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AD9251 FDA

FDA input ZIn

My question.  In the datasheet fig 41 shows a recommended differential driver (FDA) interfacing to the ADC diff inputs.  It looks like the Zin of the amplifier is appx 50ohms.  I have a case where I could drive this FDA with another op amp but using a higher load than 50ohms would be easier.  Most op amps cannot drive 50ohms directly so I would need to use a series 50ohm to limit current.  Is it possible to reconfigure the FDA input a bit to make it maybe 200-500 ohms?  Any circuit suggestions?  Freq of operation is <= 1MHz.  

  • Thanks for your interest in AD9251. Because this is more of an amplifier question, I've moved it to the "ADC Drivers" Q&A.

    Thank you.


  • 0
    •  Analog Employees 
    •  Super User 
    in reply to DougI

    Hi ,

    The AD9251 ADC diff input is a switched capacitor arrangement:

    According to AN-742 Application Note, the ADC input impedance will have Real and Imaginary components that vary between the Track to Hold mode. From this application note, we have the following plot for AD9236 ADC (cannot find a similar plot for AD9251 that you've asked about, but there should be similarities):

    The important parameter is the Track mode impedance. From Figure 11 of the application note, shown below, that'd be 1.57kohm @ 120MHz:

    Your application of only 1MHz signals will have a much higher input impedance than what's shown for 120MHz to be 1.57kohm.

    The RC (33ohm and 10pF) of Figure 41 (AD9251 datasheet) are recommended components for best settling at the ADC input.

    I hope I've answered your questions.



  • Hello Hooman,

    Thanks for the reply.  This is not exactly what I was going after.  I was interested in reconfiguring the diff amp input so that it's not 50ohm.  I would like to raise it higher so that I could drive it direct with an op amp.   I was wondering how to configure the op amp Zin to be maybe 200-500ohms?  The gain of the diff amp can be one, or very low.  


  • Hi Dennis,

    If all you're trying to do is to raise the diff amp for higher Zin, you could employ the ADI Diff-Amp Calculator for that:

    I've configured the ADA4938 for Zin = 500ohm below:

    All you do is enter your Rs=500ohm, enter your Vocm (1V for example), supply voltages (+/-5V for example), and enter the Target Gain (1V/V) with some starting value for Rg (e.g. 1kohm). The tool computes the other stuff.

    Here is my diff-amp calculator file that you can start with if you like:

    ADA4938 EZ

    I've shown this in LTspice simulation as well below:

    Here is the LTspice file if you're interested to play with (Extract All before you open in LTspice):

    ADA4938 EZ Rin_500ohm Setup



  • Hello Hooman,

    This is exactly what i wanted to know.  Didn't know there was a program to do all of the heavy lifting.  Glad there is.  



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