Motorola MCF5282 ユーザーズマニュアル

27-74

MOTOROLA

 

• 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.

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.

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.

Ω

Ω

Ω

Ω

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 

µF

7.6 

µs

76 

µs

760 

µs

7.6  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)