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THS4500: Source impedance affecting amplifier gain

Parashar Mohapatra
Prodigy 20 points
Part Number: THS4500

Hi, 

The THS4500 datasheet's fully differential amplifier configuration assumes a 50 Ohm source impedance. However, in the presence of the 50ohm source impedance, the gain is affected by a factor of 1/2. In this circuit, I expect an output voltage Vout+ - Vout- = 2V but I see 1V. When I bypass the 50Ohm resistor after the buffer stage (R6), the circuit works fine. Mathematically this makes sense to me but seems strange that the datasheet says otherwise. 

  • +1
    TI__Mastermind 25045 points

    Hello Parashar,

      Your calculation and theory is correct. That is a good point. The reason why the result is a factor of 1/2 is because when characterizing an amplifier, a dedicated source is used at the input of the amplifier. Sources set to internal 50ohm impedance automatically accounts for the voltage divider at the input and compensates for this by providing double the signal amplitude. Feel free to continue replying to this thread for further questions/concerns! 

    Thank you,

    Sima  

  • +1
    Guru 48395 points

    Many of the higher speed char curves use network analyzers where the calibration procedure normalizes out the -6dB matching loss. But yes, it is physically there - normally you drive with a doubly terminated source and load to get the best response if the source has a cable involve in it. If the source is very close to the Rg input resistor, matching is not usually required - eliminating the -6dB loss. 

  • 0 in reply to Michael Steffes
    Prodigy 20 points

    Thank you for the explanation. It is very helpful!