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photodiode trans impedance amplifier

priya dharsini
Prodigy 160 points
Other Parts Discussed in Thread: LM339

Hello everyone,

                   Am working with distance enhancement in Visible light communication.Am using OSD-100E photodiode.Am designing a trans impedance amplifier.This photodiode has a junction capacitance of 2500pf and an active area of 100mm2.Please recommend an IC that best suits for photodiode transimpedance amplifier design

  • 0
    Guru 140200 points
    Hi Priya,

    what transimepdance and what frequency range are we talking about?

    You know that 2500pF is a huge detector capacitance?

    Kai
  • 0
    TI__Guru* 93105 points

    Hello Priya,

    Once you have fully considered and answered Kai's important questions about your TIA application, you can apply TI's WEBENCH Amplifier Designer tool. That should help you get to a good design based on your inputs. You can find the Designer tool here:

    Select the Transimpedance bullet under TI Design Tools and provide the required information.

    Once you have the basic circuit design we can help you select the TI op amp that best meets the performance requirements of the TIA circuit.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to kai klaas69
    Prodigy 160 points

    @Kai Klaas69,Thank you so much.The modulation frequency is 200 Khz(using 555 timer) using a 3watt LED to transmit the signal.I have chosen this detector since it has large active area so that it can detect light at larger distance.My aim is to increase the range.I have attached a reference paper where they have acheived 400 cm distance using OSD-100E photodiode.1794403.pdfUS_paper_dist.pdf

  • 0 in reply to Thomas Kuehl
    Prodigy 160 points
    Thank you so much
  • 0 in reply to priya dharsini
    Prodigy 160 points
    In photodiode datasheets,the junction capacitance value is given under two conditions,for eg in OSD-100E photodiode,it has junction capacitance 2500 pf at Vr=0Vand 520 pf at Vr=12V, which value of junction capacitance to be considered while using webench tool for photodiode transimpedance design?
  • 0 in reply to priya dharsini
    Guru 140200 points
    Hi Priya,

    biasing the photodiode with a reverse voltage decreases the detector capacitance and by this speeds up the photodiode. But the dark current can heavily increase then and can cause higher noise.

    Kai
  • 0 in reply to Thomas Kuehl
    Prodigy 160 points

    In photodiode datasheets,the junction capacitance value is given under two conditions,for eg in OSD-100E photodiode,it has junction capacitance 2500 pf at Vr=0Vand 520 pf at Vr=12V, which value of junction capacitance to be considered while using webench tool for photodiode transimpedance design?

  • 0 in reply to priya dharsini
    Guru 140200 points

    Hi Priya,

    this depends on your reverse bias voltage setting. With zero reverse bias take 2500pF and with -12V reverse bias take 520pF.

    Kai

  • 0 in reply to priya dharsini
    TI__Guru* 93105 points

    Hello Priya,

    If you follow the Fig. 5 receiver circuit in the WCEWS paper, it shows that the photodiode is being biased with a positive voltage applied to the cathode. Therefore, it is being operated with a reverse bias which reduces the junction capacitance as Kai has discussed. The bias circuit consists of a simple resistor string consisting of a 2.2 k resistor, in series with a 100 k potentiometer, connected to the +5 V supply.

    Since the photodiode is reverse biased, the current through the series resistances will be low and it appears the reverse voltage across the diode will likely be several volts positive. The diode is then connected to the inverting input of an LM339 comparator, and not an operational amplifier. The other LM339 input, the non-inverting input is biased by a 2 k ohm potentiometer connected between +5 V and ground; thus, the non-inverting input voltage can be set to any level between 0 V and + 5V. This establishes the voltage to which the voltage at the output of the photodiode is compared.

    A comparator is analogous to an operational amplifier running open-loop. The gain of the comparator is very high and its output will reside against one output rail, or the other i.e. close to +5V, or close to -5 V because the LM339 is powered by  +/-5 V supplies.

    If you intend to use an op amp in a TIA circuit and apply the suggested calculator, it appears that you will need to use the diode capacitance associated with the intended reverse voltage. There is limited information on line for the OSD-100E photodiode, but there is a set of capacitance vs. bias curves for the diode family. It only goes up to a diode having 50 mm^2 area, while the OSD-100E has a 100 mm^2 area. It does look like from the other curves that they have similar shapes and slopes and you may be able to simply double the capacitance of the 50 mm^2 diode, to obtain the 100 mm^2 diode capacitance at any particular bias voltage for the scale shown.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

  • 0 in reply to kai klaas69
    Prodigy 160 points
    Thank you so much.But for +12V or -12V, I should take 520pf?
  • 0 in reply to Thomas Kuehl
    Prodigy 160 points
    Thank you so much..While designing photodiode amplifier using webench,I could'nt enter the max photodiode current(Ipd) for OSD-100E,since it is not given in the datasheet..How to calculate or how to find it out experimentally?
  • 0 in reply to priya dharsini
    Guru 140200 points

    Hi Priya,

    Ipd has to do with the feedback resistance setting. Enter values between 50µA and 1000µA and see what happens...

    By the way, do you really need a modulation frequency of 200kHz? Can't you choose a much lower frequency, let's say in the 1kHz range? This would simply the TIA design.

    Kai

  • 0 in reply to priya dharsini
    TI__Guru* 93105 points
    Hello Priya,

    The OSD-100E photodiode has a very large active area, and corresponding very high Cpd capacitance of 2500 pF, with VR = 0 V. Unfortunately, the TIA calculator allows for a maximum Cpd of 1000 pF. In most cases that maximum capacitance is a reasonable limit. I think that once you have established the required TIA gain (RF) an appropriate feedback capacitance (CF) can be determined.

    It is not evident to me what levels of current you expect from the OSD-100E detector when you illuminate it with the light source. That will be needed in order to set the TIA gain.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to kai klaas69
    Prodigy 160 points

    Thank you Kai.I have chosen high frequency of 200 Khz inorder to reduce interference with ambient light frequency(100-150hz).Please clarify,so if I choose 1khz as modulation frequency,then my TIA bandwidth should be chosen 1 Khz or above 1 khz?

  • 0 in reply to Thomas Kuehl
    Prodigy 160 points
    Thank u so much for your clarification Thomas Kuel. So as given in the datasheet when Vr=12V,capacitance is 520pf.Now can I design a positive reverse bias TIA,where I can enter the reverse bias voltage (Vbias--as given in webench)as 12V and junction capacitance=520 pf? or can I choose negative reverse bias= -12v for 520 pf junction capacitance,or zero bias TIA with 2500pf(1000 pf allowed by webench tool). I think we can ignore Zero bias,since junction capacitance is large.Either Positive reverse bias or negative reverse bias can be chosen?Please let me know whether my understanding is correct.