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ISLA212P25 Arkusz danych(PDF) 4 Page - Intersil Corporation |
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ISLA212P25 Arkusz danych(HTML) 4 Page - Intersil Corporation |
4 / 25 page Application Note 1837 4 AN1837.0 May 3, 2013 The analog input signal comes in through 2-MiniCircuits transformers then into the ISL55210 then into an interstage passive RLC filter to the ADC. The blue potentiometer in the middle of the board is a common mode voltage adjustment for the average DC voltage applied to the ADC inputs for this AC coupled signal path. This board plugs into the KMB-001 motherboard to appear as in Figure 3. Here, the input signal is connected and the 500MHz clock is being applied through a TTE 500MHz Bandpass filter. The power is being applied to the motherboard as shown by the active green light just under the +5V power connector in the upper right. Again, the +5V board connector is plugged in first, then the AC plug inserted to the power allowing the power brick to filter the +5V power up transient. Similarly on power down, unplug the +5V power at the AC plug side. While the full signal path schematic is shown in Figure 46, it is best to break it into pieces for discussion of design, performance, and options. In general, the board offers numerous optional connections that are indicated in green on the schematic and/or by DNP for the standard board build. The basic signal path is intended to: 1. Terminate a single ended input with an AC-coupled, broadband, 50Ω impedance. This is of course expecting the source to also be a broadband 50Ω source and that impedance does get reflected through the input transformers to be part of the signal chain flatness characteristic. 2. Convert the single ended input to the required differential signal at the ADC inputs centered within the converter’s desired common mode voltage range. 3. Provide amplification from a nominal -7dBm (283mVP-P) single tone input to a -2dBFS (2dB below the 2VP-P full scale of the ADC) or 1.58VP-P at the ADC input pins with extremely low noise and distortion. This 5.6V/V gain (15dB) gain is a combination of the input transformer step up, ISL55210 gain, and various insertion losses in the transformers and interstage filter from the ISL55210 to the ADC. 4. As part of the interstage filter design, this board includes a VCM control loop that senses the average DC voltage at the two ADC inputs and servo’s a control voltage into the filter design to match a reference voltage applied to the servo loop op amp. The board is delivered set to 0.96V as that has shown improved SFDR for the filter source impedance and the ISLA214P50 ADC. Changing ADC selections and/or filter designs in this board might benefit from a different nominal VCM control target for best SFDR and the VCM adjust pot allows and easy means to test that. AMPLIFIER POWER SUPPLY DECOUPLING ISSUES The portion of the signal path schematic of Figure 4 shows one example of good power supply decoupling for this single 3.3V supply amplifier. Where possible, a large valued capacitor isolated towards this 4GHz amplifier with a high frequency ferrite and then another 1µF element provides a PI filter on the board. Right at the 2 device supply pins, X2Y capacitors are used to get the best high frequency decoupling. These 0.1µF elements actually are two capacitors connected in parallel to give 0.2µF decoupling with much lower ESR and higher self resonant frequency than typical ceramic SMD capacitors. Standard SMD 0.1µF ceramic capacitors can be used instead but may increase the even order harmonics at higher frequencies as they go self resonant. Those are provisioned on the board as optional elements but not populated as shown in Figure 4 by the green capacitors C1003 and C1008. FIGURE 3. ISLA214P50-55210EV1Z DAUGHTERBOARD ON KMB-001 MOTHERBOARD |
Podobny numer części - ISLA212P25 |
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Podobny opis - ISLA212P25 |
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