Datasheet

LTC2172-12/
LTC2171-12/LTC2170-12
21
21721012fb
Transformer Coupled Circuits
Figure 3 shows the analog input being driven by an RF
transformer with a center-tapped secondary. The center
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.
applicaTions inForMaTion
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.
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.
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 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
217212 F04
LTC2172-12
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
217212 F05
LTC2172-12
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
217212 F06
LTC2172-12
25Ω
25Ω
200Ω
200Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2172-12
217212 F07
–
–
+
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifier