參數(shù)資料
型號: QADCRM
英文描述: Queued Analog-to-Digital Converter Reference Manual
中文描述: 排隊模擬到數(shù)字轉(zhuǎn)換器參考手冊
文件頁數(shù): 42/122頁
文件大?。?/td> 940K
代理商: QADCRM
MOTOROLA
5-10
PIN CONNECTION CONSIDERATIONS
For More Information On This Product,
Go to: www.freescale.com
QADC
REFERENCE MANUAL
The external circuit described in
Table 5-1
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.
5.6.2 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
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 5-2
illustrates the effect of different levels of total leak-
age 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 5-2 Error Resulting From Input Leakage (IOFF)
Source
Impedance
Leakage Value (10-bit Conversions)
50 nA
0.1 counts
1 count
10 nA
0.2 counts
100 nA
0.2 counts
2 counts
1000 nA
0.2 counts
2 counts
20 counts
1 k
10 k
100 k
F
Freescale Semiconductor, Inc.
n
.
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