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Dual microphone, noise cancellation method

Seng Huat Toh
Prodigy 60 points
Other Parts Discussed in Thread: LP2966

Hi,

I had made an attempt trying simulate a dual microphone, noise cancellation method, with the Supplies to this circuit is 3.3V and GND.

Below is the snapshot of my circuit, I had also attached my schematic file 7762.PRENOISwSUMAMP_BPF_COM.TSCof my circuit.

What I'm not sure from my circuit is the voltage offset for the op-amps. From past experience, the input offset voltage get amplified at the output and the desired signals get saturated. However, as i simulate this circuit, it's not the case, the output stays at the offset voltage level. So 'im confused here.

Please kindly advise what I should take note of, whenever the op-amp circuit supplies are not dual but V+ connects to 3.3V and V- is to GND.

Awaits reply

Regards

Seng Huat

  • 0
    TI__Guru**** 189399 points

    Hi, Seng Huat,

    I didn't simulate your circuit, but by reviewing your schematic, you have stages ac-coupled with capacitors which will eliminate dc offset errors, won't they?

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Hi Don,

    Yes, at the stage with the last two op-amps, it is for the filtering purpose (2nd order band pass) this had block off the offset voltage, as such I had a 1.5M ohm resistor pull up (1.6V) right before the input.

    At the very last stage, I had followed the recommended circuit from the datasheet, to place a capacitor (dc offset stage) before the analog microphone input of an audio codec.

    Working on the op-amp circuit, I'm limited to 3.3V and GND (0V), so for an op-amp circuit, I know I need to have a offset voltage for the op-amps input so that the input signals won't get cut-off or saturated. Since I only have one supply (3.3V), my only way to 'make up' the offset voltage is through a voltage divider. What I don't understand is that what I can do so that I can obtain the same voltage offset level at every op-amp stage.

    Hope I can clear my doubts in this conversation

    Regards

    Seng Huat

  • 0 in reply to Seng Huat Toh
    TI__Guru**** 189399 points

    Hi, Seng Huat,

    There are a couple ways you can generate this voltage. We usually refer to this as Bypass or Mid-rail voltage.

    In your case, with 3.3V supplies, you would want to target 1.65V as your mid-rail voltage.

    You can generate this with a simple resistor divider, but any variations on your supply will couple thru to the audio signal.

    Or, you can use a precision reference to give you exactly 1.65V.

    -d2

  • 0 in reply to Don Dapkus
    Prodigy 60 points

    Hi Don,

    Thanks for the respond.

    I had my schematic being reviewed by my EE team in my company actually, so the feedback summary is my circuit is too complicated... I was then recommended to try to use a Differential amplifier configuration instead.

     

    Infact from this simulation, I had actually get my desired result such that the noise source get cancelled out at Vout. This is a simulation under the scenario that The amplitude of the noise source (which is a white noise wav file) is 2x larger than the noise source from the voice source.

    Like you said, I need another voltage source for the offset voltage i had chose LP2966 (1.8 and 3.3) already, thanks for sharing the TI-Burr-Brown part with me.

    I'm going to build the prototype circuit, so I may be I'll share my experiment results later here, since I started this problem..

    Regards

    Seng Huat