two channel amplifier(difference and the reference)

Discussion in 'The Projects Forum' started by Plantis, Apr 10, 2012.

  1. Plantis

    Thread Starter New Member

    Apr 9, 2012
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    Hi All,

    Please, I need some help.

    Currently, I am working on a project. I need to design and make a circuit which will have two channels. I have two photodiodes. I have one laser beam. I split it into two equal beams and then I measure their intensities by using two photodiodes. Next, I use chopper to modulate second beam. I measure two signals simultaneously. My circuit has two outputs. One is a reference(just first beam). Second is a difference between two beams. Difference output works well. However, the reference channel gives me constant potential in -2.3V + some tiny reaction on light.

    Photodiodes are connected between PAD1 and PAD2, Pad3 and PAD5

    See attached scheme. There are few additional comments and my observations.

    Additional information: I have already made prototype. Currently, I am testing it.

    Part numbers:
    Op. Amplifier: ADA 4817-1
    photodiode: Hamamatsu S3071

    Regards,
    Plantis
     
    Last edited: Apr 10, 2012
  2. Plantis

    Thread Starter New Member

    Apr 9, 2012
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    Additional information: I have already made prototype. Currently, I am testing it.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    You need to provide the part number of the amplifier.
     
  4. Plantis

    Thread Starter New Member

    Apr 9, 2012
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    Initial post was changed.
    Thank you
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    How are the photodiodes connected to the circuit?

    EDIT: I get that they are connected to the pads. Are they reverse-biased?

    EDIT: Your circuit makes no sense. Neither pad 1 nor pad 2 connects to an amplifier input.

    EDIT: Keep in mind that the amplifier will amplify any DC bias on the photodiode, as well as the current. This may be driving your amplifier into saturation.
     
    Last edited: Apr 10, 2012
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  6. Plantis

    Thread Starter New Member

    Apr 9, 2012
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    Yes, there are reverse-biased.
    The point between resistors R2 and R3 (between pads 2 and 3) has zero potential if currents through first and second photodiodes are equal. In this case, difference channel measures difference between light fluxes.

    Reference channel just measures potential on the reference photodiode.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The node between resistors R2 and R3 is connected to the output (FB) of the upper op amp. There is no connection from the photodiodes to either input of that amplifier. I think you intended to connect that node to the -input, not the FB output.

    EDIT: How much current do you expect from the lower photodiode?

    EDIT: With your circuit wired as shown in the schematic, the upper op amp's output should be zero, independent of the photodiodes.
     
    Last edited: Apr 10, 2012
  8. Plantis

    Thread Starter New Member

    Apr 9, 2012
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    Why is it so?
     
  9. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Attached is a simplified version of your "difference amplifier" as drawn in your schematic.
    1. Keep in mind that, in your amplifier, FB and OUT are the same signal.
    2. You left the PD (power down) pin floating. The datasheet says not to do this. It needs to be connected to +Vs.
    3. It appears that you have 10uF from the output to ground. Why? Even if this does not make the op amp unstable, it will totally kill the bandwidth of the expensive wideband amplifier.
     
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