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Original question:

THS4500: Source impedance affecting amplifier gain

THS4500: Is a termination resistor Rt necessary for V+ if a buffer stage is used?

Parashar Mohapatra
Prodigy 20 points
Part Number: THS4500
Other Parts Discussed in Thread: THS4561, LMV796

I plan on using a buffer at V+ of THS4500 as shown below. 

Is a termination resistor before R3 necessary as specified in the datasheet? If so, how is the value determined in the absence of a source resistance? Lastly, the datasheet recommends having R3=487Ohms and R4 = 523 Ohms. However, when I work out the math (also confirmed by tests), I get a gain of ~0.93. Simulations and equations for an ideal diff amp show that R1=R4=R2=R3 should give a gain closer to 1. Is there something about THS4500 that I am getting wrong or are the test conditions in the datasheet different as suggested in the response to my previous question

example from the datasheet for reference.

  • 0
    Guru 47865 points

    The LMV796 is only 17MHz? If you are using it as DC bias path, no you don't need the termination R, but recognized as its output impedance ramps above 17MHz you gain balance is going to go away. Maybe you should just describe what you are trying to do? Where is your signal input, if it really is the LMV796 why use such a fast FDA, perhaps look at the THS4561

  • 0 in reply to Michael Steffes
    Prodigy 20 points

    I am trying to use a hall effect sensor with an output between 0-2V DC, so, I do not really care about the high-frequency effects. The position will be changing relatively slowly and will be an analog signal.  I am adding an FDA because the output signal needs to travel over a long harness. As you said, I will check out the THS4561. 

  • 0 in reply to Parashar Mohapatra
    TI__Mastermind 23045 points

    Hello Parashar,

       Agreed, no need for termination R. For that configuration, it was used for impedance matching and maintaining balancing between the two paths. Michael brings up a really good point: maintaining balance is critical for FDA's, so in higher frequencies than specified for the LMV796, the output impedance will change which will affect the balance between the noninverting and inverting input resistance to the FDA. 

       For resistor configuration, the table focuses on balanced configuration. It is highly recommended to use the feedback resistors as specified in the datasheet, but input resistors can be tuned to your gain requirements + any source/previous device's output resistance. I would instead recommend following the setup for the datasheet figures characterizing the device at a gain of 1, which uses a feedback resistor of 392. 

    Thank you,

    Sima