Motorola MCF5282, MCF5281 user manual Settling Time for the External Circuit

Models: MCF5282 MCF5281

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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.

27.9.7.1Settling Time for the External Circuit

The values for RSRC, RF, and CF in the user's external circuitry determine the length of time required to charge CF to the source voltage level (VSRC). At time t = 0, VSRC changes in Figure 27-53while S1 is open, disconnecting the internal circuitry from the external circuitry. Assume that the initial voltage across CF is 0. As CF charges, the voltage across it is determined by the equation, where t is the total charge time:

VCF = VSRC (1 –e–t/(RF + RSRC) CF)

As t approaches infinity, VCF will equal VSRC. (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 (VSRC = full scale), Table 27-24shows the required time for CF to charge to within 1/2 of a count of the actual source voltage during 10-bit conversions. Table 27-24is 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).

Table 27-24. External Circuit Settling Time to 1/2 LSB

Filter Capacitor (CF)

 

Source Resistance (RF + RSRC)

 

 

 

 

 

 

 

100

 

1 k

10 k

 

100 k

 

 

 

 

 

 

 

 

 

 

1 ∝F

760 ∝s

 

7.6 ms

76 ms

 

760 ms

 

 

 

 

 

 

 

0.1 ∝F

76 ∝s

 

760 ∝s

7.6 ms

 

76 ms

 

 

 

 

 

 

 

0.01 ∝F

7.6 ∝s

 

76 ∝s

760 ∝s

 

7.6 ms

 

 

 

 

 

 

 

0.001 ∝F

760 ns

 

7.6 ∝s

76 ∝s

 

760 ∝s

 

 

 

 

 

 

 

100 pF

76 ns

 

760 ns

7.6 ∝s

 

76 ∝s

 

 

 

 

 

 

 

The external circuit described in Table 27-24is a low-pass filter. Measurements of an AC component of the external signal must take the characteristics of this filter into account.

27-74

MCF5282 User’s Manual

MOTOROLA

Page 658
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Motorola MCF5282, MCF5281 user manual Settling Time for the External Circuit, External Circuit Settling Time to 1/2 LSB