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AD9272 - LNA impedance matching

Question asked by logison on Jun 16, 2015
Latest reply on Jun 16, 2015 by AshrafS

Hi ,

 

I've got some doubts..  Let's consider the evaluation board schematics http://www.analog.com/media/cn/technical-documentation/evaluation-documentation/UG-001.pdf. Take a look on page 12 and let's take a look for example on Channel "A". R105 and R107 values has been calculated according to formula 2 ( AD9272 datasheet/ http://www.analog.com/media/en/technical-documentation/data-sheets/AD9272.pdf / page 23). The value of the resistor R107 is calculated for transducer impedance 50Ohm@LNA Gain=21.3dB (Table 7 on datasheet page 23). Question: What is purpose of a 49.9Ohm resistor R101 on a evaluation board? The input impedance is already matched by R107. Adding R101 will IMO degrade it. Whats your opinion about it?

 

Best Regards,

 

Mariusz

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