Motorola ColdFire MCF5281 - Manual (Page 605)

Queued Analog-to-Digital Converter (QADC)
Freescale Semiconductor 28-67
28.9.7.1 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 28-53 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 28-24
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 28-24 is based on the RC network in Figure 28-53 .
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 28-24 is a low-pass filter. Measurements of an AC component of
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
Chapter 33,
“Electrical Characteristics ”. Input leakage is greater at higher operating temperatures. In the temperature
range from 125°C to 50°C, the leakage current is halved for every 8°C to 12°C reduction in temperature.
Assuming V
RH
–V
RL
= 5.12 V, 1 count (with 10-bit resolution) corresponds to 5 mV of input voltage. A
typical input leakage of 200 nA acting through 10 k
Ω of external series resistance results in an error of 0.4
count (2.0 mV). If the source impedance is 100 k
Ω 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 illustrates the effect of different levels of total leakage on accuracy for different values of
source impedance. The error is listed in terms of 10-bit counts.
Table 28-24. External Circuit Settling Time to 1/2 LSB
Filter Capacitor (CF)
Source Resistance (R
F
+ R
SRC
)
100
Ω
1 k
Ω
10 k
Ω
100 k
Ω
1
μ
F 760
μ
s 7.6 ms 76 ms 760 ms
0.1
μ
F 76
μ
s 760
μ
s 7.6 ms 76 ms
0.01
μ
F 7.6
μ
s 76
μ
s 760
μ
s 7.6 ms
0.001
μ
F 760 ns 7.6
μ
s 76
μ
s 760
μ
s
100 pF 76 ns 760 ns 7.6
μ
s 76
μ
s
VCF = VSRC (1 –e–t/(RF + RSRC) CF)
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3