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THS4121: THS4121CD amplifier circuit

dinesh gupta
Intellectual 310 points
Part Number: THS4121

I am using THS4121CD for one design. Input to opamp is from capacitive acoustic sensor.

We require noise floor of 10 uVrms or better. But for current circuit we get noise floor of 100 uVrms.

I want to know if there is problem with my circut. (Attached the circuit below)

Thanks,

Dinesh

  • 0
    Guru 47535 points

    There is a problem in that you do not describe anywhere near enough to go on - try a full TINA schematic to go further - (also, a real load would be preferred and specifically you show no feedback elements - no one can answer this as is). 

  • 0 in reply to Michael Steffes
    Intellectual 310 points

    Hello,

    Thanks for your help! I tried the TINA schematic simulation and I am getting the below noise.

    With the below circuitry we are getting the noise floor of 100 uVrms.

    Can you please suggest the feedback elements and the real load that you mentioned in the schematic.

    Sorry, but I am new to circuit development and would appreciate your help.

    FYI, there is an input parasitic capacitance of 5.5 pF. Can you please confirm if it is required to be added in the TINA schematic simulation.

  • 0 in reply to dinesh gupta
    TI__Genius 15555 points

    Hello Dinesh,

    The parasitic capacitance should be included in the THS4121 model. Could you clarify would you are trying to achieve with the circuit? Since it is clearly not in a typical gain configuration.

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    Guru 47535 points

    Hello Dinesh, 

    Surprised it simulates at all, 

    1. You must first validate your DC operating point - consider the effect of those 10G feedback R's with the input bias currents - that will peg the inputs to the rail opening up the feedback loop - not a valid circuit. 

  • 0 in reply to Michael Steffes
    Guru 47535 points

    Oops - so this is that one CMOS FDA, next layer

    1. Those 10G feedback R will introduce a very low pole in the loop due to input C - that would become a stability issue and of course does peak the the noise considerably. 

    Essentially, combining a 10G feedback with a 100Mhz part is going to be difficult. 

  • 0 in reply to dinesh gupta
    Guru 140200 points

    Hi Dinesh,

    the THS4121 might be a bit too fast to be used in a beginner project. Have you experience with OPAmp circuits?

    Kai