LTC2272CUJ LINER [Linear Technology], LTC2272CUJ Datasheet - Page 23

• Manufacturer Part Number: LTC2272CUJ
• Description: 16-Bit, 80Msps/65Msps Serial Output ADC
• Manufacturer: LINER [Linear Technology]

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 cir-
cuitry and provide isolation from ADC S/H switching. The
LTC2273/LTC2272 have very broadband S/H circuits, DC
to 700MHz; it can be used in a wide range of applications;
therefore, it is not possible to provide a single recom-
mended 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 LTC2273/
LTC2272 do 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 LTC2273/LTC2272 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
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
8.2pF
8.2pF
5Ω
5Ω
V
A
A
CM
IN
IN
+
–
CM
, setting the
LTC2273/
LTC2272
22732 F03
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.
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
LTC2273/LTC2272
25Ω
25Ω
25Ω
25Ω
0.1μF
0.1μF
10Ω
10Ω
4.7pF
2.2μF
2.2pF
2.2pF
5Ω
5Ω
2.2μF
4.7pF
4.7pF
5Ω
5Ω
V
V
A
A
A
A
CM
CM
IN
IN
IN
IN
+
–
+
–
LTC2273/
LTC2273/
LTC2272
LTC2272
23
22732 F04a
22732 F04b
22732f