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TINA/Spice/THS4121: Specification indicates open-loop gain of 66dB. What is the bandwidth for which this gain is valid?

James Fyne
Prodigy 110 points
Part Number: THS4121
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

I'm trying to use the THS4121 in a differential transimpedance application. The required bandwidth is less than 100 kHz, but I require much more gain than 66dB, which is what's found in the spec.. The TI SPICE model indicates more gain is possible, but I want to make sure this is valid. Even feedback resistors of 1 Meg Ohms seem to work fine.

  • 0
    Guru 140200 points

    Hi James,

    as you speak of a transimpedance application, do you mean by "gain" the "transimpedance gain"?

    Can you show us a schematic of your application?

    Kai

  • 0
    TI__Genius 15975 points
    Hi James,

    The 66-dB specified in the datasheet is for the open-loop gain (Vo/Vi), instead of the transimpedance (TIA) gain (Vo/Iin) which is primarily determined by the feedback resistor (Rf). Since the THS4121 device has very low input bias currents (~1.2pA), I think it should be possible to use an Rf of 1Mohms.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 110 points

    Hi Rohit,

    I was thinking the same thing, that with external resistors controlling the gain, a value greater than 2000 (66dB) should be achievable. That's why I was wondering what circumstances pertained to the 66dB value, e.g. advertised bandwidth of operation. A gain-BW product graph would have been helpful.

    Thanks for your reply.

    jfyne 

  • 0 in reply to James Fyne
    TI__Genius 15975 points

    Hi James,

    It is possible to simulate the open loop frequency response of the THS4121 using TINA simulator (attached below), which should give the gain-bandwidth product graph.

    THS4121_OpenLoopResp.TSC

    I would recommend you to go through this app-note (should be applicable for differential input as well), in-order to understand the GBP relationship with the RF value. 

    www.ti.com/.../sboa122.pdf

    Best Regards,

    Rohit