AD620 Instrumentation Amplifier Circuit w/ Microphone

Discussion in 'General Electronics Chat' started by tennisrulz, Mar 4, 2010.

  1. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    Hello,

    I am working on a circuit for a project that requires an audio input into an electret microphone to feed into an instrumentation amplifier (AD620A), which has a high CMRR and also provide a gain of 50 V/V. My problem is that the two of these components integrated does not work. At first the microphone output did not feed into the AD620A op-amp (it was in the range of mV) so a buffer (LM741) was added. It still didn't work, but after much research a 10 k ohm resistor was added, which I'm also not exactly why it worked. I guess it provided an alternative path for the current to flow to ground. And, I kept the buffer because it gave a much better output than without it. Now, although there is an output from the AD620 op-amp, it only yields a gain of 2 V/V versus 50 V/V. I have been working on this for quite some time. Any help would be very much appreciated. Thank you and have a good day! I've attached the circuit.
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You don't need common mode rejection unless you have a differential input signal with common mode noise on it.
    What problem are you really trying to solve?
     
  3. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    Hi. The microphone has two inputs - one is grounded and the other one is the output, which is fed into the circuit. And, since the microphone is picking up a lot of noise, I am using the AD620 to remove some of that noise. I'm trying to find why the the AD620 op-amp isn't providing a gain of 50 V/V. It only gives a gain of 2 V/V.

    Thank you in advance!
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
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    Your plan to reduce noise will not work. You will have to look elsewhere. Can you post a schematic of your setup as it is exact now. That will help us.
    Quite often the noise in circuits are caused by using a not optimal cabling method, and/or grounding problems. Sometimes even the power supply may pump noise into your design. The fix may quite simple in many cases.
     
  5. tennisrulz

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    Mar 3, 2010
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    I have attached the circuit in the word document above, but here it is again.

    Thanks in advance.
     
  6. t06afre

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    May 11, 2009
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    I saw you circuit in your first post. But i thought this was a circuit for simulation only. No decoupling caps as one example.
    Also what do you use as power supply, and how far is the power supply located from the circuit. Do you use shielded cables, and is the shield grounded in both ends. That may cause ground loops.
     
  7. t06afre

    AAC Fanatic!

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    I also noticed now that you use the LM741. This amplifier will not work well with a supply voltage equal to +/- 3 volt. You should get a more modern amplifier that can work with low supply voltages. The LM741 is a relic, not to be used in our days.
     
  8. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    Hello,

    I'm using Agilent E3631A Triple Output DC Power Supply (http://www.ece.seas.gwu.edu/~ecelabs/labtour/pwrsupag301.html), which is located less than 2 feet away from the circuit. I do use shielded banana coaxial cables.

    About the LM741, those are the op-amps we have typically used in labs. I looked at the spec sheet and it doesn't give you the power supply range. I need the power supply to be relatively low because I tend to use batteries for my device instead of using external power supplies to power up my circuit.

    Thanks in advance.
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Look at the excerpts from the National datasheet. You have to add your output swing to the headroom requirements in the datasheet. This is how you determine power supply requirements.
    However, you still face the fact that common mode rejection in the inamp will do you no good if you don't have common mode noise. I'm not convinced you do.
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    Is it any reason for your low supply voltage? The 741 opamp prefer +/- 15 volt. And how about decoupling caps. Have you installed those. Use around 100nF placed very close to each chip supply voltage. Have you looked at the data sheet for the microphone. What is the recommended component values and supply voltage. Attention to details is very important then working with electronics. I have learned the hard way. Also trust us then we say that the AD620 in the setting you are using it do not remove any noise at all. You can replace it with a 741, the result will be the same. But on the other hand it will not introduce more noise in your setup
     
  11. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    The reason it's low is that I prefer to use battery to provide power supply for my device rather than using external power supply since the device is intended to be used on patients who have arthritis. The LM741 was implemented as a buffer because the signal from the microphone was not feeding into the AD620 op-amp, but turns out a 10 k ohm resistor was missing from my microphone circuit. But, when I tested it without the buffer, the signal was more distorted. I've attached the microphone circuit. This is the electret condenser microphone I'm using (http://www.radioshack.com/product/index.jsp?productId=2062215) The power supply for the microphone is 2 - 10 Vdc. I have not tried the decoupling capacitors, but I shall do that.
    I've also noticed that the circuit is picking up AM frequencies. Since, I'm working inthe audible frequency range..I was going to incorporate a low pass RC filter.
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Your 741 opamp is not an amplifier and is doing nothing in your circuit except it is adding hiss. It is probably reducing the signal from the microphone because its supply voltages are much too low.

    Your AD620A intrumentation amplifier has a single input so it is not using any common-mode rejection. An ordinary audio opamp could replace your entire circuit. A single supply voltage can be used.

    Is the noise audio noise or is it hum picked up by an unshielded input cable? Both noise types are not common-mode.
     
  13. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    Thank you. The signal is the sound from the speaker and the input cables are shielded. I'm using a coaxial cables.
     
  14. kenethh7

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    Feb 4, 2010
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    intrumentation amplifier has a single input so it is not using any common-mode rejection. An ordinary audio opamp could replace your entire circuit. I have learned the hard way. Also trust us then we say that the AD620 in the setting you are using it do not remove any noise at all.

    -keneth
     
    Last edited by a moderator: Mar 7, 2010
  15. Audioguru

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    Dec 20, 2007
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    If your microphone is listening to a speaker (why?) and is also listening to background noises then an intrumentation amplifier will not magically cancel the noises because they are not common-mode.

    Instead of acoustical-coupling, why not connect directly to the input or output of the amplifier that drives the speaker?

    I asked about missing shielding on the input cable from the mic because I have made many mic preamp circuits (in unshielded plastic enclosures), I have a very strong AM radio station nearby and my circuits never pick it up.

    Oh. I never use a lousy old 741 opamp. Maybe it picks up radio stations.
     
  16. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    The microphone is ideally listening to joint sounds from human subjects, but the speaker is mimicking sound of low frequencies in which joint sounds occur.

    Thank you all for your help. I understand my the differential input is not doing anything, but just amplifying. I decided to replace that op-amp with an audio amplifier (LM386), which used a low power supply voltage (4 - 12V), which is good. So, I remember while testing that the signal was picking up high frequency noise, typically AM, so I will incorporate an RC bandpass filter (20 - 10k Hz) before the amplification stage. So, the stages will look like this.

    Microphone input ---> RC BPF ---> LM386 audio amplifier w/ 20 gain

    The output will be fed into an NI elvis system where I can acquire the signal to analyze further. I hope this works out. Thank you all again!
     
  17. Audioguru

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    Dec 20, 2007
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    An LM386 is a fairly noisy power amplifier (hiss). A gain of only 20 is too low for a microphone.
    You should use a low noise opamp like a TL071 instead with a gain of 100 to 200.

    My TL071 preamps with a lot of gain do not pickup the strong AM radio station nearby.
     
  18. tennisrulz

    Thread Starter New Member

    Mar 3, 2010
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    Thank you. I looked at the spec sheet and it lists the maximum absolute power supply rating of 18 V. What is the minimum power supply it can handle? I wanted to avoid using an external power supply since it will be a portable device.
     
  19. tennisrulz

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    Mar 3, 2010
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  20. Audioguru

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    No.
    The TL071 and most other opamps have a max allowed supply of plus and minus 18V which is a total of 36V. The minimum total supply for a TL071 is 7V.

    There are many better opamps. The OPA134 is a good audio preamp with a minimum total supply of 5V.
     
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