Datasheet
LTC2205-14
17
220514fb
Figure 3. Single-Ended to Differential Conversion Using a Transformer.
Recommended for Input Frequencies from 5MHz to 150MHz
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 150MHz to 250MHz
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
Direct Coupled Circuits
Figure 5 demonstrates the use of a differential amplifi er to
convert a single ended input signal into a differential input
signal. The advantage of this method is that it provides
low frequency input response; however, the limited
gain bandwidth of any op amp or closed-loop amplifi er
will degrade the ADC SFDR at high input frequencies.
Additionally, wideband op amps or differential amplifi ers
tend to have high noise. As a result, the SNR will be
degraded unless the noise bandwidth is limited prior to
the ADC input.
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 transmission
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.
APPLICATIONS INFORMATION
35Ω
5Ω
35Ω
10Ω
10Ω
5Ω
5Ω
0.1μF
A
IN
+
A
IN
–
8.2pF
2.2μF
8.2pF
8.2pF
V
CM
T1
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
220514 F03
LTC2205-14
0.1μF
A
IN
+
A
IN
–
4.7pF
2.2μF
4.7pF
4.7pF
V
CM
LTC2205-14
ANALOG
INPUT
0.1mF
0.1mF
T1
1:1
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2mF
220514 F04a
5Ω10Ω
25Ω
25Ω
10Ω
5Ω
0.1μF
A
IN
+
A
IN
–
2.2μF
2.2pF
2.2pF
V
CM
LTC2205-14
ANALOG
INPUT
0.1μF
0.1μF
T1
1:1
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
22054 F04b
5Ω
25Ω
25Ω
5Ω