I am designing a circuit for driving a differential input ADC. As operational amplifier I use LTC 6254. My design works fine when simulating in LT Spice. However, when it comes to analyzing one of my outputs using a spectrum analyzer voltage level collaps

I simulated my circuit in LTSpice, as the spectrum analyzer I placed 50 ohm resistors. It works fine. 

  • 0
    •  Analog Employees 
    on Apr 26, 2018 8:37 AM


    A schematic would help us analyze what's wrong. What ADC are you using?


  • 0
    •  Analog Employees 
    on Apr 27, 2018 10:11 AM

    Hi Mustafa,

    For a simpler solution, use ADC drivers. To optimize the 65MSPS sampling rate of LTM9008-14, you can use either LTC6404-1 or AD8139. However, if you will only operate at 5 MHz (shown in your simulations), please use ADA4940 or LT1994 instead. It is much simpler to implement than using to op-amps to convert single-ended to differential signals.

    For the attenuation you observed in the output, do you have other components after the amplifier outputs? It seems to have created a voltage divider network with the analyzer. In 1 Mohm termination, it's not a big factor.


  • Hi,

    Let me be more clear.

    1) I have a single ended signal and I want to convert it to digital using LTM9008-14. Since  LTM9008-14  accepts differential input I use a driver circuit whose schematic is below.As operational amplifiers I used LTC6254. 50 ohm resistors represent Spectrum analyzer's input.

    When I simulate this circuit I get the result below, which is quite good enough for me.

    I simulate the same circuit, changing 50 ohm resistors with 1mega ohm resistors to simulate the input of the oscilloscope, nothing changes as seen below.

    2) I produced the circuit's pcb and then tested it by connecting the probe with one of the differential lines. When the oscilloscope's input impedance is set to 1 Megaohm everything seems fine as you can see below.

    3) However, when I set the input impedance as 50ohm voltage level collapses. 

    Such a result makes me think that the operational amplifier cannot supply the necessary current. This situation affects 2 things.

                a. Input impedance of the spectrum analyzer is 50 ohm. Since the voltage level collapses dramatically I see very weak signals. Harmonics cannot be seen, probably because they stay below noise floor. As a result I cannot analyze the quality of my circuit pproperly.

               b. I am not sure whether or not LTC6254 can supply the necessary current to drive LTM9008-14.

    Do you habe any idea about the situation?

    You can give advice for another operational amplifier with a really high GBP, or another single ended to differential converter.

    Thank you very much for your interest.

  • Yes my input signal is always below 5MHz.
    I have looked through the differential amplifiers you suggested  AD8139,    LTC6404-1,  ADA4940,  LT1994.
    One other requirement in my design is that the input source is a high impedance source and can only drive capacitive loads of ~15pF.
    Therefore I require the configuration I send in the above schematics. 
    1) Is it possible to drive the amplifiers you suggested in a similar way?
    I will check my circuit again about the 50 ohm load driving problem.
  • 0
    •  Analog Employees 
    on May 2, 2018 5:11 AM

    Hello Mustafa,

    Give me some time to work on the solution.