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LMH6881: Input referred noise condition

Part Number: LMH6881
Other Parts Discussed in Thread: TINA-TI

Hello team,

Input referred voltage noise of LMH6881 is specified as typ 2.3nV/rtHz in its datasheet.

Which differential input or single end input is valid for the noise spec?

Best regards,

  • Hello Taketo-san,

    The input-referred voltage noise is considering the voltage noise (referred from the output) at the noninverting input terminal to the op-amp.

    Please see the following TI Precision Labs training, as well as the linked op-amp specifications document:

    https://training.ti.com/ti-precision-labs-op-amps-noise-spectral-density

    https://www.ti.com/lit/an/sloa011b/sloa011b.pdf

    Best,

    Alec

  • Hello Alec-san,

    Thank you for your comment. Understood that the value is considering single end mode, not differential mode.

    Is it correct that INPS pin is noninverting input terminal what you meant?

    Best regards,

  • Hello Taketo-san,

    I understand your question better now.  I would like to add clarity onto why there is a single input-referred voltage noise specification, even though there are two input modes.

    Please refer to Section 7.2 diagram, which I will copy below at the end of my message.  In this image, there are four resistors present on the input network for the LMH6881.  This symbolizes the input impedance setup, which is described in the datasheet as 100 Ohms differential or 50 Ohms each for the single-ended inputs.  

    The noise calculation does not take the single-ended or differential nature of the input into account.  The input impedance is either 100 Ohms differential (with 50 Ohm resistors floating) or 50 Ohms + 50 Ohms for each single-ended input (with 100 Ohms differential floating).  Since the resulting impedance is the same, the voltage noise specification will apply to either input setup.

    Thank you for your great question!

    Best,

    Alec

  • Hello Alec-san,

    With condition of 26dB gain(x12), Input referred voltage noise is specified with typ 2.3nV/rtHz and output voltage noise is specified with typ 47nV/rtHz.

    How can we calculate output noise from the input referred noise?

    2.3n * (1+12) = 29.9n and it doesn't reach to 47nV.

    Could you let me know there is other big noise source I should take into account on the output noise calculation or my calculation is wrong?

    Best regards,

  • Hello Taketo-san,

    I would be happy to work through calculations and help with understanding the noise analysis.  I did receive your email and I have been busy in the lab this week so far; please allow me a bit of time to wrap up measurements & return to your question.

    Best,

    Alec

  • Hello Taketo-san,

    I have observed a small error with the calculation you provided:

    When considering 26dB of gain represented in V/V, the conversion is 10^(26dB / 20) which is approximately 20V/V, or 19.95 V/V more precisely.  If we continue onward with the remainder of the calculation, the 2.3nV/sqrt(Hz) value multiplied by the gain (19.95) yields the expected output voltage noise of ~46nV/sqrt(Hz).  This slight difference is due to rounding error in my calculation; however there can be confident in the noise analysis of the LMH6881 you have performed so far.

    Please wait for an email response from me regarding your additional question over email.

    Best,

    Alec 

  • Hello Alec-san,

    I am embarrassed. Yes, 26dB is x20... Now it is clear.

    I wait for your email feedback on other question. Thank you.

    Best regards,

  • Hello Alec-san,

    Sorry for multiple questions.

    I would like to confirm "Output Voltage Noise" definition in electrical characteristics table.

    Is it differential noise? in other words, is it noise seen Vop with referencing Von? 

    Or, is it single end noise? in other words, noise seen Vop(or Von) with referencing GND?

    Best regards,

  • Hello

    Could you please comment on "Output Voltage Noise" spec definition?

    Best regards,

  • For the forum: Please see a snippet of the resolution conversation below:

    """

    Hello Sato-san,

    1. This is a differential noise specification. That is correct, it would be noise seen by Vop. 

    2. Typical application for characterization follows Figure 37. The loading is correct, but the input sources should be set to either 50ohms internal or via representation by series 50 ohm resistors. The Tina-TI simulation below shows output voltage noise at 47nV/√Hz at maximum gain below 1MHz.     


         

             LMH6881 Model Notes from netlist are below:

    This could be a LTSpice issue if the setup matches the Tina-TI circuit shown above. We have run into issues with customer's obtaining different results in LTSpice compared to Tina or PSpice with our other models. 

    LMH6881 Noise.tsc

    Thank you,
    Sima

    """

    I will update this thread with any additional information relevant to analyzing LMH6881 noise.

    Best,

    Alec

  • Here is the final piece of information from the discussion, illustrating how to reconfigure SPICE model to model output voltage spectral density appropriately:

     The Noise simulation in the reference circuit is not the correct output for output voltage noise spectral density. The two images below show the circuit used (removed DC blocking input caps), and the output voltage noise trend which is the same as shown in Tina-TI. 

             

       To setup the simulation profile, make a new simulation profile with the below settings under AC Sweep/Noise:

       Then press the run button. The simulation will have no trends on the plot. Right click anywhere on the plot and select add new trace. Below are the equations to set for either noise profile. The one on the right is the non-integrated sqrt(Hz) noise value shown in the datasheet.

    This work-through is done in PSPICE.

    Best,

    Alec