LTC2309 Linear Technology Corporation, LTC2309 Datasheet - Page 10

• Manufacturer Part Number: LTC2309
• Description: 12-Bit SAR ADC
• Manufacturer: Linear Technology Corporation

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LTC2309
APPLICATIONS INFORMATION
10
Table 1. Channel Confi guration
S/D O/S S1
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
1
+
–
+
–
+
–
+
–
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
1
(
(
(
(
(
(
(
(
–
+
–
+
–
+
–
+
)
)
)
)
)
)
)
)
{
{
{
{
Figure 1a. Example of MUX Confi gurations
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
1
4 Differential
S0
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
1
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
+
–
+
0
Combinations of Differential
+
–
–
+
+
+
+
+
{
{
1
–
+
+
and Single-Ended
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM (
+
–
+
2
–
–
+
+
3
)
+
+
+
+
+
+
+
+
2309 F01a
8 Single-Ended
+
–
+
4
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM (
–
+
+
5
–
)
6
+
–
+
–
+
+
7
COM
–
–
–
–
–
–
–
–
mode, the analog inputs draw only a small leakage cur-
rent. If the source impedance of the driving circuit is
low, the ADC inputs can be driven directly. Otherwise,
more acquisition time should be allowed for a source
with higher impedance.
Input Filtering
The noise and distortion of the input amplifi er and
other circuitry must be considered since they will add
to the ADC noise and distortion. Therefore, noisy input
circuitry should be fi ltered prior to the analog inputs to
minimize noise. A simple 1-pole RC fi lter is suffi cient
for many applications.
The analog inputs of the LTC2309 can be modeled as
a 55pF capacitor (C
(R
selected input once during each conversion. Large fi lter
RC time constants will slow the settling of the inputs.
It is important that the overall RC time constants be
short enough to allow the analog inputs to completely
settle to 12-bit resolution within the acquisition time
(t
ACQ
ON
Figure 1b. Changing the MUX Assignments “On the Fly”
Figure 2. Driving COM in Unipolar and Bipolar Modes
) as shown in Figure 3a. C
) if DC accuracy is important.
+
+
–
–
{
{
Unipolar Mode
COM
1st Conversion
CH2
CH3
CH4
CH5
COM
(UNUSED)
IN
) in series with a 100Ω resistor
REFCOMP/2
–
+
+
+
{
{
IN
+
–
gets switched to the
2nd Conversion
COM (
CH2
CH3
CH4
CH5
Bipolar Mode
COM
–
)
2328 F01b
2328 F02
2309f