Question about amplifying a sound signal from an electret mic

[David Gold]

Hello everyone,
I'm a 4th year undergraduate student in EE and I have a question for my graduation project.

My project is to measure elephant calls and do some DSP on it with MATLAB.
The the block diagram/Signal chain looks something like this:

Elephant calls -> Electret microphone -> Preamplifier -> Filter-> DAQ ( NI 6008 USB ) -> Laptop (MATLAB)

I have a question about the Preamplifier in the signal chain above:

I don't know which Amplifier I should use. From the searches I've conducted on the Internet, some people say I should use an Instrumentation amplifier
(which is a diff amplifier with high CMRR) like "AD AD620" or "TI INA217" , and some say I should use an op amp like the "TI TL071"

The mic is powered by a battery and I think that I have a thing called "floating ground" because the system isn't connected to earth :



My question is this:

which amplifier should I use and how do I connect it correctly?

Thank you very much.

 

[goldsmith]

Hi David
I can suggest AD620 as a high gain preamplifier .

Best Wishes
Goldsmith

 

[David Gold]

Thank you very much, Goldsmith.

The Amplifier will also be powered with batteries (different ones of course).
Does the amplifier need to be in reference to the same ground as the mic or it has to have a ground of its' own?

I'm kind of confused about the different grounds because I read that in some conditions "ground loops" can occur.

 

[ahmad1954]

you should amplify your signal because it is weak. you can amplifying signal with both opamp or Ic. you should try that but I think opamp can help you in this project.

 

[betwixt]

Electrets produce quite a large output voltage compared to most types. If the Elephant is within a few hundred metres you may see voltages as high as 100mV from them so you don't need a lot of gain. I assume the battery is inside the microphone so you have a shield (ground) and 'live' wire from it. The amplifier probably isn't critical and an an op-amp should be more than sufficient. All it as to do is lift the amplitude enough that it overcomes the filter losses and still lets enough through to give a good indication on the DAC.

In general, tie the microphone ground to the amplifier ground and the amplifier ground to the filter and DAC. Keep then electrically joined but in a chain from mic to DAC. You only get ground loops where there are more than one ground connection and they are to points with different potentials on them. If you keep things in a chain, any potential difference is only referenced to a single stage.

Brian.

 

[David Gold]

Thank you very much, Brian!


And what about the CMRR? as far as I know only differential amplifiers have this ability.
Isn't high CMRR an important issue in Data Acquisition? Or you simply think it's an "overkill" for this situation?

 

[betwixt]

Absolute overkill for this application and in any case the amplifier will be used as a single ended gain block of about x50 voltage gain so any common mode errors will be insignificant.

Almost all amplifiers have a CMRR figure but it is only important if the amplifier is used in certain configurations. In this case you will probably be using it in 'standard' inverting or non-inverting mode (it doesn't really matter for your application) and common mode errors play a very tiny part in the output.

Brian.

 

[David Gold]

Thank you so much, Brian!
You are extremely helpful!