2003 Proper Input Network Selection Achieves Optimum Dynamic Perf

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2003 Proper Input Network Selection Achieves Optimum Dynamic Perf

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CONVERSION/SAMPLING CIRCUITS BASESTATIONS WIRELESS INFRASTRUCTURE HIGH-SPEED SIGNAL PROCESSING
2003
Proper Input Network Selection Achieves Optimum Dynamic Performance Excellent Gain Flatness High-Speed ADCs
following application note provides guidelines select appropriate transformer passive components achieve gain flatness over wide range input frequencies without sacrificing dynamic performance high-speed data converter. Proper selection board components essential factor meet demanding high dynamic performance gain flatness requirements high-IF analog-to-digital converters (ADCs). following technical note will lend some insight into appropriate selection input networks designed easy single-ended differential input signal conversion with help wide-band transformer, termination resistors filter capacitors. MAX1449 chosen demonstrate analyze possible input configuration designs. Figure shows typical AC-coupled, single-ended differential conversion design using wide-band transformer such Mini-Circuit's T1-1T-KK81 (200MHz) with primaryside termination /22pF filter network. this configuration, single-ended signal from -impedance source taken converted differential signal through transformer. Primary-side termination into allows excellent matching between signal source transformer. However this also means that there mismatch between primary secondary side transformer. primary side looks into combined impedance while secondary side experiences large impedance mismatch with input resistance shunted 22pF. This will impact frequency response input network, ultimately affecting frequency response converter. transformer's nominal leakage inductance range anywhere from 25nH 100nH. Combined with input filter capacitor 22pF this will create disturbing resonance frequency:
located between 110MHz 215MHz resulting undesired gain peaking this frequency range.
Figure Figure depicts similar AC-coupled configuration, however this circuit designed with better performing wide-band transformer such Mini-Circuits ADT1-1WT (800MHz) with primary-side termination /10pF filter network. Although, ADT1-1WT impedance, lower leakage inductance yields significantly better frequency response -1dB 400MHz, compared only 50MHz T1-1T-KK81.
Figure Figure shows results both termination schemes selections filter network components transformers. significant improvement observed between graphs. input bandwidth trace T1-1T-KK81 transformer (blue) clearly shows gain peaking about 0.5dB between 90MHz 110MHz, while curve ADT1-1WT transformer (magenta) stays flat within tenth frequencies 300MHz. dynamic performance this condition (ADT1-1WT transformer,
Figure primary-side termination 10pF input filter capacitors INN) still yielded excellent 58.4dB fIN=50MHz. Though Figure only displays tested input frequencies 80MHz 260MHz (ADT1-1WT only), tests have proven that gain remains flat within 0.1dB input frequencies well beyond Nyquist region. Taking effort better match secondary side impedance transformer help further enhance gain flatness. this using secondary-side termination rather than primary-side termination. This approach will covered separate application note based some input network designs their analysis performed with Maxim's recently introduced MAX1122/23/24 family. Refer application note link reference section (below) further details primary-side secondary-side termination. REFERENCES MAX1448EVKit Datasheet, Rev1, 7/2001, Maxim Integrated Products, Sunnyvale, MAX1449 Datasheet, Rev0, 10/2000, Maxim Integrated Product, Sunnyvale, Application Note: Secondary-Side Transformer Termination Improves Gain Flatness HighSpeed
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2003 Proper Input Network Selection Achieves Optimum Dynamic Perf

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