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OPA170: Photodiode Amp

Part Number: OPA170

Hi,

Good day.

Our customer watched the "How to Design Transimpedance Amplifier Circuits" () and they have a question on photodiode application. I copied their inquiry below. I would appreciate if you could help us o this.

"I want to know how to convert an input current from an array of for example 50 photodiodes to an output voltage that ranges from 0V to 5V or 0 to 10V.
The array of photodiodes should be on some kind of frame 1m x 1m. This supposed to be a sensing area. One side will be an array of IC emitters and the other side (receiver side) array of IC photodiodes (BPV10NF).
I found TI example with transimpendance amplifier circuit for one photodiode and when the photodiode is illuminated amplifier output reflects as voltage jump at the output of the amplifier. In my case they are constantly illuminated, also there is an array of them and when objects pass this area it should be detected - voltage on the amplifier should increase.
I am wondering can I make the parallel connection of photodiodes and is it better to use other kind of amp. and how to design it."

Thank you for your kind support.

Regards,

Cedrick


  • Hi Cedrick,

    what is the desired signal bandwidth?

    Kai

  • Hi Cedrick,

    I do not see why you couldn't put multiple photodiodes in parallel. The thing I would be concerned with is having 50 diodes in parallel will put a lot of capacitance directly on the inverting input pin leading to potential stability issues. Each diode can have 10's or 100's of picofarads of capacitance, multiply this by 50 and it will be like hanging a 500pF to 5nF cap directly on the inverting input pin. The bandwidth would have to be significantly reduced for it to remain stable.

    I recommend taking a look at this reference design. It discusses in detail how to design the TIA circuit and takes the stability of the amplifier into consideration.

    Thank you,

    Tim Claycomb