Datasheet

LTC2268-14/
LTC2267-14/LTC2266-14
20
22687614fa
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 optimal
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 distor-
tion.
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
Figure 7. Front End Circuit Using a High Speed
Differential Amplifier
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
226814 F04
LTC2268-14
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
226814 F05
LTC2268-14
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
226814 F06
LTC2268-14
25Ω
25Ω
200Ω
200Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2268-14
226814 F07
–
–
+
+
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF