Help! Transimpedance circuit

Discussion in 'General Electronics Chat' started by Anhtien17, Mar 18, 2015.

  1. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Dear Experts,

    I am working on a project related to transmission signal between LED and Photodiode.
    1. About transmitter:
    - Use a high power LED to transmit a square wave in frequency range from 1kHz to 10kHz.

    2. About receiver:
    - Use a high speed PD (datasheet in attached file)
    - Use a high speed Op Amp (OPA354) in Transimpedance mode for the 1st stage.
    - Use a common Op Amp (LM2904) in non-inverting mode to amplify output of the 1st stage.
    Pls, see circuit and datasheets in attached files.

    But I have a problem when I transmit signal on high distance 1.2m. It seems to be disappeared 1 period in timeline (see photo).
    Please, help me to find out what is going on with my received signal.
    Thanks a lot!

    1st_OPA354_2nd_LM2904.PNG
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    unless you use solme optical componants, your signal will get weaker over distance. it spreads comming out of the transmit led, wakening over distance. also, you will get noise from any light source in the room. try it with a couple of parablic mirrors, or small telescopes. you might try a laser, they have a smaller beam size, and are coherent, thus travel farther.
     
    Last edited: Mar 18, 2015
  3. crutschow

    Expert

    Mar 14, 2008
    13,003
    3,232
    So which receiver are you using for the tests?
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    This thing looks fine on every other pulse. Something very strange is going on. Can you post a schematic of the rest of the circuit?

    What does the signal on the output of the first stage look like?
     
  5. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    You can check datasheet of photodiode in attached file "PS3.6-5-TO5-7605.pdf"
     
  6. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Here is my schematic. I am thinking about capacitors. It might be a reason, because raise side of pulse is cut off.
    But I don't know how to fix it.
    upload_2015-3-18_23-37-54.png
     
  7. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Yes. Surely, one reason is about distance or power of LED. But this phenomenon is quite strange @@
     
  8. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    What supply voltage are you using? According to the datasheet you linked to the LM2904 has a maximum output voltage swing of only 20mV for Vcc=5V and an output load of 10k (surely that can't be right :confused:) !
     
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    Something periodic is going on, so its good to look at the capacitors.

    The OPA354 has a gain-bandiwidth product that is far in excess of your 10 kHz needs; it might be oscillating at 100 MHz or higher. The first thing I would try putting 0.1 uf or .01 uf multilayer ceramic capacitors from the Vcc pins to ground. The next thing I would try is to increase that 1 pf capacitor to about 47 pf. With such a high frequency-gain product the layout and grounding also falls under suspicion.

    Another easy thing to try at this point is since the OPA354 has a standard pin-out would be to swap out the OPA354 for a lower performance amp and see if the "every other" problem goes away, or at least changes.

    Toward Alec_t's point, try putting a 1k resistor from the output of the LM2904 to ground (to allow the output to work closer to ground).
     
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  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    For the 2904, the *low* output voltage is 20 mV max, 5 mV typ. The high output voltage for 5 V Vcc is given on page 7, fig. 5. FOr 1 mA output current, the delta-V from Vcc looks like around 1/4 volt. THIS does not seem right for a darlington NPN output stage, but still you should see at least 3 Vpeak.

    ak
     
  11. alfacliff

    Well-Known Member

    Dec 13, 2013
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    try a smaller value for c7
     
  12. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Yes. I use 12V to supply for LM2904, but forget output load @@ I will try it.
     
  13. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    I have two questions:
    1. How did you calculate for Cf value (1pF -> 47pF)? I understand your mean that increase Cf to decrease Fcut-off to focus on my freq range (1->10kHz). I am sorrry, I am a newbie in this field.

    2. Similar to LM2904, Does the OPA354 need Rload or not? If yes, where is it placed? and how much value of it?
    Thanks.
     
  14. DickCappels

    Moderator

    Aug 21, 2008
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    1. Value for Cf was chosen to bring the rolloff frequency of the feedback network to about 20 kHz.
    Cf = 1/(2 Pi Rf Fc)
    Where Fc is the corner frequency.
    For more complete calculation of the bandwidth, see page 5 of the OPA354 datasheet.

    While you are on page 5, you might want to read the PC layout guidelines on that page. I would not try to use this opamp without it being on a printed circuit board with the recommended decoupling capacitors.

    2. Load resistor for OPA354: It might help.

    While on the subject of output voltage, it is very strange to see a high gain-bandwidth amplifier running with the output so close to the power supply rail. Things get nonlinear in that region very quickly and that might be part of why you get this low frequency periodic loss of signal.

    Raising the noninverting input of the OPA354 to more than 500 mv DC might help solve the problem.

    Questions for you:

    Is the funny periodic signal visible at the output of the OPA354?

    Do you have the recommended bypass capacitors?

    Is the circuit on a printed circuit board?

    Are all of the conductors that connect to the inverting input of the OPA354 short?
     
  15. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Dear,

    Here is my test circuit. I made it for easy and quick implementation.
    I also answer your questions
    1. It has at output of OPA354
    2. I use recommended value in datasheet (Cd = 4pF, Cf = 1pF)
    3 + 4. Answer is photos.
     
  16. DickCappels

    Moderator

    Aug 21, 2008
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    It is hard to tell what is what in the photographs, but things look like they are spread out. Where, in the photos is the OPA354?

    Corrections to my earlier post today, the decoupling and PCB layout guidelines are on page 15 of the datasheet, not page 5.

    Do you have the bypass capacitors according to the recommendation on page 15 of the datasheet?
    A 10nF ceramic bypass capacitor is the minimum
    recommended value; adding a 1μF or larger tantalum
    capacitor in parallel can be beneficial when driving a
    low-resistance load.

    By the way, you should also bypass the LM2904 as a good measure.

    Is the funny periodic signal visible at the output of the OPA354?
     
  17. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
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    Yes. I will try with your suggestions now.
    Yes. It has same the funny periodic signal at the output of OPA354.
     
  18. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
    12
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    Dear,

    I already tried with above comments and also increase power of LED 3 times. But the strange signal still appears at high frequency.
    You can see photo for this problem.
    1. At low frequency 1KHz, it is true for 2 stages of Opamps.
    files: Low_1st stage.png; Low_2nd stage.png

    2. At middle and high frequecy, one period disappear.
    files: Middle_1st stage.png; Middle_2nd stage.png; High_1st stage.png; High_2nd stage.png

    Another problem is improving rising side of pulse, Do you have any idea to improve it?

    Thank you so much.
     
  19. DickCappels

    Moderator

    Aug 21, 2008
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    What is channel 1 and what is channel 2? (I think channel two is the output of the opamps...)
     
  20. Anhtien17

    Thread Starter New Member

    Mar 18, 2015
    12
    0
    Dear,

    - the 1st stage is output of OPA354
    - the 2nd stage is output of LM2904

    I think the problem is concerned to OPA354 transimpedance function. There is something we don't know about this function.
    Ah. One more question, I would like to ask you about type of this opamp "rail to rail", because I thought that the received signal will be changed its amplitude when we move the receive to close to the LED or vice visa, but for this omamp, it change about duty cycle. It is not my goal. My goal is measure the received amplitude when the receiver moving to localize its position.
     
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