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Compiler: electret condesor mic Preamp recommendation

V Pot
Expert 1975 points
Other Parts Discussed in Thread: OPA172, OPA320, TLV6741

Tool/software: TI C/C++ Compiler

Hi,

1)I'm looking to capture heart sounds using a electret condesor mic. I need to have a very low noise preamp and antiliasing filter that passes frequencies below 4000hz, and then connects to and adc then digital filtering happens using dsp. So i'm looking for a low noise amplifier for preamp and antiliasing application here. i'm not sure which one to choose.

2) also can you provide a low noise circuit with preamp and anti-aliasing

3) my voltage requirement is below 3.7v

  • 0
    TI__Expert 6250 points

    Hi V Pot,

    TIDU-765 describes an electret microphone amplifier using the OPA172. You might also want to look at Figure 60 in the OPA1692 datasheet. 

    For general information on noise in op amp circuits, consider looking at our TI Precision Lab series on noise, which can be found here.

  • 0 in reply to Alexander Davis
    Expert 1975 points
    Alex,

    I’ve already looked into that implementation. As said in my post abovd I’m looking for a low noise opamp that was designed to work with supply lower than 3.3v. I’m more interested in antialiasing filter filtering off anything above 4000hz. Rather than preamp because I’m gonna pass this to an adc with 8000hz sampling rate and it has preamp inside it which I’ll use it.

    So please recommend a good low noise opamp implementation with supply below 3.3v
  • 0 in reply to V Pot
    TI__Expert 6250 points
    Hi V Pot,
    You might want to consider OPA320 or TLV6741.

    The exact meaning of low noise is going to depend on your circuit configuration. A bipolar input op amp with very low voltage noise may have significant current noise, which will dominate the noise performance of many amplifier circuits. CMOS or FET input op amps can achieve relatively low voltage noise, but typically have very low current noise, so may work better in a transimpedance amplifier like that in TIDU-765.
  • 0 in reply to Alexander Davis
    Expert 1975 points
    Alex,

    1)Thankyou. Opa320 looks promising for connecting to adc. Does the other opamp can also drive the adc? I’m using aic3206 codec.

    2) is there a good antialiasing low pass filter circuit with opa320 or similar for reference?
  • 0 in reply to V Pot
    TI__Expert 6250 points
    V Pot,
    You might consider following the design provided in SPRAB38A:

    www.ti.com/.../sprab38a.pdf
  • 0 in reply to Alexander Davis
    Expert 1975 points
    Alex,
    Thanks for the doc. My implementation is also stethoscope and I’ve seen this doc before. Can you please clarify below
    1) on page 7 figure 6 . What is that NO-0e and NC-0e? Also what is mic_L1 and mic_L_in ?

    2) I’m planning to use the pga inside the codec to preamplify. So it is ok to apply lowpass filter then preamplify? Usually people do the other way around right? Any disadvantage?
  • 0 in reply to V Pot
    Expert 1975 points
    Alex,

    Can you reply to my above question?
  • 0 in reply to V Pot
    TI__Expert 6250 points
    V Pot,
    It appears that both should be labeled NC 0E, which appears to be a Not-Connected or Do Not Populate component, judging by the schematics further on in the document.

    For good noise performance, it would be better to put as much of your signal gain as early as possible in your signal chain. Otherwise, you will have noise from your microphone interface and low pass filter getting gained up (possibly significantly) by the codec PGA. Aside from the noise consideration, I don't think there are any other issues associated with that configuration.
  • 0 in reply to Alexander Davis
    TI__Genius 11135 points
    V Pot

    We haven't heard back from you so we assume this answered your questions. If not, just post another reply below.

    Thanks
    Dennis
  • 0 in reply to Dennis Goeke
    Expert 1975 points

    Sorry i have a follow up question to what alex said.

    If that is no connection what they meant by nc-0E, then what should i connect to pin 4 in- like it shown in picture of the document for low pass filter schematic

  • 0 in reply to V Pot
    TI__Expert 6250 points
    Hi V Pot,
    Looking at Figure 20, it looks like the intent of R2 and R3 (both NC-0E) was to allow the preamp and filter op amp to be bypassed for debugging. If the filter buffer isn't bypassed, pin 4 should connect to the op amp output, forming a unity-gain buffer.