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I want to interface two electret microphones with a stereo audio codec. One mic will capture audio from one side of a box and the other one from the opposite side. I want to use TLV320AIC3254 from Texas Instruments (Stereo Audio Codec).

My microphones have two terminals (+ and -) that I will connect to the corresponding adaptation circuit for electret microphones (one resistor and two capacitors). After this, I will have one electrical point to interface (the signal itself). How can I connect that point to the Stereo Audio Codec? In the scheme I can read IN1, IN2, IN3 and L and R each.

Let's think my right microphone is IN1, I will connect it to IN1_R, what should I connect to IN1_L? Should I connect the other mic there? Should I connect it to GND and connect the left mic in IN2_L?

As a summary, I don't know why there are three pairs of L-R connections in the codec pinout and how to interface them with just two microphones.

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  • \$\begingroup\$ The second image (Figure 5-1) is the circuit you are looking for. The chip itself provides bias for electret mics as you can see, so you don't need the biasing circuit shown in the last image. \$\endgroup\$
    – Rohat Kılıç
    Jun 13 at 13:23
  • \$\begingroup\$ @RohatKılıç Thanks for your reply. To see if I have understood your answer. I have some questions: 1. Is internally the MICBIAS pin in the chip connect to +3,3V referred to GND? 2. So... the pull-up resistor of the third picture is the pull-up resistor connected to MICBIAS in the second picture, right? 3. Why the pinout says "L" and "R" if it is "+" and "-" for just one microphone? There is no left or right (there is no stereo recording) if you use just one mic. That naming was what confused me. IN1 is not IN1_L or IN1_R, it's IN1, refferring to microphone 1, which is going to be L or R \$\endgroup\$
    – Developing Electronics
    Jun 14 at 10:20
  • \$\begingroup\$ 1. Check the datasheet. There's a table showing details about MICBIAS voltage. It's configurable. 2. Yes. 3. The inputs are configurable (single ended or differential) this means that you can apply 6 single-ended or 3 differential inputs. And they are not supposed to accept mic signals only so you can apply line level signals as well. Depending on the configuration you can apply stereo signals as single ended or mono differential signals. It's totally up to you. \$\endgroup\$
    – Rohat Kılıç
    Jun 14 at 10:39
  • \$\begingroup\$ @RohatKılıç Thanks a lot, so helpful. I want to thank your comment, I am new in the platform, how can I give a like to your comment? or points or something to mark it as the best answer. \$\endgroup\$
    – Developing Electronics
    Jun 14 at 12:45
  • \$\begingroup\$ I'm going to post an answer so you can accept it and therefore the question will not remain open. \$\endgroup\$
    – Rohat Kılıç
    Jun 14 at 13:18

1 Answer 1

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The typical application diagram (Fig. 5-1 in the datasheet) is what you are looking for. So you don't need to build an extra circuit for biasing your electret mics.

The _L and _R convention may look confusing, that's because the inputs are configurable (single-ended or differential) so you can apply either 6 single-ended signals (e.g. 3 line-level stereo) or 3 differential signals (e.g. electret microphone or pre-amplified dynamic microphone signals). The chip is not supposed to accept only microphone signals or only line level signals. Check the datasheet for configuration details.

As for biasing the electret mics, datasheet provides a detailed info about MICBIAS which is a configurable voltage-output pin. Remember that you'll still need the external resistors since they set the output voltage of the microphone.

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  • \$\begingroup\$ an electret mic usually has its - pin connected to its metal case and to ground. But here the - pin is used as part of a differential signal then it will be an antenna that picks up hum and other interference. \$\endgroup\$
    – Audioguru
    Jun 14 at 14:36
  • \$\begingroup\$ @Audioguru One of the inputs of the diff amp could be connected to GND, but I couldn't find what input voltage range is allowed in the datasheet \$\endgroup\$
    – bobflux
    Jun 14 at 14:44
  • \$\begingroup\$ The mic inputs are already biased (inverting opamp input) which is why input coupling capacitors are used and the datasheet limits for the input voltage are 0.3V below ground to 0.3V above ground so you can short an unused input to ground with a wire or with a coupling capacitor. \$\endgroup\$
    – Audioguru
    Jun 14 at 16:13
  • \$\begingroup\$ @Audioguru So i should connect the mic as the accepted answer from Rohat said and short the unused inputs to ground, right? \$\endgroup\$
    – Developing Electronics
    Jun 20 at 6:14
  • \$\begingroup\$ The unused inputs must be AC grounded through the series input capacitor. Then their DC remains properly biased. \$\endgroup\$
    – Audioguru
    Jun 21 at 12:40

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