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OPA836 as a microphone preamp

Richard Meidl
Intellectual 370 points
Other Parts Discussed in Thread: OPA836

I looked at the specifications for the OPA836 and the specifications seemed to meet my needs.  It has a good bandwidth product and is low noise, 4.6nV/sqrtHz.  But when I use it, the THD+N is high at the low inputs and great when the input gets around 30mVrms.  Is this just the wrong part for this application?  Shouldn't a part with low noise be able to pull audio signals out of the noise floor.  I want to go down to 1 or 2 mVrms in.  thank you for your help.

  • 0
    TI__Genius 13935 points

    Hello Richard,

    I would need to know a few more details before I can be sure what is causing the noise issues.  What is the impedance of the source?  How are you measuring the THD+N - is it a bandlimited measurement?   

    On Page14 of the OPA836 datasheet Figure 16 shows that the current noise begins it's 1/f corner around 30 to 50 kHz, so for a high impedance input the noise could be much worse than the headline number of 4.6nV/rt Hz. 

    Also, with a bandwidth of over 200 MHz it's possible you are getting the accumulated broadband noise of the amplifier unless you are using an audio passband filter. 

    I would recommend against using high speed amplifiers (BW > 50MHz) for audio applications.  I would suggest you look in our Audio section for an amplifier.

    http://www.ti.com/lsds/ti/audio-ic/audio-operational-amplifier-product.page?familyAliasId=1200585

     

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Intellectual 370 points

    Hello Loren,

    Thank you for your reply.  The source impedance of the microphones varies between 150 to 2.5k Ohms depending upon what type of microphone is used.  I measured the THD+N with the band limited from 22 Hz to 22kHz..  I used FilterPro for the design and I believe that the OPA836 was the suggested part.  I will look at using a different part for this application.

    Best regards,

    Rich

  • 0 in reply to Richard Meidl
    TI__Genius 13935 points

    Hello Rich,

    I talked with someone with more audio experience than myself and he indicated that some of our high speed parts are fine for audio.

    Can you give me some more details on the measurements you are making?  What gain is the amplifier?  What is the THD+N at the lower amplitudes?  I was presuming that noise was the issue and not distortion, is that correct?  What is the smallest signal voltage you expect?  Are you measuring at the amplifier output or with an ADC? 

    At 1kHz the current noise is 3pA / squrt Hz.  At 2.5k source impedance this would give a current noise contribution that would be similar in magnitude to the voltage noise (~8nV/ rt Hz).  Is this still in the range you would expect to be adequate?

    Regards,
    Loren

  • 0 in reply to Loren Siebert 1
    Intellectual 370 points

    Hello Loren,

    Attached is a pdf of the circuit I am using.  The gain is 40.  My THD+N at low amplitudes, 10mVrms was around 0.5%.  At 5mVrms it was closer to 1.0%.  When the input is in the 30mV range, distortion is less than 0.1%.  So, my guess is that it is noise and not distortion that is giving me my problem.  I would like to be able to go down to 1mVrms input.  I was measuring at the output of the OPA836. 

    I think 8nV/rt Hz would be adequate.

    Thanks again for your help!

    Rich

  • 0 in reply to Richard Meidl
    TI__Genius 13935 points

    Hi Richard,

     

    I will have to look into this a bit more to see what the 1mV preformance should be.  I think you have given me enough information, I just need to run the numbers.

     

    Regards,

    Loren