• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 30 subscribers
  • Views 460 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Offset nulling and topology (op amp)

HOFF Remy
Prodigy 50 points
Other Parts Discussed in Thread: OPA1611, LME49990

Hello,

i am working to design a audio preamp with the lowest THD+N (obviously ;-) ) and look around LME49990 or OPA1611.

Do you think adding a nulling offset topology (voltage divider with a pot) will improve the THD+N or it will only cancel the offset voltage.

Indeed, if my mind is good, there is already a balance current system in the chip. So, ?

Second question about the general topology. I see in the opa1611 datasheet (page 6) that the inverter topotoly is very much better after 3 kHz. How can we explain that ? topogy, resistance source ? Could you suggest something ?

Thank you very much.

Remy.

  • 0
    TI__Guru**** 189401 points

    Hi, Hoff,

    Both are excellent choices!

    Nulling offset just helps pop, I don't think you'll see any improvement in THD. It could be useful if you've got another stage with significant gain after it.

    As for your second question, I don't get what you're asking. The THD goes up (gets worse) after 3 kHz.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 50 points

    Thank you for your reply, Don.

    About my second question, i tried to explain that on the opa1611 datasheet, figure 7, page 6, we can see that the THD+N don't does up after 3kHz when used with G = -1,  instead with G= +1.

    Then, could we conclude that inverter topology is significanly better (in F>3kHz) than non-inverter topology ?

     

  • 0 in reply to HOFF Remy
    Guru 21975 points

    Hoff,

    The G=+1 and G=-1 have two different effects that combine. The noise gain is 2 for G=-1 which increases one source of distortion. The G=1 configuration has a common-mode signal source of distortion. The two sources can combine in different ways (adding, subtracting). This may cause one op amp to behave differently than another in terms of the variation between G=1 vs. G=-1.

    I agree with Don, a properly implemented DC servo loop should not affect distortion.

    Regards, Bruce

  • 0 in reply to Bruce Trump
    Prodigy 50 points

    Ok, so i will try to calculate the noise with the two formulas (inverter and non inverter) in the datasheet.

    Thank you.