NXP Semiconductors MC68CK16Z1, User Manual

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M68HC16 Z SERIES ANALOG-TO-DIGITAL CONVERTER
USER’S MANUAL 8-23
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 k
Ω
of external
series resistance results in an error of less than 1 count (5.0 mV). If the source imped-
ance is 1 M
Ω
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 8-10 External Circuit Settling Time (10-Bit Conversions)
Filter Capacitor
(C
F
)
Source Resistance (R
F
)
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
Table 8-11 Error Resulting From Input Leakage (IOFF)
Source
Impedance
Leakage Value (10-Bit Conversions)
10 nA 50 nA 100 nA 1000 nA
1 k
Ω
— — — 0.2 counts
10 k
Ω
— 0.1 counts 0.2 counts 2 counts
100 k
Ω
0.2 counts 1 count 2 counts 20 counts
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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