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OPA657: Simulation of transimpedance amplifier direction of current source

HARI T O1
Expert 1395 points
Part Number: OPA657
Other Parts Discussed in Thread: LMH34400

Tool/software:

Dear Team

I need to design a circuit which help me to interface a current output sensor to ADC.The sensor is not ready yet.The IC design team is working on it.Only thing I know is the sensor output will be a current.

So I started learning about transimpedance amplifiers and it´s simulation.I was checking about some simulations.

One question came to my mind is the direction of current source.In some reference the current direction is from top to bottom.

Some other places the current direction is from bottom to top.

May I know does this have any significance.

Which direction do I need to select.

Below is the the my initial block diagram.

In all the below images the arrow shows the direction of sensor current flow

  • 0
    TI__Genius 12750 points

    Hi,

    There is no true difference between the amplifier sourcing current to the photodiode versus sinking current from the photodiode assuming there is nothing being violated with the device specifications in one configuration versus the other. The one major consideration is the output voltage expected. If the device is sinking current from the photodiode, that will require the output voltage to drop from its bias point. If you source current from the device to the photodiode, the output voltage will need to increase to account for the voltage drop across the feedback resistor. This is something to account for when designing a TIA to ensure you are in the voltage ranges required by the amplifier.

    One other comment that might not apply to you is if you are using an integrated solution like the LMH34400, this amplifier can source or sink current but is optimized to source current to the photodiode. But in general, there is no true preference as long as the circuit accounts for the output and input common-mode voltage ranges required by the device like was mentioned above.

    Best Regards,

    Ignacio

  • 0
    TI__Intellectual 1125 points

    Hi,

    Ignacio explained it very well, that the current direction decides what output voltage swing you are expecting to feed the ADC. Given that a small duration light pulse is applied on the photodiode-

    1) If the photodiode is sourcing the current, the output voltage will be a negative pulse (voltage drop)
    2) If the photodiode is sinking the current, the output voltage will be a positive pulse (voltage rise)

    We have actually developed a Photodiode Circuit Design Tool: https://dev.ti.com/gallery/view/TIA/TIA_GFN/ver/1.0.4/ 
    You can try changing the PD polarity and the see the effect on the output voltage, and also the bias voltage of photodiode. 

    Best Regards,

    Aditya Gosavi

  • 0
    TI__Intellectual 1125 points

    https://dev.ti.com/gallery/view/TIA/Analog_Circuit_Design_Studio_1/ver/1.0.2/
    You can find the photodiode circuit design tool here also, along with the other tools like Shunt Current Sensing and FDA calculator.

  • 0
    Expert 1395 points

    Dear Team,

    Thank you very much for your answers.

    Let me ask my question in other way.We can forget about the amplifier part.

    I have a sensor which output current .Please see the below image.

    In this case what is the direction of current  coming from the sensor.

    Will it be like shown with green line or red line

  • 0 in reply to HARI T O1
    TI__Intellectual 1125 points

    Hi,

    If your sensor is a photodiode, I believe the current direction will depend on the polarity of the diode. As photodiode is generally reversed biased, the direction of current will be reverse to that of the diode direction. 

    Thanks

    Aditya Gosavi

  • 0 in reply to Aditya Gosavi
    Expert 1395 points

    Hi Aditya,

    Thank you very much.

    For photo-diode I understood the direction.

    If some other sensor,may I know how it will be.

    Regards

    HARI

  • +1 in reply to HARI T O1
    TI__Intellectual 1125 points

    Hi,

    I believe there is no standard way of telling the direction of current in the sensor. If you could know the part number of sensor, then maybe we can tell the current direction.

    Thanks
    Aditya Gosavi