the internal circuitry from the external
circuitry. Assume that the initial voltage across C
it is determined by the equation, where t is the total charge time:
As t approaches infinity, V
10-bit resolution, 1/2 of a count is equal to 1/2048 full-scale value. Assuming worst case
(V
= full scale), Table 27-24 shows the required time for C
SRC
a count of the actual source voltage during 10-bit conversions. Table 27-24 is based on the
RC network in Figure 27-53.
The following times are completely independent of the A/D
converter architecture (assuming the QADC is not affecting the
charging).
Table 27-24. External Circuit Settling Time to 1/2 LSB
Filter Capacitor (CF)
The external circuit described in Table 27-24 is a low-pass filter. Measurements of an AC
component of the external signal must take the characteristics of this filter into account.
27-74
, R
, and C
in the user's external circuitry determine the length of time
F
F
to the source voltage level (V
F
VCF = VSRC (1 –e–t/(RF + RSRC) CF)
will equal V
CF
1 µF
0.1 µF
0.01 µF
0.001 µF
100 pF
MCF5282 User's Manual
). At time t = 0, V
SRC
is 0. As C
F
. (This assumes no internal leakage.) With
SRC
NOTE
Source Resistance (R
100 Ω
1 kΩ
760 µs
7.6 ms
76 µs
760 µs
7.6 µs
76 µs
7.6 µs
760 ns
76 ns
760 ns
changes in
SRC
charges, the voltage across
F
to charge to within 1/2 of
F
+ R
)
F
SRC
10 kΩ
100 kΩ
76 ms
760 ms
7.6 ms
76 ms
760 µs
7.6 ms
76 µs
760 µs
7.6 µs
76 µs
MOTOROLA
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