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LTC2207-14/LTC2206-14
19
220714614fc
INPUT DRIVE CIRCUITS
Input Filtering
A rst order RC lowpass 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 LTC2207-
14/LTC2206-14 have 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 recom-
mended RC lter.
Figures 3, 4a and 4b show three examples of input RC
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 LTC2207-
14/LTC2206-14 do not require any input lter to achieve
data sheet specications; however, no ltering will put
more stringent noise requirements on the input drive
circuitry.
Transformer Coupled Circuits
Figure 3 shows the LTC2207-14/LTC2206-14 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
0.1μF
AIN
+
AIN
–
4.7pF
2.2mF
4.7pF
VCM
LTC2207-14/
LTC2206-14
ANALOG
INPUT
0.1μF
T1
1:1
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
2207614 F04a
5Ω
50Ω
10Ω
25Ω
10Ω
5Ω
Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 100MHz to 250MHz
0.1μF
AIN
+
AIN
–
2.2μF
2.2pF
VCM
LTC2207-14/
LTC2206-14
ANALOG
INPUT
0.1μF
T1
1:1
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
2207614 F04b
5Ω
50Ω
25Ω
5Ω
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
than 50
Ω can reduce the input bandwidth and increase
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
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.
Figure 3. Single-Ended to Differential Conversion
Using a Transformer. Recommended for Input
Frequencies from 5MHz to 150MHz
35Ω
50Ω
35Ω
10Ω
5Ω
0.1μF
AIN
+
AIN
–
8.2pF
2.2μF
8.2pF
VCM
LTC2207-14/
LTC2206-14
T1
T1 = MA/COM ETC1-1T
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
2207614 F03
APPLICATIONS INFORMATION
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