20KHz Instrumentation Amplifier

Discussion in 'General Electronics Chat' started by pal114525, Aug 22, 2015.

  1. pal114525

    Thread Starter Member

    Jul 3, 2015
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    Hi,

    I've designed an Instrumentation amplifier to receive a 20KHz signal from another coil at the Tx side such that this would give ZERO output if there is NO METAL between the TX and RX. And this would give an output voltage if there is any metal in between TX and RX.
    Would you please guide me to select the proper values of resistor for that?

    Thanks & Regards.
     
  2. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,538
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    I'm sure you know what you are talking about, but none of us do. Your question is missing a LOT of information. And, the terms TX and RX do not appear on it anywhere. What is the signal amplitude you want to amplify? And why are you building an IA from scratch? To get any kind of common mode rejection and gain accuracy you need at least 0.1% tolerance resistors.

    Separate from all of that. at 20 kHz a TL074 has a little over 40 dB of open loop gain. That probably is not enough to make the gain you need and have a minimum of 20 dB of negative feedback to stabilize the first stage output impedance into the second stage.

    ak
     
  3. pal114525

    Thread Starter Member

    Jul 3, 2015
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    0
    Hi,

    Thanks a bunch for your valuable feedback.
    TX means - Transmitter and
    RX means - Receiver

    I have two RX- coils which are in series to cancel out the induced emf created by the another coil at the TX side. The Tx coil and Rx coils are face to face with each other and their separation gap is around 2.5 ft.
    I have used an Instrumentation Amplifier using TL074 to cancel out. I've designed it on breadboard. But, not not getting exact cancel out.
    Would you please guide me to choose the proper value of resistors and IC for that?

    Thanks & Regards.
     
  4. pal114525

    Thread Starter Member

    Jul 3, 2015
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    Would you please explain me the point -
    " That probably is not enough to make the gain you need and have a minimum of 20 dB of negative feedback to stabilize the first stage output impedance into the second stage."

    Thanks & Regards.
     
  5. #12

    Expert

    Nov 30, 2010
    16,284
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    That is the reason why AK thinks the TL074 is probably the wrong chip for this job.
     
  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,538
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    A couple of things. First , yes, I know what TX and RX mean. Do you? My point was that you throw the terms around in your first post without any context. Not only is there no transmitter or receiver in your schematic, there is no reference of any kind to them yet you expect us to magically know by osmosis or Ouija what you are talking about. And none of your later posts clarify things.

    Yes, there are two coils on your schematic. But a) there is nothing to indicate that these are part of the "receiver"; and b) they are not in series. There is a ground connection between them, so they do not share the same current and thus are not in series. Also, since there are no polarity dots on the coils, there is nothing to indicate the current polarities out of them, which is *critical* to the success of this design.

    You ask why you are not getting things to cancel out, but again there is no information to work with. How much cancelling are you getting now? How much cancelling do you need? What is the amplitude of the input signals? What is the required output amplitude? What is the shape of the input signals? What is the shape of the remaining signal after cancellation? What is the output signal used for? What kind of circuit does the output drive?

    Here is a quote from post #2: "To get any kind of common mode rejection and gain accuracy you need at least 0.1% tolerance resistors." On your breadboard, what is the tolerance of each of the six fixed resistors?

    ak
     
  7. pal114525

    Thread Starter Member

    Jul 3, 2015
    46
    0
    Hi,

    The attached schematic is a typical instrumentation amplifier.
    This circuit should cancel out any emf induced in the coils such that the emfs are equal in magnitude.
    The two coils have the same inductance (1.7mH).
    The Transmitter (Tx, which is not shown here) is transmitting an EM field of 16KHz continuously. The two RX coils are geometrically
    placed such that they should cancel out this field and at the output of the instrumentation amplifier there should be ZERO volt ac signal.
    Would you please tell me if any changes are required in the circuit?

    Thanks & Regards.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    Can you show the actual placement of the TX and RX coils in relation to each other? A photo would be great. What is the amplitude that you get at each receiving coil?
     
  9. pal114525

    Thread Starter Member

    Jul 3, 2015
    46
    0
    Hi,

    Please see the attachment. The triangle ABC is an isosceles triangle.
    Please let me know the changes required in the circuit.

    Thanks & Regards.
     
  10. pal114525

    Thread Starter Member

    Jul 3, 2015
    46
    0
    The receiving signal depends on the field strength. But, it should be of the order of millivolt in the coil.
     
  11. pal114525

    Thread Starter Member

    Jul 3, 2015
    46
    0
    Hi,

    Thanks for your valuable feedback.
    I have two RX coils, each of which has inductance of 1.7mH and which are connected series opposite. I have connected them with an instrumentation amplifier.
    There is a Tx coil ( which is not shown here), which is transmitting the EM field of 16KHz continuously. The two RX coils should cancel out this signal and give a ZERO output at the output of the instrumentation amplifier output terminal. But, should give a NON-ZERO value if there is a metal in-between the TX and RX coils.
    I have designed it in Hardware, but I am getting ZERO output from the at the output of the instrumentation amplifier both the cases ( NON metal and presence of metal).
    I am using +12V and -12V supply.
    I would be highly obliged if you tell me the following points.

    1. What should be the value of Vref in this regard?
    2. What would be the value of bias resistor?
     
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