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ISLA214P13 bảng dữ liệu(PDF) 5 Page - Intersil Corporation |
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ISLA214P13 bảng dữ liệu(HTML) 5 Page - Intersil Corporation |
5 / 25 page Application Note 1837 5 AN1837.0 May 3, 2013 INPUT SIDE CIRCUIT DESIGN AND OPTIONS As delivered, the interface uses an input transformer that is very flat to low frequencies followed by a common mode choke transformer that is a DC short for biasing with <0.3dB insertion loss to >1GHz for the differential signal. Either may be removed and replaced by shorts or resistor elements using the bypass resistors. The input signal is coupled through a 1μF blocking cap (to protect against accidental DC shorts) to an input step up transformer and then directly into a common mode choke transformer. The 1:2 turns (1:4Ω) ratio step up transformer offers many advantages combined with an FDA as shown here. The second transformer provides a differential signal short but a common mode open circuit. Differentially, the two 100Ω gain resistors appear directly at the output of the input step up transformer. Briefly, the 2-100Ω series resistors feed into a differential virtual ground at the FDA summing junctions (R1009&R1010 in Figure 4) and form a 200Ω differential termination impedance for the ADT4-6T. That is input referred as a broadband 50Ω termination impedance out of the 1μF blocking cap. Scaling these resistors up, while still maintaining a 200Ω secondary termination, can be done using the Rterm1 element. This is sometimes useful at lower gain targets to allow the feedback resistors (R1008 & R1011) to be scaled up when they might be adding significant loading to the output stage of the ISL55210. Detailed specifications for 4GHz gain bandwidth product, 0.85nV/√Hz input noise ISL55210 may be found at: http://www.intersil.com/products/ISL55210 The ISL55210 provides duplicates of the differential outputs on the input side for tighter signal path layout. The feedback resistors are the 2 -500Ω elements where the connection back into the inverting summing junctions are the 0Ω elements. Neglecting transformer insertion losses, the gain from the input port to the amplifier outputs should be 2 (in the first transformer) X 5 (in the FDA) = 10 (or 20dB). The ADT4-6T was selected primarily for its excellent flatness and distortion down to 100kHz. Its measured response showed a -1dB flatness span from 40kHz to 178MHz when driven from 50Ω to 200Ω load with a -0.18dB midband insertion loss. This is a suitable frequency span for the intended 100kHz to 100MHz digitizer bandwidth in this board. It does show a bit of rolloff at 100MHz which is partially equalized by the 10pF capacitors to ground at the summing junctions in Figure 4. Adding those capacitors does start to peak the output noise of the ISL55210, but this stage will be followed by a passive filter rolling that noise off. A detailed discussion of the input referred noise figure for this transformer coupled FDA topology may be found at: http://www.edn.com/design/analog/4400484/Accurately- predict-measured-noise-figures-for-transformer-coupled- differential-amplifiers--Part-1-of-2-- For the default configuration on this ISLA214P50-55210EV1Z board using the components shown in Figure 4, the estimated noise figure will be 7.2dB from a 50Ω source. Converting that to an input referred spot noise voltage including a 50Ω source noise gives a very low 1.02nV/√Hz. This is only for the amplifier stage, and not including any noise in the original source signal. Delivering this to the ADC inputs through the full signal path gain FIGURE 4. AMPLIFIER POWER SUPPLY DECOUPLING AND INPUT SIGNAL UP TO THE AMPLIFIER INPUTS |
Số phần tương tự - ISLA214P13 |
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Mô tả tương tự - ISLA214P13 |
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