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OPA375: How to interface microphone input

yunsik kim
Prodigy 140 points
Part Number: OPA375
Other Parts Discussed in Thread: LMV1012

Hi, Ti.

I constructed an electret microphone amplification circuit using LMV1012 and OPA375.
The schematic is below.
I want to connect this circuit to a Bluetooth module(BM20, Microchip).
The module is powered by a 3.7 V battery and above circuit is powered by LDO in the module.
I don't know how to interface this circuit to the Mic input pin of the module.
In the BM20 user's guide, they designed a interface circuit with a traditional audio jack terminal.
The MIC_BIAS is an output of 3.1 V.

My question is two.

1. In the circuit above, what is the purpose of C23 capacitor?

2. Is the interface circuit design below correct? Is there any risk of clipping the AC coupled signal?

Best Regard.

Yunsik.

  • +1
    TI__Genius 9366 points

    Hello Yunsik,

    I am not familiar with the inclusion of this capacitor. The signal will be AC coupled from the pre-amp, so this capacitor should not be influencing operation too significantly. 

    The interface circuit should work as desired, being that it will use the restive divider to introduce a DC offset on the AC-coupled audio signal. 

    Clipping however could occur if the preamplifier is set with too high a gain for the sensitivity of the microphone. Alternatively, clipping could also occur in the interface block if the MIC_Bias voltage was too low relative to the amplitude of the audio signal.

    We have a design guide for microphone Pre-amplifiers here: TIA microphone amplifier circuit.

    This design guide covers the design flow for creating a desired audio output voltage given a selected microphone and sound pressure. 

    Please let me know if you have any questions,

    Best,

    Jacob

  • 0
    Guru 140200 points

    Hi Yunsik,

    figure 5-1 of datasheet of BM20 shows two different inputs: MIC1 and LINE INPUT. In your case -since you provide some amplification for your microphone- you will need to use the LINE INPUT, not the MIC1 input. Or why do you want ot connect the output of OPA375 to the MIC1 input?

    yunsik kim said:
    1. In the circuit above, what is the purpose of C23 capacitor?

    C23 shall suppress HF-noise coming from the RF-sender of bluetooth module.

    Kai