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TIPD134: Op Amp noise consideration

Benjamin
Prodigy 250 points
Part Number: TIPD134
Other Parts Discussed in Thread: OPA1679, OPA1604, , OPA1678, OPA1656, TL074

I have developed a 3 band active audio filter, and I want to experiment with different op-amps to optimise the noise level.

I have skimmed through TIPD134 and it seems like input noise voltage (nV/√Hz) is the main parameter to consider when selecting for noise. So I am considering OPA1604 and the lower cost OPA1679

Is there something else I should consider (besides having a high enough slew rate) for this design? Unfortunately lowering the resistor values is not an option.

Each of the three outputs drives a 20kΩ load.

  • 0
    TI__Genius 11775 points

    Hello Benjamin,

    For resistors whose thermal noise exceeds the broadband noise of the amplifier (i.e. greater than ~ 1 kOhm) the current noise becomes more important than the voltage noise.  This is why low current noise (which translates to MOS or JFET input amplifiers) amplifiers are used for this design.

    Figure 6-3 in the OPA1678/9 data sheet intends to show this effect;  for a MOS input device, the current noise never becomes the dominant noise source.  The noise is only dominated by the resistor noise itself.  If the input current noise was higher, the input current noise multiplied by the source (input) resistance would become dominant at higher source resistances:

    I would not recommend OPA1604; this is bipolar with higher current noise.  Other good options would be OPA164X (JFET) or OPA1656 (MOS).

    Hope this helps.

    Regards,
    Mike

  • 0 in reply to Michael Hartshorne
    Prodigy 250 points

    Hi Mike,

    Thanks for the reply, I think I understand the noise a bit better now.

    Just so I understand correctly. The three main noise considerations I should be concerned about are:

    1. Input Voltage noise:
    Negligible with my high resistance design

    2. Input Current noise
    Optimised by higher impedance op-amp inputs, and technically lower with higher value input resistors.

    3 Resistor thermal noise
    Significant in my design. For example approx 30nV√Hz with 50kΩ resistors.

    Did I understand that correctly? That would explain why I was seeing similar results even with a TL074.

    Thanks
    - Ben

  • +1 in reply to Benjamin
    TI__Genius 11775 points

    Hi Ben,

    Yes on #1 and #3 - for #2, I would say this - the input current noise times the input resistance creates voltage noise.  The current noise itself will not be lower with higher value input resistance, but, the resulting overall voltage noise will be dominated by a high-input current noise device for designs that use high-value resistors.  For your design,  I would anticipate that the resistor noise alone will be the dominant noise source, so as long as you pick MOS-input amplifiers, you won't see much difference because the amplifier voltage or current noise do not end up being the primary contributing noise source.  However if you were to pick a bipolar input amplifier with high current noise, the current noise times the resistance could result in an overall higher noise.

    Regards,
    Mike