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High speed Transimpedance Amplifier design

moki fonk
Prodigy 220 points
Other Parts Discussed in Thread: LMH6629, OPA847

Hello All,

I am working on a project about a wavemeter. the wavemeter have 16 photodiodes. I want to design a transimpedance amplifer that I will interface with an ADC that I will later select.

- My problem is selecting a transimpedance amplifier with a speed of 100MHz at 10ns with low noise. this is because the photodiode are working in the photovoltaic mode which is a problem at high frequencies. With a high signal to noise  ratio. My question are: its possible to have transimpedance amplifier that can perform this task. Do I need a two stages. design one for the transimpedance amplifier and the other for the additional gain .

 the photodiode current is 0.04mA and capacitance is 100pF

- the  ADC will be 12 bits with a sample rate of 100MHz

  • 0
    TI__Genius 12631 points

    Hello,

     What is the actual BW of the input signal to the Transimpedance Amplifier? The OPA847 or LMH6629 are both good choices for this application. The LMH6629 has lower noise than the OPA847. It will be difficult to achieve high BW and high gain, especially with the high value of input capacitance. You will most probably need to have a 2nd voltage gain stage after the TIA stage.

    Samir

  • 0 in reply to Samir Cherian
    Prodigy 220 points

     

    Hi Samir,

    Thanks for your reply,What do you mean by the actual BW of the input signal, the maximum laser bandwith is 1Ghz, and measurement time is 10ns at 100MHz.  

     I saw the LMH6629 and I am going through the datasheet.

  • 0 in reply to moki fonk
    TI__Genius 12631 points

    Hello, 

      The optimum bandwidth needed for a pulsed input actually depends on the rise/fall time of the pulse as these are the highest frequency components of the pulse signal. The relationship is F(GHz)=0.35/Tr(ns). The rise and fall time of the pulse could be restrcited by the laser driver which may not have the 1GHz BW that the laser has.

    Samir.

  • 0 in reply to Samir Cherian
    Prodigy 220 points

    Hello,

    I have check the laser driver, it has the 1GHz BW. I have used the LMH6629 IC for the simulation and the results are ok. 

    I want to used the  synchronous detection to reduce noise.

  • 0 in reply to moki fonk
    TI__Genius 12631 points

    Hello,

      Good to know the LMH6629 is working well for you. Which laser driver are you using? Is your input to the transimpedance amplifier going to be a pulse? What is the end application?

    Samir

  • 0 in reply to Samir Cherian
    Prodigy 220 points

    Hello,

    It is AWG photodiodes chip with 10 channels, that means the input to the transimpedance amplifier will current from the diodes which is DC light level and AC modulated light incident on photodiodes. As I said, I want to use synchronous detection after the transimpedance stage to eliminate the noise and have a DC level representing the amplitude that will be fit in the ADC to have a high resolution and accuracy.

     

  • 0 in reply to moki fonk
    TI__Genius 12631 points

    Hello Moki,

      Thanks for the explanation. I have been trying to design a system which generates a continuous wave output from an optical system, however my laser drivers dont seem to be fast enough for doing several MHz of continuous waveforms, eg sine wave. At present I can only get high speed square pulses from my optical system. Can you please share what type of laser driver you are using in your system.

    Thanks,

    Samir

  • 0 in reply to Samir Cherian
    Prodigy 220 points

    Hello Samir,

    I sorry for the late reply, I did not design the laser driver, I am using a tunable laser.