MC68HC16Z1CAG16 Datasheet, PDF (209/500 Pages) Freescale Semiconductor, Inc – M68HC16Z Series users manual

Freescale Semiconductor, Inc.
Table 8-10 External Circuit Settling Time (10-Bit Conversions)
Filter Capacitor
(C_F)
1 µF
.1 µF
.01 µF
.001 µF
100 pF
100 Ω
760 µs
76 µs
7.6 µs
760 ns
76 ns
Source Resistance (R_F)
1 kΩ
10 kΩ
7.6 ms
76 ms
760 µs
7.6 ms
76 µs
760 µs
7.6 µs
76 µs
760 ns
7.6 µs
100 kΩ
760 ms
76 ms
7.6 ms
760 µs
76 µs
The external circuit described in Table 8-10 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.
8.8.6.2 Error Resulting from Leakage
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_RH – V_RL = 5.12 V, 1 count (assuming 10-bit resolution) corresponds to 5
mV of input voltage. A typical input leakage of 50 nA acting through 100 kΩ of external
series resistance results in an error of less than 1 count (5.0 mV). If the source imped-
ance is 1 MΩ 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 8-11 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.
CAUTION
Leakage from the part of 10 nA is obtainable only within a limited tem-
perature range.
Table 8-11 Error Resulting From Input Leakage (IOFF)
Source
Impedance
1 kΩ
10 kΩ
100 kΩ
10 nA
—
—
0.2 counts
Leakage Value (10-Bit Conversions)
50 nA
100 nA
—
—
0.1 counts
0.2 counts
1 count
2 counts
1000 nA
0.2 counts
2 counts
20 counts
M68HC16 Z SERIES
USER’S MANUAL
ANALOG-TO-DIGITAL CONVERTER
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