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LTC2207-14 Datasheet(PDF) 18 Page - Linear Technology |
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LTC2207-14 Datasheet(HTML) 18 Page - Linear Technology |
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18 / 32 page  LTC2207-14/LTC2206-14 18 220714614fa 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 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 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 LTC2207- 14/LTC2206-14 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 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 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 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 |
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