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THS4531: balanced output

Connie Lu1
Intellectual 2080 points
Part Number: THS4531

Hi experts,

My customer have a question regarding the THS4531. Figure 75 in the datasheet sets the gain equation up as -Rf/Rg, as expected.

Their application is a little closer to Figure 68 with an input terminator of 75 ohms.

What they would like to understand is how balance is designed in this amplifier.  What is the purpose of the 0.22uF/49.9 ohm R for “balancing the amplifier”?  If there is no connection at Vin-, why wouldn’t the implementation just be DC-coupled and the Rit on Vin- be whatever value provides balance?

Thanks,

Connie 

  • 0
    TI__Mastermind 24145 points

    Hello Connie,

      The reason the Figure includes the 0.22uF and 49.9Ohm at the input to the no connection Vin- is due to the need to balance the inputs of the FDA. Since, the document is assuming the input to Vin+ will be from a signal generator that is AC coupled and a 50 ohm source, then this added impedance is seen at the positive input of the FDA which will need to be replicated at the negative input of the FDA in order to reduce reflections and preserve signal integrity. You are correct for mentioning being able to implement the balance at the inputs by using Rit if your single-ended source is not AC coupled. This figure demonstrates this case which was grabbed from this technical article: 

    The technical article linked includes in-depth detail about input impedance of FDAs and the importance of input impedance matching. Let me know if you have further questions on this topic. 

    Thank you,

    Sima 

  • 0
    Guru 48395 points

    Hey Connie, aside from what Simi was saying, the easy way to think about this is that you would like the voltage divider from each FDA output back to the input pins of the FDA to be balanced,

    often, these test setups are using a network analyzer where there is a large blocking cap looking back into that source. Hence the THS4531 drawing - that is more of a test setup issue and not so much application recommendation.