LTC1405 データシートの表示（PDF） - Linear Technology
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LTC1405 Datasheet PDF : 16 Pages
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LTC1405
APPLICATIO S I FOR ATIO
Table 2. Comparison of Analog Input Configurations
SUPPLIES
COUPLING
VREF
GAIN
±5V
DC
4.096V
1x
5V
DC
4.096V
2x
5V
DC
2.048V
1x
5V
DC
4.096V
1x
5V
DC
4.096V
1x
±5V
AC
4.096V
1x
(Transformer)
5V
AC
4.096V
1x
(Transformer)
AIN +
± 2.048
2.5 ± 1.024
2.5 ± 1.024
2.5 ± 2.048
0 to 4.096
±1.024
2.5 ± 1.024
AIN –
0
2.5
2.5
2.5
2.048
±1.024
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
2.5 ± 1.024 Very Best SNR, THD for Single Supply
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 AIN– to ground and con-
nect VSS to – 5V (Figure 4). In a single power supply
system with the input signal centered around 2.5V, con-
nect AIN– to VCM and VSS to ground (Figure 5). If the input
signal is not centered around ground or 2.5V, the voltage
for AIN – must be generated externally by a resistor divider
or a voltage reference (Figure 6).
5V
0V
VIN
+AIN
LTC1405
–AIN
VCM VSS
1µF
1405 F04
–5V
Figure 4. DC Coupling a Ground
Centered Signal (Dual Supply System)
5V
2.5V
VIN
1µF
+AIN
LTC1405
–AIN
VCM VSS
1405 F05
4.096V
0V
5V
VIN
2.048V
5V
+AIN
LTC1405
–AIN
SENSE
VSS
1405 F06
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 AIN+ and AIN– to maintain
offset accuracy.
5V
C
0V
VIN
+AIN
LTC1405
–AIN
C
RR
VCM VSS
1µF
1405 F07
Figure 5. DC Coupling a Signal Centered
Around 2.5V (Single Supply System)
Figure 7. AC Coupling to the LTC1405. Note That the Input Signal
Can Almost Always Be Directly Coupled with Better Performance
9
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