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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.
External
R
SOURCE
V
+
-
During the sample window, the external input circuit must be able to charge the sample capacitor
(C
) through the series combination of the input source resistance (R
S
sistance (R
), and the comparator feedback resistance (R
1
(R
) is calculated using the following formula, where A
T
R
R
=
+
T
SO URCE
Typically, the (R
/ A
F
V
determines the minimum sample time specified in the datasheet.
Internal
~1KΩ
R1
I
LI1
Leakage
Figure 11-7. Idealized A/D Sampling Circuitry
R
F
----------------
R
+
1
A
+
1
V
+ 1) term is the major contributor to the total resistance and the factor that
ANALOG-TO-DIGITAL CONVERTER
Sample
R
F
~2pF
A
V
C
S
), the input series re-
SOURCE
). The total effective series resistance
F
is the gain of the comparator circuit.
V
A0243-02
11-11
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