LTC2205 Linear Technology, LTC2205 Datasheet - Page 22

• Manufacturer Part Number: LTC2205
• Description: (LTC2204 / LTC2205) 65Msps/40Msps
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC2205/LTC2204
Transformer Coupled Circuits
Figure 3 shows the LTC2205/LTC2204 being driven by
an RF transformer with a center-tapped secondary. The
secondary center tap is DC biased with V
ADC input signal at its optimum DC level. Figure 3 shows
a 1:1 turns ratio transformer. Other turns ratios can be
used; however, as the turns ratio increases so does the
impedance seen by the ADC. Source impedance greater
than 50Ω can reduce the input bandwidth and increase
high frequency distortion. A disadvantage of using a
transformer is the loss of low frequency response. Most
small RF transformers have poor performance at frequen-
cies below 1MHz.
Figure 3. Single-Ended to Differential Conversion Using a Transformer.
Recommended for Input Frequencies from 5MHz to 150MHz
22
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
T1
0.1µF
10Ω
10Ω
5Ω
35Ω
35Ω
8.2pF
ANALOG
2.2µF
INPUT
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
8.2pF
8.2pF
5Ω
5Ω
0.1µF
0.1µF
V
A
A
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
CM
IN
IN
+
–
CM
1:1
T1
LTC2205/
LTC2204
, setting the
22054 F03
25Ω
25Ω
0.1µF
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 70MHz to 250MHz
Center-tapped transformers provide a convenient means
of DC biasing the secondary; however, they often show
poor balance at high input frequencies, resulting in large
2nd order harmonics.
Figure 4a shows transformer coupling using a transmis-
sion line balun transformer. This type of transformer has
much better high frequency response and balance than
fl ux coupled center tap transformers. Coupling capacitors
are added at the ground and input primary terminals to
allow the secondary terminals to be biased at 1.25V. Figure
4b shows the same circuit with components suitable for
higher input frequencies.
ANALOG
INPUT
2.2µF
2.2pF
2.2pF
5Ω
5Ω
0.1µF
0.1µF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
V
A
A
CM
IN
IN
+
–
1:1
T1
LTC2205/
LTC2204
22054 F04b
25Ω
25Ω
0.1µF
10Ω
10Ω
4.7pF
2.2µF
4.7pF
4.7pF
5Ω
5Ω
V
A
A
CM
IN
IN
+
–
LTC2205/
LTC2204
22054fb
22054 F04a