Dear friends,

I would like to interface four electret microphones to a Texas Instruments TLV320ADC6140 ADC chip. My question is, what is the best way to go about this for optimal performance?

Should I use a pre-amp circuit, or can I bias the MIC and connect directly to the ADC via AC coupling capacitors?

Additional context:

[1] All four microphones will be mounted to the same PCB as the ADC

[2] low noise / high SNR is a key factor... Achieving low phase distortion is even more important.

[3] I would also like to minimise size and cost, so component count is a consideration

[4] Required bandwidth - 50 Hz to circa. 16 kHz.

[5] I don't want to use an OpAmp because of point [2]

[6] I don't want to use MEMs as the SNR is inferior to electret currently.

In summary, should I use a transistor preamp circuit, as shown in the attachments, or can I connect directly to the ADC?
Note that the ADC datasheet example shows MEMs microphones connected without preamps. These are presumably low output impedance.
What do you reckon experts?

Thx!
[/QUOTE]
Connecting Electret Microphones to TLV320ADC6140 – Preamp or Direct Connection?

Greetings all,

I'm currently working to connect four electret microphones to the TLV320ADC6140 and would appreciate any expert advice concerning the concerns of performance, specifically phase distortion, and noise.

My Primary Needs Are:

Every microphone integrated into one PCB with the ADC.

Lowest noise floor and High SNR.

Critical to the design is the requirement for minimal phase distortion.

The operating range is from 50 Hz to ~16 kHz.

Total size, cost, and the number of components needs to be minimized.

The use of OpAms should be avoided due to the aforementioned concerns relating to distortion and phase shift.

They should not be MEMS microphones because my tests show their SNR is much weaker than electret microphones.

The Principal Inquiry Is:

Am I able to connect electret microphones directly to the TLV320ADC6140 via AC coupling capacitors and bias resistors, or use a transistor preamp circuit?

The reason the TLV320ADC6140 shows MEMS mics connected directly is probably their low output impedance and also because they have integrated amplifiers. With electret microphones, it is usually several millivolts due to their high output impedance. So my questions are:

Is the input impedance sufficient and does the internal biasing scheme permit direct connection using only biasing and AC coupling?

Or is it still possible to require a discrete low noise preamplifier to bring the signal into optimal range for the ADC?

Are there applicable trade-offs in phase linearity with a simple BJT preamp which need to be kept in mind?

What I'm Looking For:

Use-cases or authoritative literature explaining the interfacing of electret microphones with audio ADCs, especially the TLV320.

Any thoughts on if a minimalist preamp adds anything (like phase accuracy) or is better omitted, say JFET or BJT common-emitter?

Thank you for your thoughts!

 

The microphone amplifier that I see is intended to be used with a higher voltage phantom power. It does not relate to your application at all.

 

This is a copy of an earlier thread: Electret Microphone to TI ADC Interface Circuit | All About Circuits

 

I thought that was the case. Especially with the reference to the draawing, which I have seen, but was not on this thread. My comment still holds.

 

The microphone amplifier that I see is intended to be used with a higher voltage phantom power. It does not relate to your application at all.

Thanks for this. I wrote the original thread. I'm not sure why 'kami100' has lifted it, re-written it and then posted it to a separate thread?!

Anyway, to your point, the circuit example shown is for a battery powered discrete transistor preamplifier e.g. those used in shotgun mics, etc. I agree though that similar types of circuits are used for phantom powered mics.

The thrust of my( / 'kami100''s) question is do I need a preamp circuit at all, or can I just connect the microphone capsule directly to the ADC via coupling capacitors. This would be a good outcome from a component cost point of view. But performance matters, so I will put a buffer in if I need to.

The application is for an acoustic phased array module for far field target detection and direction finding. The target signals are often weak with reference to the acoustic noise floor. The DF algorithm uses phase information from an FFT and discards amplitude data, so random inter-channel phase error or noise is of concern. Amplitude errors less so.

Cheers.

 

To see if you need a pre- amplifier you need to know the input voltage span of your A/D converter, and the anticipated output voltage when your microphone "Hears" the sound you are seeking to detect. My guess is that the microphone array produces a smaller signal with a smaller dynamic range. So my guess is that you DO NEED an amplifier. I suggest using a very low noise integrated circuit amplifier because they can be a much smaller package for the gain. AND if you want the response from each channel to be similar it is a lot simpler to produce identical IC amplifiers.

 

My question is, what is the best way to go about this for optimal performance?

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You will find that this type of device is highly dependent upon
the Physical-Construction and Orientation of ALL of the associated Components.

Yes, its really amazing what Software-Tweaks can do to alleviate some of those problems,
but the Software can't perform miracles and completely "bypass" Physics.
Physics will have to be dealt-with.

Careful consideration should be given to Filtering the Output of the Microphones,
and the "Target" Sound-Generator,
and to the Directional-Horns, which may be required in a challenging, Noisy-Environment.
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