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Manipulate voltage output in transimpedance amp circuit

  1. Jan 29, 2013 #1
    I have a circuit (attached) that is providing too much voltage into my FPGA. I attached the circuit.

    The output of the transimpedance amp is sending 4.2V to my FPGA which only needs 3V. I h ave reason to believe that the 4.2V input into the FPGA is causing some weird anomalies to occur while testing my FPGA with 4.2V input from amp. I was wondering if I increase the voltage drop at R80 would that help bring the voltage down to 3V into my FPGA. Or is the voltage output controlled by both R80 and R78?

    I just need to understand what it will take to get the output of the amp to be 3V instead of 4.2V

    Please advise
     

    Attached Files:

  2. jcsd
  3. Jan 29, 2013 #2
    Assuming input to FPGA is digital?

    R80 is not controlling output voltage of TIA.

    I would just use a voltage divider since you are already low pass filtering to 3MHz.
     
  4. Jan 30, 2013 #3
    its a 4.2 V square wave.

    so a voltage divider to the right of C128?
     
  5. Feb 3, 2013 #4
    Generally, a series resistor, such as your 1K, will work with the ESD diodes to allow level conversion.
    However, high speed logic will typically break into oscillation during extreamly slow rise and fall times. Typically your data sheet will have a maximum rise / fall time quoted.
    To get around this, install a schmitt trigger between the analog and logic. As I recall, a 74xx14 will do this and is available as tiny logic.

    - Mike
     
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