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BUF634A: BUF634A current noise in Tina is inconsistent with data sheet

Terry Noe
Prodigy 20 points
Part Number: BUF634A
Other Parts Discussed in Thread: TINA-TI

I'm running a simple simulation of the BUF634A in Tina, looking at noise.  To measure the current noise, I put a 1 Mohm resistor on the input, and measure the output noise.

Simulating the output noise, I get

At 10 kHz, output noise is 3 uV/rtHz, so the input noise is 3 uV / 1 megohm = 3 pA/rtHz.  The datasheet says the current noise is 1 pA/rtHz, 10 dB lower.  Any idea what's going on here?  I tend to believe the datasheet before the model, but some measurements on my application circuit on my bench suggest the noise is higher than expected, so maybe the model is right?

I should mention that the BUF634A has a bandwidth control pin, this pin is not accessible in the Tina model.  I think the model is configured for wideband mode, based on the frequency response and voltage noise measurements.  But if I'm wrong, and the model is configured for low bandwidth mode, current noise should be 0.3 pA/rtHz, even lower than in wideband mode.

Thanks,

Terry

  • 0
    TI__Genius 17306 points

    Hello Terry,

    I can work on replicating your input-referred current noise simulation; I can confirm the model is setup for wideband mode, as low-bandwidth mode can be simulated by setting an upper bandwidth limit on your simulation signals and parameters.

    If you have measurements you can easily share, I could also examine those data values.

    Best,

    Alec

  • 0
    Guru 140200 points

    Hi Terry,

    by double clicking onto the OPAmp symbol in the TINA-TI simulation you can easily see what's modelled in the Spice model and what is not. It turns out that only the "input noise voltage density" is modelled but not the "input noise current density":

    When you compare figures 7-9 and 7-10 of datasheet you will notice that (for frequencies above 1kHz) in the low bandwidth mode the input noise voltage dominates over the input noise current for source impedances below 8nV / 0.3pA = 27kOhm and in the high bandwidth mode for source impedances below 3nV / 0.9pA = 3.3kOhm. And since in the most applications the BUF634A sees (way) smaller source impedances than these, the input noise current density of BUF634A usually plays a minor role. This may be the reason why the input noise current density isn't modelled in the Spice model of BUF364A.

    Kai

  • 0
    Guru 48395 points

    Hey Terry, I already had this file for some reason, the new model at higher Iq looks perfect to me on input current noise, ALso, I looked at your file, your 1Mohm source is bandlimiting the noise response due to input parasitic C which is why it is rolling off so starkly

    here is the datasheet plot, yes about 0.9pA at flatband, 

    And this test file, 

    BUF634A current noise test including 2000 model.TSC

  • 0 in reply to Michael Steffes
    Guru 48395 points

    Hey Terry, I already had this file for some reason, the new model at higher Iq looks perfect to me on input current noise, ALso, I looked at your file, your 1Mohm source is bandlimiting the noise response due to input parasitic C which is why it is rolling off so starkly

    here is the datasheet plot, yes about 0.9pA at flatband, 

    And this test file, 

    BUF634A current noise test including 2000 model.TSC

    And then isolating on the voltage noise, the new model for the BUF634A looks real good, the old one not so much, 

  • 0 in reply to Alec Saebeler
    Prodigy 20 points

    Hi Alec,

    Thanks for your help.  If you look at my previous post, the schematic and noise data show the problem I'm discussing.  FYI, I believe the model is by default in wideband mode.  When I measure the output voltage noise (by setting the resistance at the amplifier input to zero) the voltage noise curve corresponds with the datasheet values for Wide-BW mode (figure 7-9, red trace).

  • 0 in reply to Michael Steffes
    Prodigy 20 points

    Hi Michael,

    Thanks for the help.  Could you re-upload the file BUF634A current noise test including 2000 model.TSC?  When I try to open it, TI Tina is giving an error.  I don't know if the file is corrupt, or if the problem is on my end.

    I don't want to waste your time on newbie questions, but where did you find the updated BUF634A model?  I tried downloading the file from the BUF634A page, file SBOMAY2B.TSC , but when I open it and enter the macro, I see:

    * BUF634A - Rev. A
    * Created by Sean Cashin; 2020-02-12

    It looks like it's the rev A file, even though the web page describes it as rev B.

    Regards,

    Terry

  • 0 in reply to Terry Noe
    Guru 48395 points

    Well I had attached as a V7 file, here is a V9 version (I use V11, but that library is not necessary up to date, just has industrywide library)

    6685.BUF634A current noise test including 2000 model.TSC

    If I download this one, 

    And open the macro, I see this, 2020 by Sean Cashin

     

  • +1 in reply to Michael Steffes
    Prodigy 20 points

    Thank you, MIchael