LTC2175-12_15 Datasheet, PDF (20/34 Pages) Linear Technology – 12-Bit, 125Msps/105Msps/ 80Msps Low Power Quad ADCs

LTC2175-12/
LTC2174-12/LTC2173-12
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.
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
0.1µF
ANALOG
INPUT
T1
0.1µF
50Ω
T2
25Ω 0.1µF
25Ω
T1: MA/COM MABA-007159-000000
T2: MA/COM MABAES0060
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
V_CM
0.1µF
A_IN+
LTC2175-12
4.7pF
A_IN–
217512 F04
Figure 4. Recommended Front End Circuit for Input
Frequencies from 70MHz to 170MHz
0.1µF
ANALOG
INPUT
T1
0.1µF
50Ω
T2
25Ω 0.1µF
25Ω
T1: MA/COM MABA-007159-000000
T2: COILCRAFT WBC1-1LB
RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
V_CM
0.1µF
A_IN+
LTC2175-12
1.8pF
A_IN–
217512 F05
Figure 5. Recommended Front End Circuit for Input
Frequencies from 170MHz to 300MHz
50Ω
ANALOG
INPUT
0.1µF
T1
0.1µF
2.7nH
25Ω
0.1µF
25Ω _2.7nH
T1: MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
V_CM
0.1µF
A_IN+
LTC2175-12
A_IN–
217512 F06
Figure 6. Recommended Front End Circuit for Input
Frequencies Above 300MHz
ANALOG
INPUT
HIGH SPEED
DIFFERENTIAL
AMPLIFIER
0.1µF
++
200Ω 200Ω
25Ω
– ^{– 0.1µF}
25Ω
V_CM
0.1µF
A_IN+
LTC2175-12
12pF
A_IN–
217512 F07
Figure 7. Front End Circuit Using a High Speed
Differential Amplifier
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