mcf5282 Freescale Semiconductor, Inc, mcf5282 Datasheet - Page 605

• Manufacturer Part Number: mcf5282
• Manufacturer: Freescale Semiconductor, Inc

28.9.7.1 Settling Time for the External Circuit
The values for R
charge C
open, disconnecting the internal circuitry from the external circuitry. Assume that the initial voltage across
C
As t approaches infinity, V
1/2 of a count is equal to 1/2048 full-scale value. Assuming worst case (V
shows the required time for C
conversions.
The external circuit described in
the external signal must take the characteristics of this filter into account.
28.9.7.2 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
“Electrical
range from 125°C to 50°C, the leakage current is halved for every 8°C to 12°C reduction in temperature.
Assuming V
typical input leakage of 200 nA acting through 10 kΩ of external series resistance results in an error of 0.4
count (2.0 mV). If the source impedance is 100 kΩ and a typical leakage of 100 nA is present, an error of
2 counts (10 mV) is introduced.
In addition to internal junction leakage, external leakage (for example, if external clamping diodes are
used) and charge sharing effects with internal capacitors also contribute to the total leakage current.
Table 28-25
source impedance. The error is listed in terms of 10-bit counts.
Freescale Semiconductor
F
is 0. As C
F
to the source voltage level (V
Characteristics”. Input leakage is greater at higher operating temperatures. In the temperature
RH
illustrates the effect of different levels of total leakage on accuracy for different values of
F
Table 28-24
charges, the voltage across it is determined by the equation, where t is the total charge time:
The following times are completely independent of the A/D converter
architecture (assuming the QADC is not affecting the charging).
–V
SRC
RL
Filter Capacitor (CF)
, R
= 5.12 V, 1 count (with 10-bit resolution) corresponds to 5 mV of input voltage. A
F
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3
, and C
0.001 μF
is based on the RC network in
0.01 μF
100 pF
0.1 μF
Table 28-24. External Circuit Settling Time to 1/2 LSB
CF
1 μF
VCF = VSRC (1 –e–t/(RF + RSRC) CF)
F
will equal V
to charge to within 1/2 of a count of the actual source voltage during 10-bit
F
Table 28-24
in the user's external circuitry determine the length of time required to
SRC
SRC
). At time t = 0, V
. (This assumes no internal leakage.) With 10-bit resolution,
is a low-pass filter. Measurements of an AC component of
760 μs
760 ns
100 Ω
7.6 μs
76 μs
76 ns
NOTE
Source Resistance (R
Figure
7.6 ms
760 μs
760 ns
7.6 μs
76 μs
1 kΩ
28-53.
SRC
changes in
Queued Analog-to-Digital Converter (QADC)
7.6 ms
760 μs
10 kΩ
76 ms
7.6 μs
76 μs
F
SRC
+ R
SRC
= full scale),
Figure 28-53
)
100 kΩ
760 ms
7.6 ms
760 μs
76 ms
76 μs
Table 28-24
Chapter 33,
while S1 is
28-67