LTC2274 Linear Technology, LTC2274 Datasheet - Page 19

• Manufacturer Part Number: LTC2274
• Description: 105Msps Serial Output ADC
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
INPUT DRIVE CIRCUITS
Input Filtering
A fi rst order RC lowpass fi lter at the input of the ADC can
serve two functions: limit the noise from input circuitry and
provide isolation from ADC S/H switching. The LTC2274
has a very broadband S/H circuit, DC to 700MHz; it can
be used in a wide range of applications; therefore, it is not
possible to provide a single recommended RC fi lter.
Figures 3, 4a and 4b show three examples of input RC
fi ltering at three ranges of input frequencies. In general
it is desirable to make the capacitors as large as can be
tolerated—this will help suppress random noise as well
as noise coupled from the digital circuitry. The LTC2274
does not require any input fi lter to achieve data sheet
specifi cations; however, no fi ltering will put more stringent
noise requirements on the input drive circuitry.
Transformer Coupled Circuits
Figure 3 shows the LTC2274 being driven by an RF trans-
former with a center-tapped secondary. The secondary
center tap is DC biased with V
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
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
T1
Figure 3. Single-Ended to Differential Conversion
Using a Transformer. Recommended for Input
Frequencies from 5MHz to 150MHz
0.1μF
10Ω
10Ω
50Ω
35Ω
35Ω
8.2pF
2.2μF
CM
, setting the ADC input
8.2pF
8.2pF
5Ω
5Ω
V
A
A
CM
IN
IN
+
–
LTC2274
2274 F03
the input bandwidth and increase high frequency distor-
tion. A disadvantage of using a transformer is the loss of
low frequency response. Most small RF transformers have
poor performance at frequencies below 1MHz.
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
ANALOG
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 100MHz to 250MHz
INPUT
INPUT
0.1μF
0.1μF
0.1μF
0.1μF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
1:1
1:1
T1
T1
25Ω
25Ω
25Ω
25Ω
0.1μF
0.1μF
10Ω
10Ω
4.7pF
2.2μF
2.2pF
2.2pF
LTC2274
5Ω
5Ω
2.2μF
4.7pF
4.7pF
5Ω
5Ω
V
V
A
A
A
A
CM
CM
IN
IN
IN
IN
+
–
+
–
LTC2274
LTC2274
19
2274 F04a
2274 F04b
2274f