LTC1740_15 Datasheet, PDF (11/16 Pages) Linear Technology – 14-Bit, 6Msps, Sampling ADC

APPLICATIO S I FOR ATIO
5V
0V
V_IN
+A_IN
LTC1740
–A_IN
V_{CM VSS}
1µF
1405 F04
–5V
Figure 4. DC Coupling a Ground Centered Signal
(Dual Supply System)
2.5V
V_IN
5V
+A_IN
LTC1740
–A_IN
V_{CM VSS}
1µF
1740 F05
LTC1740
5V
C
0V
V_IN
+A_IN
LTC1740
–A_IN
C
RR
V_{CM VSS}
1µF
1740 F07
Figure 7. AC Coupling to the LTC1740. Note That the Input Signal
Can Almost Always Be Directly Coupled with Better Performance
MINI CIRCUITS
T1-1T
15Ω
V_IN
1000pF
15Ω
1µF
5V
+A_IN
LTC1740
–A_IN
V_{CM VSS}
1740 F08a
Figure 5. DC Coupling a Signal Centered Around
2.5V (Single Supply System)
2.500V
0V
5V
V_IN
1.25V
1µF
5V
+A_IN
LTC1740
–A_IN
V_REF
SENSE
V_SS
1740 F06
Figure 6. DC Coupling a 0V to 2.5V Signal
AC Coupling the Input
The analog inputs to the LTC1740 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.
Figure 8a. Single Supply Transformer Coupled Input
MINI CIRCUITS
T1-1T
15Ω
V_IN
1000pF
15Ω
1µF
5V
+A_IN
LTC1740
–A_IN
V_{CM VSS}
1740 F08b
–5V
Figure 8b. Dual Supply Transformer Coupled Input
Differential Operation
The THD and SFDR performance of the LTC1740 can be
improved by using a center tap RF transformer to drive the
inputs differentially. Though the signal can no longer be
DC coupled, the improvement in dynamic performance
makes this an attractive solution for some applications.
Typical connections for single and dual supply systems
are shown in Figures 8a and 8b. Good choices for trans-
formers are the Mini Circuits T1-1T (1:1 turns ratio) and
T4-6T (1:4 turns ratio). For best results the transformer
should be located close to the LTC1740 on the printed
circuit board.
1740f
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