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LTC1405I Datasheet(PDF) 10 Page - Linear Technology |
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LTC1405I Datasheet(HTML) 10 Page - Linear Technology |
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10 / 16 page  10 LTC1405 Differential Operation The THD and SFDR performance of the LTC1405 can be improved by using a center tap RF transformer to drive the inputs differentially. Though the signal can no longer be DC coupled, the improvement in dynamic performance makes this an attractive solution for some applications. Typical connections for single and dual supply systems are shown in Figures 8a and 8b. Good choices for trans- formers are the Mini Circuits T1-1T (1:1 turns ratio) and T4-6T (1:4 turns ratio). For best results the transformer should be located close to the LTC1405 on the printed circuit board. must be greater than 50MHz to ensure adequate small- signal settling for full throughput rate. If slower op amps are used, more settling time can be provided by increasing the time between conversions. The best choice for an op amp to drive the LTC1405 will depend on the application. Generally applications fall into two categories: AC applications where dynamic specifica- tions are most critical and time domain applications where DC accuracy and settling time are most critical. Input Filtering The noise and the distortion of the input amplifier and other circuitry must be considered since they will add to the LTC1405 noise and distortion. The small-signal band- width of the sample-and-hold circuit is 100MHz. Any noise or distortion products that are present at the analog inputs will be summed over this entire bandwidth. Noisy input circuitry should be filtered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is sufficient for many applications. For example, Figure 9 shows a 1000pF capacitor from +AIN to –AIN and a 30Ω source resistor to limit the input bandwidth to 5.3MHz. The 1000pF capacitor also acts as a charge reservoir for the input sample-and-hold and isolates the amplifier driving VIN from the ADC’s small current glitch. In undersampling applications, an input capacitor this large may prohibitively limit the input band- width. If this is the case, use as large an input capacitance as possible. High quality capacitors and resistors should be used since these components can add distortion. NPO and silver mica type dielectric capacitors have excellent linearity. Carbon surface mount resistors can generate distortion from self-heating and from damage that may occur during soldering. Metal film surface mount resis- tors are much less susceptible to both problems. APPLICATIO S I FOR ATIO 1405 F08a +AIN VSS VIN 1000pF 15 Ω 15 Ω MINI CIRCUITS T1-1T 1 µF LTC1405 5V –AIN VCM 1405 F08b +AIN VSS VIN MINI CIRCUITS T1-1T 1 µF LTC1405 5V –5V –AIN VCM 1000pF 15 Ω 15 Ω Figure 8a. Single Supply Transformer Coupled Input Figure 8b. Dual Supply Transformer Coupled Input +AIN VIN LTC1405 1405 F09 –AIN 1000pF 30 Ω Choosing an Input Amplifier Choosing an input amplifier is easy if a few requirements are taken into consideration. First, to limit the magnitude of the voltage spike seen by the amplifier from charging the sampling capacitor, choose an amplifier that has a low output impedance (<100 Ω) at the closed-loop bandwidth frequency. For example, if an amplifier is used in a gain of 1 and has a unity-gain bandwidth of 50MHz, then the output impedance at 50MHz must be less than 100 Ω. The second requirement is that the closed-loop bandwidth Figure 9. RC Input Filter |
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