Datasheet

LTC2175-12/
LTC2174-12/LTC2173-12
20
21754312fa
applicaTions inForMaTion
tap is biased with V
CM
, setting the A/D input at its opti-
mal DC level. At higher input frequencies a transmission
line balun transformer (Figures 4 to 6) has better balance,
resulting in lower A/D distortion.
Amplifier Circuits
Figure 7 shows the analog input being driven by a high
speed differential amplifier. The output of the amplifier is
AC-coupled to the A/D so the amplifier’s output common
mode voltage can be optimally set to minimize distortion.
Figure 4. Recommended Front End Circuit for Input
Frequencies from 70MHz to 170MHz
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
4.7pF
0.1µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T2
T1: MA/COM MABA-007159-000000
T2: MA/COM MABAES0060
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
217512 F04
LTC2175-12
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
1.8pF
0.1µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T2
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1LB
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
217512 F05
LTC2175-12
25Ω
25Ω
50Ω
0.1µF
2.7nH
2.7nH
A
IN
+
A
IN
–
0.1µF
V
CM
ANALOG
INPUT
0.1µF
0.1µF
T1
T1: MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
217512 F06
LTC2175-12
25Ω
25Ω
200Ω
200Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2175-12
217512 F07
–
–
+
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
At very high frequencies an RF gain block will often have
lower distortion than a differential amplifier. If the gain
block is single-ended, then a transformer circuit (Figures
4 to 6) should convert the signal to differential before
driving the A/D.
Reference
The LTC2175-12/LTC2174-12/LTC2173-12 has an internal
1.25V voltage reference. For a 2V input range using the
internal reference, connect SENSE to V
DD
. For a 1V input
range using the internal reference, connect SENSE to
Figure 5. Recommended Front End Circuit for Input
Frequencies from 170MHz to 300MHz
Figure 6. Recommended Front End Circuit for Input
Frequencies Above 300MHz
Figure 7. Front End Circuit Using a High Speed
Differential Amplifier