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THS4505: THS4505

Arkadi Trestman
Prodigy 50 points

Part Number: THS4505
Other Parts Discussed in Thread: THS4504, THS4551, THS4561,

In accordance with the data sheet the common-mode input range of THS4504/4505 at Vs=5V (single supply) is from -0.1 to 2V over temperature (see page 7 of the data sheet). In the second side there is an example of a fully differential line driver  with high output swing (see figure 81 of the data sheet) where the common mode is 7.5V at Vs=15V. My circuit is a transformer driver based on the fully differential configuration with input common-mode voltage is 2.5V (see attached file). If this OK?

  • 0
    Guru 48395 points

    Morning Arkadi, 

    Your circuit did not come through, use the little paperclip symbol in the ribben to attach files. 

    But yes, the transformer input FDA is a very nice approach - done correctly, the input CM for the device pins is the output CM voltage for the FDA.This ckt can improve both the noise figure and distortion.

  • 0 in reply to Michael Steffes
    Prodigy 50 points

    Thank you, Michael,

    I inserted my circuit. May I provide +2.5V input common-mode voltage to this amplifier?

    Regards,

    Arkadi.THS4505.docx

  • 0 in reply to Arkadi Trestman
    Guru 48395 points

    Oh, I see now Arkadi, you have an output transformer, I had been assuming an input transformer. 

    In any case - 

    1. You are AC coupled I/O - so yes, the output CM controlled by your 2.5V input will appear at both input and output FDA pins, works fine

    2. Your implied BW by that output 18.2ohm to 0.1uF diff cap is pretty low, not for sure as you need to also consider the reflected load coming in from the xfmr secondary - but looks to be <100kHz. Why use such a fast part? a part like the THS4551 or THS4561 should be plenty fast enough.

    3. Eventually, I would be concerned about that 10pF feedback - yes its a signal path pole, but also a noise gain shape to unity gain. Whatever device/ckt you end up with, would want to check LG phase margin.

    4. looking back towards the source, can only see that Rf = 33.2kohm - either a very high gain, or unnecessarily high noise. Again, that stage implies pretty low BW - but without the rest of the circuit hard to be sure. That AD8691 is only 10MHz GBP, emphasizes low offset, but with AC coupling not needed.

  • 0 in reply to Michael Steffes
    Guru 48395 points

    I also looked at the THS4505 datasheet, you do know that is a circa 2002 device, quite a lot has changed solution wise since then. It is of course a very credible early FDA, many many subsequent developments have improved things quite a lot in the area of performance vs quiescent current  - etc.