Datasheet

LTC2258-12
LTC2257-12/LTC2256-12
19
225812fd
For more information www.linear.com/LTC2258-12
applicaTions inForMaTion
Figure 5. Recommended Front-End Circuit for Input
Frequencies from 170MHz to 270MHz
Figure 6. Recommended Front-End Circuit for Input
Frequencies Above 270MHz
INPUT DRIVE CIRCUITS
Input filtering
If possible, there should be an RC lowpass filter right at
the analog inputs. This lowpass filter isolates the drive
circuitry from the A/D sample-and-hold switching, and
also limits wideband noise from the drive circuitry. Figure 3
shows an example of an input RC filter. The RC component
values should be chosen based on the application’s input
frequency.
Figure 4. Recommended Front-End Circuit for Input
Frequencies from 70MHz to 170MHz
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.
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
225812 F05
LTC2258-12
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
225812 F04
LTC2258-12
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
225812 F06
LTC2258-12
25Ω
25Ω
25Ω
25Ω
50Ω
0.1µF
A
IN
+
A
IN
–
12pF
0.1µF
V
CM
LTC2258-12
ANALOG
INPUT
0.1µF
T1
1:1
T1: MA/COM MABAES0060
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
225812 F03
Figure 3. Analog Input Circuit Using a Transformer.
Recommended for Input Frequencies from 5MHz to 70MHz