LTC2172-12 Linear Technology, LTC2172-12 Datasheet - Page 20

• Manufacturer Part Number: LTC2172-12
• Description: (LTC217x-12) 65Msps/40Msps/ 25Msps Low Power Quad ADCs
• Manufacturer: Linear Technology

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LTC2172-12/
LTC2171-12/LTC2170-12
APPLICATIONS INFORMATION
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
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.
ANALOG
20
INPUT
ANALOG
INPUT
Figure 4. Recommended Front-End Circuit for Input
Frequencies from 70MHz to 170MHz
T1: MA/COM MABA-007159-000000
T2: MA/COM MABAES0060
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
0.1μF
0.1μF
T1: MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
Figure 6. Recommended Front-End Circuit for Input
Frequencies Above 300MHz
0.1μF
0.1μF
T1
T1
CM
, setting the A/D input at its opti-
T2
25Ω
25Ω
25Ω
25Ω
50Ω
2.7nH
2.7nH
0.1μF
0.1μF
50Ω
0.1μF
V
A
A
CM
IN
IN
0.1μF
4.7pF
+
–
V
A
A
CM
IN
IN
+
–
LTC2172-12
LTC2172-12
217212 F06
217212 F04
ANALOG
Amplifi er Circuits
Figure 7 shows the analog input being driven by a high
speed differential amplifi er. The output of the amplifi er is
AC-coupled to the A/D so the amplifi er’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 amplifi er. 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.
INPUT
ANALOG
INPUT
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1LB
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
Figure 5. Recommended Front-End Circuit for Input
Frequencies from 170MHz to 300MHz
0.1μF
0.1μF
DIFFERENTIAL
HIGH SPEED
+
–
AMPLIFIER
Figure 7. Front-End Circuit Using a High Speed
Differential Amplifi er
T1
+
–
0.1μF
0.1μF
T2
200Ω
25Ω
25Ω
200Ω
25Ω
25Ω
0.1μF
50Ω
12pF
0.1μF
A
A
V
CM
IN
IN
+
–
0.1μF
1.8pF
V
A
A
CM
IN
IN
LTC2172-12
+
–
LTC2172-12
217212 F07
21721012f
217212 F05