Microcontroller User's Manual

27-74 MCF5282 User’s Manual MOTOROLA
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
, R
F
, and C
F
in the user's external circuitry determine the length of time
required to charge C
F
to the source voltage level (V
SRC
). At time t = 0, V
SRC
changes in
Figure 27-53 while S1 is open, disconnecting the internal circuitry from the external
circuitry. Assume that the initial voltage across C
F
is 0. As C
F
charges, the voltage across
it is determined by the equation, where t is the total charge time:
As t approaches infinity, V
CF
will equal V
SRC
. (This assumes no internal leakage.) With
10-bit resolution, 1/2 of a count is equal to 1/2048 full-scale value. Assuming worst case
(V
SRC
= full scale), Table 27-24 shows the required time for C
F
to charge to within 1/2 of
a count of the actual source voltage during 10-bit conversions. Table 27-24 is based on the
RC network in Figure 27-53.
NOTE
The following times are completely independent of the A/D
converter architecture (assuming the QADC is not affecting the
charging).
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.
Table 27-24. External Circuit Settling Time to 1/2 LSB
Filter Capacitor (CF)
Source Resistance (R
F
+ R
SRC
)
100 Ω 1 kΩ 10 kΩ 100 kΩ
1 µF 760 µs 7.6 ms 76 ms 760 ms
0.1 µF 76 µs 760 µs 7.6 ms 76 ms
0.01 µF7.6 µs 76 µs 760 µs7.6 ms
0.001 µF 760 ns 7.6 µs 76 µs 760 µs
100 pF 76 ns 760 ns 7.6 µs 76 µs
VCF = VSRC (1 –e–t/(RF + RSRC) CF)