LTC1405 Linear Technology, LTC1405 Datasheet - Page 9

• Manufacturer Part Number: LTC1405
• Description: 12-Bit/ 5Msps/ Sampling ADC
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Table 2. Comparison of Analog Input Configurations
SUPPLIES
5V
5V
5V
5V
5V
DC Coupling the Input
In most applications the analog input signal can be directly
coupled to the LTC1405 inputs. If the input signal is
centered around ground, such as when dual supply op
amps are used, simply connect A
nect V
system with the input signal centered around 2.5V, con-
nect A
signal is not centered around ground or 2.5V, the voltage
for A
or a voltage reference (Figure 6).
5V
5V
IN
IN
SS
–
–
must be generated externally by a resistor divider
to V
to – 5V (Figure 4). In a single power supply
Figure 5. DC Coupling a Signal Centered
Around 2.5V (Single Supply System)
2.5V
0V
Figure 4. DC Coupling a Ground
Centered Signal (Dual Supply System)
(Transformer)
(Transformer)
COUPLING
CM
DC
DC
DC
DC
DC
AC
AC
and V
U
V
V
SS
IN
IN
to ground (Figure 5). If the input
U
1 F
1 F
4.096V
4.096V
2.048V
4.096V
4.096V
4.096V
4.096V
V
REF
IN
+A
–A
V
+A
–A
V
CM
W
–
CM
IN
IN
IN
IN
LTC1405
LTC1405
to ground and con-
V
–5V
V
5V
5V
SS
SS
1405 F04
1405 F05
GAIN
1x
2x
1x
1x
1x
1x
1x
U
2.5 1.024
2.5 1.024
2.5 2.048
2.5 1.024
0 to 4.096
A
1.024
2.048
IN
+
AC Coupling the Input
The analog inputs to the LTC1405 can also be AC coupled
through a capacitor, though in most cases it is simpler to
directly couple the input to the ADC. Figure 7 shows an
example where the input signal is centered around ground
and the ADC operates from a single 5V supply. Note that
the performance would improve if the ADC was operated
from a dual supply and the input was directly coupled (as
in Figure 4). With AC coupling the DC resistance to ground
should be roughly matched for A
offset accuracy.
Figure 7. AC Coupling to the LTC1405. Note That the Input Signal
Can Almost Always Be Directly Coupled with Better Performance
0V
2.5 1.024
Figure 6. DC Coupling a 0V to 4.096V Signal
2.048
4.096V
A
1.024
2.5
2.5
2.5
IN
0
0V
5V
–
V
IN
COMMENTS
Best SNR, THD
Best SINAD, THD for Single Supply
Worse Noise than Above Case
Best Single Supply Noise, THD Is Not Optimal
Same As Above
Very Best SNR, THD
Very Best SNR, THD for Single Supply
C
R
V
IN
2.048V
R
C
1 F
IN
+
+A
–A
SENSE
and A
IN
IN
LTC1405
LTC1405
V
+A
–A
V
5V
SS
CM
IN
IN
IN
LTC1405
1405 F06
V
5V
–
SS
to maintain
1405 F07
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