LTC2205-14 LINER [Linear Technology], LTC2205-14 Datasheet - Page 17

• Manufacturer Part Number: LTC2205-14
• Description: 14-Bit, 65Msps ADC
• Manufacturer: LINER [Linear Technology]

APPLICATIONS INFORMATION
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
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 150MHz to 250MHz
INPUT
0.1mF
0.1mF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2mF
1:1
T1
25Ω
25Ω
0.1µF
10Ω
10Ω
Figure 3. Single-Ended to Differential Conversion Using a Transformer.
Recommended for Input Frequencies from 5MHz to 150MHz
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
4.7pF
T1
4.7pF
5Ω
5Ω
2.2µF
4.7pF
0.1µF
10Ω
10Ω
V
A
A
CM
IN
IN
+
–
5Ω
LTC2205-14
220514 F04a
35Ω
35Ω
2.2µF
8.2pF
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 de-
grade 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.
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
ANALOG
INPUT
8.2pF
8.2pF
5Ω
5Ω
0.1µF
0.1µF
V
A
A
CM
IN
IN
+
–
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2µF
LTC2205-14
1:1
T1
220514 F03
25Ω
25Ω
0.1µF
LTC2205-14
2.2µF
2.2pF
2.2pF
5Ω
5Ω
V
A
A
CM
IN
IN
+
–
LTC2205-14
17
22054 F04b
220514fa