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Unbalanced to Balanced (XLR) with Controlled Gain

Victor Levandovsky
Intellectual 575 points
Other Parts Discussed in Thread: OPA1632, THS4131, OPA211, OPA1612, OPA1611

Hello,

I`m designing XLR balanced output.

Could you advice me a good\best topology for converting unbalanced to balanced, please.

Power is a Dual rails (+/-15V)

I need gain adjustment in this stage (with DIP switches, for example.)

Probably using Diff Opamp (like OPA1632) is a best way, but need to switch 2 resistors in feedback simultaneous.

-- Thank you.

  • 0
    TI__Genius 15975 points

    Hi Victor,

    As you have noted, using a fully differential amplifier (FDA) like OPA1632 or THS4131 would be the best way for converting unbalanced to balanced output.

    Would it be possible for you to have an attenuator network in-front of the FDA input, instead of the FDA feedback path? You may be adding nonlinearity otherwise in the signal path, when including the DIP switches in the FDA feedback loop.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 575 points

    Thank you for the answer.

    I think possible solution is OPA1612 (or OPA211, OPA1611) in Instrumental OpAmp configuration (using input stage)

    with one of diff input grounded. Gain can be adjusted changing only one resistor.

    What you think?

  • 0 in reply to Victor Levandovsky
    TI__Genius 15975 points

    Hi Victor,

    You could use the OPA1612 in Instrumentation op amp (or OPA211, OPA1611) configuration. However, I thought you wanted to do unbalanced (single ended input) to balanced output (diff output), right? I don't think you will be able to do that with an Instrumentation op-amp configuration.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 575 points

    Hello,

    Thank you for the answer.

    Any OpAmp`s (including InAmp`s and FDA`s) work with voltage difference on its inputs.

    So, If I connect ground to one of inputs I get SE to DIFF conversion, isn`t it?

    First stage (of classic three OpAm`s InAmp) looks pretty well because I need adjust only one resistor for gain setting

    and have both in-phase and out-of-phase outputs.

    Best Regards,

    Victor

  • 0 in reply to Victor Levandovsky
    TI__Genius 15975 points
    Hi Victor,

    Yes, you should be able to have a SE to Diff conversion if you ground one of the inputs of the first stage of Instrumentation Amplifier.
    However, you would not have control over the output common mode voltage in this approach, which can be easily accomplished with an FDA.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Intellectual 575 points

    Yes, need perform some SPICE modelling for worst case.

    But I think this will me minimized with dual OPAMP.