Datasheet

11
LTC1405
1405fa
Table 2. Comparison of Analog Input Configurations
SUPPLIES COUPLING V
REF
GAIN A
IN
+
A
IN
–
COMMENTS
±5V DC 4.096V 1x ±2.048 0 Best SNR, THD
5V DC 4.096V 2x 2.5 ± 1.024 2.5 Best SINAD, THD for Single Supply
5V DC 2.048V 1x 2.5 ± 1.024 2.5 Worse Noise than Above Case
5V DC 4.096V 1x 2.5 ± 2.048 2.5 Best Single Supply Noise, THD Is Not Optimal
5V DC 4.096V 1x 0 to 4.096 2.048 Same As Above
±5V AC 4.096V 1x ±1.024 ±1.024 Very Best SNR, THD
(Transformer)
5V AC 4.096V 1x 2.5 ± 1.024 2.5 ± 1.024 Very Best SNR, THD for Single Supply
(Transformer)
APPLICATIO S I FOR ATIO
WUU
U
1405 F05
1µF
+A
IN
V
SS
V
IN
2.5V
LTC1405
5V
–A
IN
V
CM
1405 F06
+A
IN
V
SS
V
IN
4.096V
0V
5V
2.048V
LTC1405
5V
–A
IN
SENSE
Figure 4. DC Coupling a Ground
Centered Signal (Dual Supply System)
Figure 5. DC Coupling a Signal Centered
Around 2.5V (Single Supply System)
Figure 6. DC Coupling a 0V to 4.096V Signal
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
IN
+
and A
IN
–
to maintain
offset accuracy.
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
IN
–
to ground and con-
nect V
SS
to – 5V (Figure 4). In a single power supply
system with the input signal centered around 2.5V, con-
nect A
IN
–
to V
CM
and V
SS
to ground (Figure 5). If the input
signal is not centered around ground or 2.5V, the voltage
for A
IN
–
must be generated externally by a resistor divider
or a voltage reference (Figure 6).
1405 F07
+A
IN
V
SS
V
IN
1µF
0V
C
C
RR
LTC1405
5V
–A
IN
V
CM
Figure 7. AC Coupling to the LTC1405. Note That the Input Signal
Can Almost Always Be Directly Coupled with Better Performance
1405 F04
1µF
+A
IN
V
SS
V
IN
0V
LTC1405
5V
–5V
–A
IN
V
CM