Datasheet

LTC2308
11
2308fb
Figure 2. Driving COM in UNIPOLAR and BIPOLAR Modes
COM
REFCOMP/2
COM
Unipolar Mode Bipolar Mode
2308 F02
+
–
Figure 1a. Example MUX Confi gurations
Figure 1b. Changing the MUX Assignment “On the Fly”
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM (
–
)
8 Single-Ended
+
+
+
+
+
+
+
4 Differential
+ (
–
)
+
+ (
–
)
+ (
–
)
+ (
–
)
–
(
+
)
–
(
+
)
–
(
+
)
–
(
+
)
COM (
–
)
Combinations of Differential
and Single-Ended
+
+
+
+
+
+
–
–
{
{
{
{
{
{
2308 F01a
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
CH0
CH1
CH2
CH3
CH4
CH5
CH6
CH7
COM
(UNUSED)
COM (
–
)
1st Conversion 2nd Conversion
+
–
+
–
+
–
+
+
{
{
{
{
CH2
CH3
CH4
CH5
CH2
CH3
CH4
CH5
2308 F01b
Driving the Analog Inputs
The analog inputs of the LTC2308 are easy to drive. Each
of the analog inputs can be used as a single-ended input
relative to the COM pin (CH0-COM, CH1-COM, etc.) or in
differential input pairs (CH0 and CH1, CH2 and CH3, CH4
and CH5, CH6 and CH7). Figure 2 shows how to drive COM
for single-ended inputs in unipolar and bipolar modes.
Regardless of the MUX confi guration, the “+” and “–”
inputs are sampled at the same instant. Any unwanted
signal that is common to both inputs will be reduced by
the common mode rejection of the sample-and-hold circuit.
The inputs draw only one small current spike while charg-
ing the sample-and-hold capacitors during the acquire
APPLICATIONS INFORMATION
mode. In conversion mode, the analog inputs draw only
a small leakage current. 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 LTC2308 can be modeled as
a 55pF capacitor (C
IN
) in series with a 100Ω resistor
(R
ON
) as shown in Figure 3a. C
IN
gets switched to the
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
) if DC
accuracy is important.
When using a fi lter with a large C
FILTER
value (e.g. 1μF),
the inputs do not completely settle and the capacitive input
switching currents are averaged into a net DC current
(I
DC
). In this case, the analog input can be modeled by an
equivalent resistance (R
EQ
= 1/(f
SMPL
• C
IN
)) in series with
an ideal voltage source (V
REFCOMP
/2) as shown in Figure 3b.
The magnitude of the DC current is then approximately
I
DC
= (V
IN
– V
REFCOMP
/2)/R
EQ
, which is roughly propor-
tional to V
IN
. To prevent large DC drops across the resistor
R
FILTER
, a fi lter with a small resistor and large capacitor
should be chosen. When running at the minimum cycle