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Original question:

THS4551: Need to know how much maximum gain can be set for THS4551

THS4551: Impedance details needed for differential output of THS4551

Sooraj Surendran
Prodigy 110 points
Part Number: THS4551


Hi All 
Am using THS4551 for converting single ended to differential output , what should be the impedance details of differential output signals . Does it need to be length matched . need suggestion on this 

  • 0
    TI__Intellectual 1090 points

    Hi Sooraj,

    Thank you for your question. Impedance matching is going to be important depending on the frequency of your signal and the length of your trace. For example, for a 1MHz signal, a trace length of 52.5 m and above will need impedance matched lines. Compare that to a 1GHz signal where traces of 52.5mm and above need to be matched. The higher frequency the signal, the shorter the distance that requires special consideration. Reference.

    A more detailed analysis into the theoretical background can be found within the application notes, Input impedance matching with fully differential amplifiers and Output impedance matching with fully differential operational amplifiers.

    In general, we recommend to taking basic measures to impedance match for high-speed applications at minimum to help ensure signal integrity as outlined in the THS4551 datasheet on page 57 in section 12. 

    Thank you,
    Rachel

  • 0 in reply to Rachel Scheller
    Prodigy 110 points

    Hi 
    our signal is less than 50khz and for this do we need to length matching at differential side of this op amp . Each Differential lines need 50E ?

  • 0 in reply to Sooraj Surendran
    TI__Intellectual 1090 points

    Hello Sooraj,

    With a signal less than 50kHz, extreme measures to impedance match the paths are not necessary. However in general, good practice for fully-differential amplifiers is to have both outputs of the amplifier as close to the same impedance as possible. Notice in the figure 8-1 below that both sides of the amplifier have the same RG, RF, and Rout values even when performing a single-ended to differential conversion.

    For a reference, we have an evaluation module for each package of the THS4551 available on TI.com to help evaluate the device. Each board comes with a user guide that has layout recommendations and examples for how to best design with the device. Linked here is an example for the VQFN (RGT) package. 

    50Ω termination is not necessary for the functionality of the device independently, but could be important for depending on the application (such as if you are using the device with lab equipment that is 50Ω terminated as in Figure 8-1). If you have any additional information about your application or schematic design, I could help make additional recommendations into the necessity. 

    Thank you,
    Rachel