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8XC196NT USER’S MANUAL
11-12
11.6.1.1
Minimizing the Effect of High Input Source Resistance
Under some conditions, the input source resistance (R
SOURCE
) can be great enough to affect the
measurement. You can minimize this effect by increasing the sample time or by connecting an
external capacitor (C
EXT
) from the input pin to ANGND. The external signal will charge C
EXT
to
the source voltage level. When the channel is sampled, C
EXT
acts as a low-impedance source to
charge the sample capacitor (C
S
). A small portion of the charge in C
EXT
is transferred to C
S
, re-
sulting in a drop of the sampled voltage. The voltage drop is calculated using the following for-
mula.
If C
EXT
is 0.005 µF or greater, the error will be less than –0.4 LSB in 10-bit conversion mode. The
use of C
EXT
in conjunction with R
SOURCE
forms a low-pass filter that reduces noise input to the
A/D converter.
High R
SOURCE
resistance can also cause errors due to the input leakage (I
LI1
).
I
LI1
is typically much
lower than its specified maximum (consult the datasheet for specifications). The combined effect
of I
LI1
leakage and high R
SOURCE
resistance is calculated using the following formula.
where:
R
SOURCE
is the input source resistance, in ohms
I
LI
1
is the input leakage, in amperes
V
REF
is the reference voltage, in volts
External circuits with R
SOURCE
resistance of 1 K
Ω
or lower and V
REF
equal to 5.0 volts will have
a resultant error due to source impedance of 0.6 LSB or less.
Sampled Voltage Drop, %
C
S
C
E XT
C
S
+
---------------------------
100%
×
=
error (LSBs)
R
S OURCE
I
LI1
×
1024
×
V
RE F
------------------------------------------------------------
=
Содержание 8XC196NT
Страница 1: ...8XC196NT Microcontroller User s Manual...
Страница 2: ...8XC196NT Microcontroller User s Manual June 1995 Order Number 272317 003...
Страница 22: ...1 Guide to This Manual...
Страница 23: ......
Страница 35: ......
Страница 36: ...2 Architectural Overview...
Страница 37: ......
Страница 49: ......
Страница 50: ...3 Programming Considerations...
Страница 51: ......
Страница 66: ...4 Memory Partitions...
Страница 67: ......
Страница 104: ...5 Standard and PTS Interrupts...
Страница 105: ......
Страница 147: ......
Страница 148: ...6 I O Ports...
Страница 149: ......
Страница 176: ...7 Serial I O SIO Port...
Страница 177: ......
Страница 194: ...8 Synchronous Serial I O SSIO Port...
Страница 195: ......
Страница 211: ......
Страница 212: ...9 Slave Port...
Страница 213: ......
Страница 231: ......
Страница 232: ...10 Event Processor Array EPA...
Страница 233: ......
Страница 270: ...11 Analog to digital Converter...
Страница 271: ......
Страница 291: ......
Страница 292: ...12 Minimum Hardware Considerations...
Страница 293: ......
Страница 306: ...13 Special Operating Modes...
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Страница 317: ......
Страница 318: ...14 Interfacing with External Memory...
Страница 319: ......
Страница 362: ...15 Programming the Nonvolatile Memory...
Страница 363: ......
Страница 408: ...A Instruction Set Reference...
Страница 409: ......
Страница 476: ...B Signal Descriptions...
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Страница 493: ......
Страница 494: ...C Registers...
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Страница 565: ......
Страница 566: ...Glossary...
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Страница 580: ...Index...
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