mc68hc916y3 Freescale Semiconductor, Inc, mc68hc916y3 Datasheet - Page 232

• Manufacturer Part Number: mc68hc916y3
• Description: Mc68hc16y3 16 Bit Microcontroller
• Manufacturer: Freescale Semiconductor, Inc

10.8.6.2 Error Resulting from Leakage
10-24
MOTOROLA
The external circuit described in Table 10-10 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.
A series resistor limits the current to a pin, therefore input leakage acting through a
large source impedance can degrade A/D accuracy. The maximum input leakage cur-
rent is specified in APPENDIX A ELECTRICAL CHARACTERISTICS. Input leakage
is greatest at high operating temperatures and as a general rule decreases by one half
for each 10 C decrease in temperature.
Assuming V
mV of input voltage. A typical input leakage of 50 nA acting through 100 k of external
series resistance results in an error of less than 1 count (5.0 mV). If the source imped-
ance is 1 M and a typical leakage of 50 nA is present, an error of 10 counts (50 mV)
is introduced.
In addition to internal junction leakage, external leakage (e.g., if external clamping di-
odes are used) and charge sharing effects with internal capacitors also contribute to
the total leakage current. Table 10-11 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.
Filter Capacitor
Table 10-10 External Circuit Settling Time (10-Bit Conversions)
Impedance
.001 F
Source
100 pF
100 k
.01 F
10 k
.1 F
1 F
1 k
(C
Leakage from the part of 10 nA is obtainable only within a limited
temperature range.
Table 10-11 Error Resulting From Input Leakage (IOFF)
F
RH
)
– V
RL
Freescale Semiconductor, Inc.
= 5.12 V, 1 count (assuming 10-bit resolution) corresponds to 5
For More Information On This Product,
0.2 counts
760 s
760 ns
7.6 s
10 nA
100
76 s
76 ns
ANALOG-TO-DIGITAL CONVERTER
—
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Leakage Value (10-Bit Conversions)
0.1 counts
1 count
7.6 ms
760 s
760 ns
7.6 s
50 nA
76 s
Source Resistance (R
1 k
CAUTION
—
0.2 counts
2 counts
100 nA
760 s
7.6 ms
7.6 s
10 k
76 ms
76 s
—
F
)
0.2 counts
20 counts
MC68HC16Y3/916Y3
1000 nA
2 counts
100 k
760 ms
7.6 ms
760 s
76 ms
76 s
USER’S MANUAL