Why a Rail to Rail op-amp is required for a IR sensor?

Discussion in 'General Electronics Chat' started by trytolearn, Apr 25, 2010.

  1. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    Hi all:

    i have been advised to uses a Rail to Rail op-amp to build a IR distance detection sensor.

    The amplitude of the IR signal will be inputed to a ADC port and a microprocessor is going to calculate the distance.

    But could anyone please explain to me that why a rail to rail op-amp is an ideal option?

    [​IMG]

    PS: the diode is represents a photodiode, i couldnt find a symbol for it so i just used a diode instead
     
    Last edited: Apr 25, 2010
  2. hgmjr

    Moderator

    Jan 28, 2005
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    While sometimes you can get by with a non rail-to-rail opamp, by using a rail-to-rail opamp you will be able to get the maximum useable signal.

    Provide a schematic so that we may better understand your application.

    hgmjr
     
  3. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    thanks

    i have provide it
     
  4. Jaguarjoe

    Active Member

    Apr 7, 2010
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    What is the function of diode D2?
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    I would be interested in a bit more details concerning how you plan to use the IR detector to measure distance. Your schematic seems to be missing an IR-detector.

    hgmjr
     
  6. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    Hi,
    sorry, the diode is infact a photodiode,
    i couldnt find a symbol for it so i just used a normal diode to present it.

    A infrared light source will shine on this photodiode
    the current pass through the high pass filter resistor to create a voltage and send it to be amplifed
     
  7. hgmjr

    Moderator

    Jan 28, 2005
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    It seems as though an ADC is not much use to you. The photodetector circuit you have is blocking dc so it appears that your IR transmitter is being pulsed.

    hgmjr
     
  8. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    yes, a 5% , 2Khz will pulse the emitter
    and the high pass filter to get rid of ambient light
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    I am still having problems envisioning how you are using this to measure distance. The speed of light is approximately 186,000 miles per second. Exactly what technique are you using to determine distance?

    hgmjr
     
  10. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    It appears you will be using non-modulated (i.e., continuous) IR and basing your distance on a decrease in intensity of the reflected beam.

    I do not think that method will work very well. What accuracy do you need? Over what distance will you be measuring?

    Some of the reasons are: 1) Continuous IR will experience lots of interference from other sources; 2) Range will be very limited; 3) Intensity of reflected beam will depend on several factors besides distance, e.g., reflectivity, angle, dust and other interferences.

    I suggest you consider either triangulation or time of flight. There are packaged IR devices that use triangulation, such as the Sharp GP2D12 to GP2D15 series of sensors.

    John

    Edit: Above was written before your comment about a modulated beam appeared.
     
  11. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    i will pulse the IR signal, sends a pulse for about 20us long, every 1ms

    ie, enable the emitter the for 20us, measure the output
    switch of the emitter, and stop measuing



    so the detector will detect the pulse
     
  12. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    Thanks

    the detection range is only up to 6 cm
     
  13. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    At 6 cm, triangulation would be by far the easiest and probably most accurate.

    John
     
  14. hgmjr

    Moderator

    Jan 28, 2005
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    Time of flight for light traveling 6cm assuming that the total distance traveled is 12cm (6cm out and 6cm back) is on the order of 0.4 nanoseconds. At that speed I am not sure I see how your IR detection scheme would work.

    hgmjr
     
  15. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    I meant switch the emitter on for 20us

    so the detector can measure it while the emitter is switched on

    [​IMG]

    here is the output from the scope
     
  16. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    Thanks :D

    i have used both my self made ones and sharp series for longer range.

    due to a bugget issue really

    i am just trying to understand why a rail to rail is perfered
     
  17. hgmjr

    Moderator

    Jan 28, 2005
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    Does the scope you are using have the ability to measure on the order a few picoseconds?

    hgmjr
     
  18. hgmjr

    Moderator

    Jan 28, 2005
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    It seems to me like the challenge of your project has less to do with whether you use a rail-to-rail opamp or not. The real challenge is to get picosecond resolution from a circuit that has device propagation times on the order of nanosecond.

    hgmjr
     
  19. trytolearn

    Thread Starter Active Member

    Mar 1, 2008
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    from the screenshot

    you can see the divison is 5us

    so the length of the pulse is about 15us

    what i am saying is that

    switch on the light for 20us

    and the detector could detect pulse because the light source is switched on for 20 us
     
  20. hgmjr

    Moderator

    Jan 28, 2005
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    So you are intending to use "time of flight"?

    hgmjr
     
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