Motorola MPC533 Reference Manual page 520

QADC64E Integration Requirements
13.7.5.2 Settling Time for the External Circuit
The values for R
SRC
required to charge C
Figure 13-50 while S1 is open, disconnecting the internal circuitry from the external
circuitry. Assume that the initial voltage across C
it is determined by the following 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 13-16 shows the required time for C
SRC
count of the actual source voltage during 10-bit conversions. Table 13-16 is based on the
RC network in Figure 13-50.
The following times are completely independent of the A/D
converter architecture (assuming the QADC64E is not
affecting the charging).
Table 13-16. External Circuit Settling Time to 1/2 LSB (10-Bit Conversions)
Filter Capacitor
(CF)
1 µF
.1 µF
.01 µF
.001 µF
100 pF
The external circuit described in Table 13-16 is a low-pass filter. A user interested in
measuring an AC component of the external signal must take the characteristics of this filter
into account.
13.7.5.3 Error Resulting from Leakage
A series resistor limits the current to a signal, therefore input leakage acting through a large
source impedance can degrade A/D accuracy. The maximum input leakage current is
specified in Appendix E, "Electrical Characteristics." Input leakage is greater at higher
13-76
PRELIMINARY—SUBJECT TO CHANGE WITHOUT NOTICE
, R and C in the external circuitry determine the length of time
F F
to the source voltage level (V
F
V CF =
will equal V
CF
Source Resistance (R
100 Ω
760 µs
76 µs
7.6 µs
760 ns
76 ns
MPC533 Reference Manual
). At time t = 0, V
SRC
is zero. As C
F


t –

 ---------------------------------------------------------- -

(
R F + R SRC
 

V SRC 1 e –



. (This assumes no internal leakage.) With
SRC
NOTE
+ R
F
1 kΩ 10 kΩ
7.6 ms 76 ms
760 µs
7.6 ms
76 µs 760 µs
7.6 µs 76 µs
7.6 µs
760 ns
changes in
SRC
charges, the voltage across
F





)C
 
F




to charge to within 1/2 of a
F
)
SRC
100 kΩ
760 ms
76 ms
7.6 ms
760 µs
76 µs
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