LTC2272I View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2272I' PDF : 44 Pages View PDF
LTC2273/LTC2272
APPLICATIONS INFORMATION
INPUT DRIVE CIRCUITS
Input Filtering
A first order RC lowpass filter 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 filter.
Figures 3, 4a and 4b show three examples of input RC
filtering 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 filter to achieve data sheet
specifications; however, no filtering 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 VCM, setting the
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
50Ω
10Ω
T1
0.1μF
10Ω
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
VCM
2.2μF
5Ω AIN+
35Ω
8.2pF
8.2pF
35Ω
5Ω AIN–
8.2pF
LTC2273/
LTC2272
22732 F03
Figure 3. Single-Ended to Differential Conversion
Using a Transformer. Recommended for Input
Frequencies from 5MHz to 150MHz
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
flux 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.
VCM
0.1μF
ANALOG
INPUT
T1
0.1μF 1:1
10Ω
25Ω 0.1μF
25Ω 10Ω
2.2μF
5Ω AIN+
4.7pF
LTC2273/
LTC2272
4.7pF
5Ω AIN–
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
4.7pF
22732 F04a
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 100MHz to 250MHz
0.1μF
ANALOG
INPUT
T1
0.1μF 1:1
25Ω 0.1μF
25Ω
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
VCM
2.2μF
5Ω AIN+
2.2pF
LTC2273/
LTC2272
5Ω AIN–
2.2pF
22732 F04b
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
22732f
23
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