DC blocking capacitor

Discussion in 'General Electronics Chat' started by Nano001, Oct 5, 2010.

  1. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Hi everyone. I implemented a DC blocking capacitor in my charge amplifier design to remove the blackground amplification and I have a question. I added a .2uf capacitor and 110k resistor to ground to the output of my amplifier. It did great at removing the amplification of the ambient light, however now the pulses are oscillating from +/- 200mV. How can I reconfigure this circuit so the pulses go up from ground, not oscillate about ground. I need to pulse shape through a LT1016 comparator and it does not seem to respond to these type of pulses. I attached my schematic. Thanks.
     
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  2. timrobbins

    Active Member

    Aug 29, 2009
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    Are you say that with constant light (and darkness) to the photodiode, tha you get 200mVpp signal oscillation across the 110k load? If so, then check the output without the LT1016 connected.
     
  3. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    The LT1016 is not connected.
     
  4. timrobbins

    Active Member

    Aug 29, 2009
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    Are you saying that with constant light, and darkness, to the photodiode, that you get 200mVpp signal oscillation across the 110k load?
     
  5. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    I am using an IR LED as a light source and an InGaAs photodiode as a detector. I used the capacitor to block the signal produced from the amplifier of the ambient light in the room because the diode is picking this up. I am getting +200mV when the IR LED is on, and -200mV when it is off, measuring across the 110k load. I am wondering why the signal is going negative when the LED is off.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    1,728
    Instead of connecting the lower end of the 110k Ohm resistor to ground, try connecting it to the junction of a voltage divider across Vcc/GND made from two 2.2k resistors.

    Then use a 5k to 10k pot across Vcc/GND to set your inverting input threshold.
     
  7. timrobbins

    Active Member

    Aug 29, 2009
    318
    16
    Try swapping your photodiode around - you may have it in wrong for a 0V connection scheme.

    But try it all without any room light, or LED, and then slowly bring LED up.
     
  8. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    Connecting the 110k resistor to a voltage divider worked great in lifting the signal, however now when I input this into the comparator there is issues. As I raise vref of the comparator to the middle of the pulses to trigger, the signal out of the op amp as well as the comparator gets very noisy, I have been trying to figure out what it is and I can't. Is there anything I can add to my circuit?

    Also when I raise the inverting terminal of the 324 up from ground there is no output signal. It has to be at ground.
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    Try using hysteresis feedback from the inverting output of the comparator to the inverting reference side.

    Without hysteresis, your comparator will likely oscillate when the inverting and noninverting inputs are equal.
     
  10. Nano001

    Thread Starter Active Member

    Jan 12, 2010
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    What would be a suitable resistor value for this hysteresis feedback? I see little no no change between a low value (say 10k) and higher value (300k).
     
  11. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I always add a very tiny amount of positive feedback to my comparators to provide some hysteresis. Depends on what all is hooked to it though, sometimes just a 2.2 M resistor from the output to the + input works.
     
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