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11-11
ANALOG-TO-DIGITAL CONVERTER
11.6.1 Designing External Interface Circuitry
The external interface circuitry to an analog input is highly dependent upon the application and
can affect the converter characteristics. Factors such as input pin leakage, sample capacitor size,
and multiplexer series resistance from the input pin to the sample capacitor must be considered
in the external circuit’s design. These factors are idealized in Figure 11-7.
Figure 11-7. Idealized A/D Sampling Circuitry
During the sample window, the external input circuit must be able to charge the sample capacitor
(C
S
) through the series combination of the input source resistance (R
SOURCE
), the input series re-
sistance (R
1
), and the comparator feedback resistance (R
F
). The total effective series resistance
(R
T
) is calculated using the following formula, where A
V
is the gain of the comparator circuit.
Typically, the (R
F
/ A
V
+ 1) term is the major contributor to the total resistance and the factor that
determines the minimum sample time specified in the datasheet.
Leakage
V
R
SOURCE
+
-
C
S
R1
~1K
Ω
~2pF
I
LI1
External
Internal
A
V
R
F
Sample
A0243-02
R
T
R
SO URCE
R
1
R
F
A
V
1
+
----------------
+
+
=
Summary of Contents for 8XC196NT
Page 1: ...8XC196NT Microcontroller User s Manual...
Page 2: ...8XC196NT Microcontroller User s Manual June 1995 Order Number 272317 003...
Page 22: ...1 Guide to This Manual...
Page 23: ......
Page 35: ......
Page 36: ...2 Architectural Overview...
Page 37: ......
Page 49: ......
Page 50: ...3 Programming Considerations...
Page 51: ......
Page 66: ...4 Memory Partitions...
Page 67: ......
Page 104: ...5 Standard and PTS Interrupts...
Page 105: ......
Page 147: ......
Page 148: ...6 I O Ports...
Page 149: ......
Page 176: ...7 Serial I O SIO Port...
Page 177: ......
Page 194: ...8 Synchronous Serial I O SSIO Port...
Page 195: ......
Page 211: ......
Page 212: ...9 Slave Port...
Page 213: ......
Page 231: ......
Page 232: ...10 Event Processor Array EPA...
Page 233: ......
Page 270: ...11 Analog to digital Converter...
Page 271: ......
Page 291: ......
Page 292: ...12 Minimum Hardware Considerations...
Page 293: ......
Page 306: ...13 Special Operating Modes...
Page 307: ......
Page 317: ......
Page 318: ...14 Interfacing with External Memory...
Page 319: ......
Page 362: ...15 Programming the Nonvolatile Memory...
Page 363: ......
Page 408: ...A Instruction Set Reference...
Page 409: ......
Page 476: ...B Signal Descriptions...
Page 477: ......
Page 493: ......
Page 494: ...C Registers...
Page 495: ......
Page 565: ......
Page 566: ...Glossary...
Page 567: ......
Page 580: ...Index...
Page 581: ......
Page 597: ......