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Photodiode Amplifier problem

alley1122
Prodigy 20 points

hello!

I do not have a lot of basic electronics background but I need a simple photodiode amplifier circuit for my project. The optical beam received has intensity of a few microwatts and the photodiode amplifier circuit is needed to convert the optical signal into a voltage one. I am using a Hammamatsu diode, chose OPA129 for this and designed the whole circuit exactly with same component values as shown in figure 7 of OPA129 datasheet (http://focus.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=sbos026a&fileType=pdf ). It is supposed to be a simple circuit where I come in with an optical signal of some microwatts and get it in voltage after passing through the amplifer but what I get at the output is fixed -15V which is the supply voltage at pin 5. Like I said I do not have a lot of basic electronics background and after some reading on the internet i think it maybe as I designed my PCB exactly as shown in fig.7 of above datasheet and in that figure they do not have any input resistance at input 2 of amplifier. Is this the reason why the photodiode amplifier is not working as it should? Additionally it was hard for me to conceive how to design the guard ring so I just grounded inputs 1,3,4 and 8 on the board which I now think is wrong. I can not progress ahead until this problem is solved so please any hep will be sincerely appreciated.

  • 0
    Prodigy 10 points

    Hi,

         In terms of making a super-precision circuit your grounding solution of pins 1,3,4,8 is not ideal, but at microwatts of power you should be MORE than totally fine.  Your problem  is actually just that your 10^10 Ohm(10 gigaohm) resistor is FAR FAR FAR too large!!  Typical Hammamatsu photodiodes have conversion efficiencies in the range of .5 amp of current per watt of optical power, so at microwatts of power you're expecting microamps of current.  A 1 MegaOhm resistor will give you a volt per micro-amp of power, and a gigaohm will give you 1 kilavolt of power.  Because you can only supply 15 volts (really -15V, as the amplifier is actually inverting), the amplifier just sits at -15v at any light level you provide.  You should try 1 megaohm, or maybe 100 kiloohms, of resistance, and see which works better.  If neither works, then try 10kilo-ohms.  My guess, actually, is that 100kilo-ohms is what you'll want (this is 10^5 ohms).

                         Jon

  • 0 in reply to Jon Bean
    Prodigy 20 points

    hi Jon,

    I tested with both 1 Mega-ohm and 100 Kilo-ohm and the circuit works great with 100 kilo-ohm as the feedback resistance. Thank you very much for your elaborative reply.

  • 0 in reply to alley1122
    Prodigy 10 points

    Hi , Like your case i am also poor in electronics background however having reasonable control in MCU . In my project i need to use photodiode as position sensor by detecting laser light falling on the photodiode . I plan to use ADCs of  microcontroller and scan voltages across all photodiodes . One having higher voltage than rest may be treated as one having position indicating laser . Once position is detected further appropriate logical action may be initiated by MCU .

    Source of laser is 300+ meters away and in photodiode will remain in out exposed to sun (not always ) . So at the moment detection of laser in such circumstance is proving difficult for me . Further same sensor has to work in the night also .

    I will try above mentioned approach and any further help will be great for me .

  • 0 in reply to Vinay Shrivastava
    Prodigy 10 points

    It sounds as though you need an optical filter, not additional circuitry. The photodiode converts light to current, irrespective of the light source. Your very first task is to select a filter which blocks all lightnot within the specified output range of your laser. Mind the temperature effects! I suggest you Google "interference filter" or "narrowband optical filter" to find suppliers.

    In addition, you should install an optical occluder or other directional shading device so the photodiode only "sees" light coming from the direction of your laser. Does the laser light only come from one location or a narrow angle? Simply put it at the end of a tube which has been painted flat black inside. Also, pay attention to the viewing angle of your photodiode. These typically come with built-in lensing and a variety of wide and narrow viewing angles.Choose the narrowest one which will do the job. This also has the effect of magnifying relative response. You can then decrease amplifier gain to eliminate false triggers.

    These low-level amplifiers are also susceptible to RF interference. Good shielding, with only the light aperture open, may help prevent this.