
QADC64E Enhanced Mode Operation
MPC561/MPC563 Reference Manual, Rev. 1.2
Freescale Semiconductor
14-73
14.6.5.2
Settling Time for the External Circuit
The values for RSRC, RF and CF in the 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 14-50 while S1 is open, disconnecting the internal circuitry from the external circuitry. Assume that the initial voltage across CF is
zero. As CF charges, the voltage across it is determined by the following equation, where t is the total
charge time:
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 14-25 shows the required time for CF to charge to within 1/2 of a count of the actual source voltage during 10-bit
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
Table 14-25 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.
14.6.5.3
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 F,range from 125° C to 50° C, the leakage current is halved for every 8 – 12° C reduction in temperature.
Assuming VRH – VRL = 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
of external series resistance results in an
error of 0.4 count (2.0 mV). If the source impedance is 100 k
and a typical leakage of 100 nA is present,
an error of two counts (10 mV) is introduced.
In addition to internal junction leakage, external leakage (e.g., if external clamping diodes are used) and
charge sharing effects with internal capacitors also contribute to the total leakage current.
Table 14-26illustrates 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 14-25. External Circuit Settling Time to 1/2 LSB (10-Bit Conversions)
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
.1
F76 s
760
s
7.6 ms
76 ms
.01
F7.6 s76 s
760
s
7.6 ms
.001
F
760 ns
7.6
s76 s
760
s
100 pF
76 ns
760 ns
7.6
s76 s