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LTC2204 Datasheet(PDF) 22 Page - Linear Technology |
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LTC2204 Datasheet(HTML) 22 Page - Linear Technology |
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22 / 36 page  LTC2205/LTC2204 22 22054fb 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. 0.1 µF AIN + AIN – 4.7pF 2.2 µF 4.7pF 4.7pF VCM LTC2205/ LTC2204 ANALOG INPUT 0.1 µF 0.1 µF T1 1:1 T1 = MA/COM ETC1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE EXCEPT 2.2 µF 22054 F04a 5 Ω 10 Ω 25 Ω 25 Ω 10 Ω 5 Ω Figure 4a. Using a Transmission Line Balun Transformer. Recommended for Input Frequencies from 70MHz to 250MHz 0.1 µF AIN + AIN – 2.2 µF 2.2pF 2.2pF VCM LTC2205/ LTC2204 ANALOG INPUT 0.1 µF 0.1 µF T1 1:1 T1 = MA/COM ETC1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE EXCEPT 2.2 µF 22054 F04b 5 Ω 25 Ω 25 Ω 5 Ω Figure 4b. Using a Transmission Line Balun Transformer. Recommended for Input Frequencies from 250MHz to 500MHz 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. APPLICATIONS INFORMATION Figure 3. Single-Ended to Differential Conversion Using a Transformer. Recommended for Input Frequencies from 5MHz to 150MHz 35 Ω 5 Ω 35 Ω 10 Ω 10 Ω 5 Ω 5 Ω 0.1 µF AIN + AIN – 8.2pF 2.2 µF 8.2pF 8.2pF VCM T1 T1 = MA/COM ETC1-1T RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE EXCEPT 2.2 µF 22054 F03 LTC2205/ LTC2204 |
Similar Part No. - LTC2204 |
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Similar Description - LTC2204 |
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