11
LTC1740
1740f
APPLICATIO S I FOR ATIO
WU
UU
1740 F07
+AIN
VSS
VIN
1
F
0V
C
RR
LTC1740
5V
–AIN
VCM
Figure 7. AC Coupling to the LTC1740. Note That the Input Signal
Can Almost Always Be Directly Coupled with Better Performance
1740 F06
+AIN
VSS
VIN
1
F
2.500V
0V
5V
1.25V
LTC1740
5V
–AIN
VREF
SENSE
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 AIN+ and AIN– to maintain
offset accuracy.
1740 F05
1
F
+AIN
VSS
VIN
2.5V
LTC1740
5V
–AIN
VCM
Figure 5. DC Coupling a Signal Centered Around
2.5V (Single Supply System)
1405 F04
1
F
+AIN
VSS
VIN
0V
LTC1740
5V
–5V
–AIN
VCM
Figure 4. DC Coupling a Ground Centered Signal
(Dual Supply System)
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.
1740 F08a
+AIN
VSS
VIN
1000pF
15
15
MINI CIRCUITS
T1-1T
1
F
LTC1740
5V
–AIN
VCM
Figure 8a. Single Supply Transformer Coupled Input
1740 F08b
+AIN
VSS
VIN
MINI CIRCUITS
T1-1T
1
F
LTC1740
5V
–5V
–AIN
VCM
1000pF
15
15
Figure 8b. Dual Supply Transformer Coupled Input