QADC64E Legacy Mode Operation
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
13-75
13.7.5.2
Settling Time for the External Circuit
The values for R
SRC
, R
F
and C
F
in the 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
while S1 is open,
disconnecting the internal circuitry from the external circuitry. Assume that the initial voltage across C
F
is
zero. As C
F
charges, the voltage across it is determined by the following equation, where t is the total
charge time:
Eqn. 13-1
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),
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.
is based on the RC network in
.
NOTE
The following times are completely independent of the A/D converter
architecture (assuming the QADC64E is not affecting the charging).
The external circuit described in
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
.” 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 – 12° C reduction in temperature.
Assuming V
RH
– V
RL
= 5.12 V, one count (assuming 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 two counts (10 mV) is introduced.
Table 13-24. External Circuit Settling Time to 1/2 LSB (10-Bit Conversions)
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
.1
µ
F
76
µ
s
760
µ
s
7.6 ms
76 ms
.01
µ
F
7.6
µ
s
76
µ
s
760
µ
s
7.6 ms
.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
V
CF
V
SRC
1
e
–
t
–
R
F
R
SRC
+
(
)
C
F
-----------------------------------------------------------
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Summary of Contents for MPC561
Page 84: ...MPC561 MPC563 Reference Manual Rev 1 2 lxxxiv Freescale Semiconductor...
Page 144: ...Signal Descriptions MPC561 MPC563 Reference Manual Rev 1 2 2 46 Freescale Semiconductor...
Page 206: ...Central Processing Unit MPC561 MPC563 Reference Manual Rev 1 2 3 62 Freescale Semiconductor...
Page 302: ...Reset MPC561 MPC563 Reference Manual Rev 1 2 7 14 Freescale Semiconductor...
Page 854: ...Time Processor Unit 3 MPC561 MPC563 Reference Manual Rev 1 2 19 24 Freescale Semiconductor...
Page 968: ...Development Support MPC561 MPC563 Reference Manual Rev 1 2 23 54 Freescale Semiconductor...
Page 1144: ...Internal Memory Map MPC561 MPC563 Reference Manual Rev 1 2 B 34 Freescale Semiconductor...
Page 1212: ...TPU3 ROM Functions MPC561 MPC563 Reference Manual Rev 1 2 D 60 Freescale Semiconductor...
Page 1216: ...Memory Access Timing MPC561 MPC563 Reference Manual Rev 1 2 E 4 Freescale Semiconductor...