LTC2204CUK View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2204CUK' PDF : 36 Pages View PDF
LTC2205/LTC2204
APPLICATIONS INFORMATION
Transformer Coupled Circuits
Figure 3 shows the LTC2205/LTC2204 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
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 transmission
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.
5Ω
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
LTC2205/
LTC2204
22054 F03
0.1μF
ANALOG
INPUT
T1
1:1
0.1μF
10Ω
25Ω 0.1μF
25Ω 10Ω
T1 = MA/COM ETC1-1-13
RESISTORS, CAPACITORS
ARE 0402 PACKAGE SIZE
EXCEPT 2.2μF
VCM
2.2μF
5Ω AIN+
4.7pF
LTC2205/
LTC2204
4.7pF
5Ω AIN–
4.7pF
22054 F04a
Figure 3. Single-Ended to Differential Conversion Using a Transformer. Figure 4a. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 5MHz to 150MHz
Recommended for Input Frequencies from 70MHz to 250MHz
22
0.1μF
ANALOG
INPUT
T1
1:1
0.1μF
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
LTC2205/
LTC2204
5Ω AIN–
2.2pF
22054 F04b
Figure 4b. Using a Transmission Line Balun Transformer.
Recommended for Input Frequencies from 250MHz to 500MHz
22054fc
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