LTC2301 Linear Technology Corporation, LTC2301 Datasheet - Page 13

• Manufacturer Part Number: LTC2301
• Description: (LTC2301 / LTC2305) 12-Bit ADCs
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
Table 2. Channel Confi guration for the LTC2301
Figure 1b. Changing the MUX Assignment “On the Fly”
+
+
–
–
(
(
{
–
+
O/S
)
)
0
1
{
Figure 1a. Example MUX Confi gurations
1st Conversion
1 Differential
CH0
CH1
LTC2305
CH0
CH1
LTC2305
+ (–)
– (+)
{
1 Differential
CH0
CH1
IN+
+
–
LTC2301
–
+
{
+
+
23015 F01a
2 Single-Ended
2nd Conversion
GND (
CH0
CH1
CH0
CH1
GND (
LTC2305
LTC2305
–
)
–
)
23015 F01b
IN–
–
+
Driving the Analog Inputs
The analog inputs of the LTC2301/LTC2305 are easy to
drive. Each of the analog inputs of the LTC2305 (CH0 and
CH1) can be used as single-ended input relative to GND
or as a differential pair. The analog inputs of the LTC2301
(IN
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 cir-
cuit. The inputs draw only one small current spike while
charging the sample-and-hold capacitors during the acquire
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 LTC2301/LTC2305 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
accuracy is important.
When using a fi lter with a large C
the inputs do not completely settle and the capacitive input
switching currents are averaged into a net DC current(I
In this case, the analog input can be modeled by an equiva-
lent resistance (R
ideal voltage source (V
ON
+
, IN
) as shown in Figure 2a. C
–
) are always confi gured as a differential pair.
EQ
LTC2301/LTC2305
= 1/(f
REFCOMP
IN
) in series with a 100Ω resistor
SMPL
/2) as shown in Figure 2b.
• C
IN
FILTER
IN
gets switched to the
)) in series with an
value (e.g. 1μF),
ACQ
13
) if DC
23015f
DC
).