ADAU1701 ADC external resistor minimal value

ValC Question

Apr 9, 2023 00:00

Hello!

I'm explaning all things but I would be glad if someone could just confirm or disprove the following third sentence ;-).

I'm working on a wearable voice amplifier project for which volume and weight is critical.

I'm seeking to kwow if connecting an electret microphone directly on ADAU1701 ADC without a preamplifier would be an option.

I case where I don't put an external resistor at all (ADC total input impedance close to 2kOhm), I guess the ADC FS input voltage should be as low as around 0.2 Vrms / 0.28V.

In case where the sensor's impedance is below 1kOhm and it's sensivity is over -42dBV/Pa, when placed at 2cm from the singer's mouth (source estimated power: 75dBspl @1m then 109dBSPL @0.02m = 5.62Pa), I should get more than 45mV at sensor's output, and considering the bad 1:2 impedance match, more than 30mV at ADC input.

This signal would to be less than 20dB lower than ADC FS input voltage. Talking about a 24bit converter, it sounds acceptable to me, and so is the ADC SNR, raised to 80dB by the digital gain further applied during processing, kept below electret SNR that would'n be better thant 78dB.

Does it make sense?

Best,
Valentin

Edit Notes
Does my little schematic reflects reality?

[edited by: ValC at 8:23 AM (GMT -4) on 9 Apr 2023]

Audio Signal Processors

hardware

microphone

SigmaDSP Audio Processors

adc analog input voltage

adau1701

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DaveThib Reply

Apr 10, 2023 00:00

Hello ValC,

I do not have a good explanation for you but I think you will run into noise issues. I would try this on an eval board before getting too far into the design.

Now I do have to comment on this:

ValC said:

I'm working on a wearable voice amplifier project for which volume and weight is critical.

Then I respectfully suggest you are not using the best part for this application.

I suggest you use the ADAU1787. It is meant for very small and very low power applications and the microphone preamp is on the chip. You can interface to the ECM directly and provide it bias voltage. A headphone amp is also on chip.

Let me know if you need a Class-D amp as well. If this is a high volume project then send me a direct message. There may be other options.

Dave T

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ValC Reply

Apr 12, 2023 00:00

Hello Dave,

the ADAU1787 would suit my needs better, indeed.

But since we are a small workshop designing and manufacturing on demand custom audio solutions for professionnal musicians, we have to rely on commercially available dsp boards, quite all based on ADAU1701. The smallest I've found is the AMP2 from Sure Electronics.

Could you confirm that with a total ADC input resistor value of 2kOhm, the input voltage is going te be down to 0.2 Vrms?

Thank you very much for caring!

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