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OPA659EVM Modification

Alex Cimaroli
Prodigy 250 points
Other Parts Discussed in Thread: OPA659EVM, LMH5401, ADC12J4000, OPA659

Hello,

We are needing a unity gain buffer for a high-speed open-circuit voltage decay measurement system.  The unity gain buffer will take as its input the open-circuit voltage of a solar cell device.  We are looking at the OPA659EVM to fulfill this purpose.  However, the OPA659EVM has its non-inverting input pulled down with a 50-ohm resistor.  This may not work for our intended purposes since the input voltage signal must be as close to an open-circuit condition as possible.  Would removing the 50-ohm resistor cause any issues with the unity gain buffer stage?

In more detail, the unity gain buffer's single-ended output is fed to the single-ended input of a LMH5401, which is then sent as a differential signal to the inputs of a high-speed ADC (the ADC12J4000).

Thank you for your assistance in this matter.

-Alex

  • 0
    TI__Genius 13935 points

    Hi Alex,

    The 50 Ohm resistor on the EVM is for use with high speed test equipment, which typically is 50 Ohms. You may remove the resistor.

    The issues with removing the reistor are:

    Bandwidth may be reduced.
    High speed signals may show reflections.

    In order to mitigate these issues:
    Locate OPA659 as close to the signal source as possible (mount on solar cell if possible)
    Minimize any capacitance on the OPA659 input.

    Regards,
    Loren

  • 0 in reply to Loren Siebert 1
    Prodigy 250 points

    Hi Loren,

    Thanks for the assistance.  The reason you say to locate the OPA659 as close as possible to the signal source is to reduce parasitic capacitance, correct?  There will already be some capacitance (at least nanoFarads) at the input of the OPA659 from the inherent capacitance of the solar cell devices that are producing the signal.  Do you know if this will present any issues for the high-speed response of the OPA659?

    As you will find, my working knowledge of electronic circuits is somewhat limited.  I was testing the OPA659EVM after removing the 50 Ohm resistor, and it responds non-linearly to input voltages below 1.5VDC.  After 1.5V, the output response is very much linear and unity gain.  I am using a single power supply of 12V to power the OPA659, but this should not be an issue.  Do you know what the problem might be?  I checked all the connections and on-board resistors, and everything is fine in that regard.

    Thanks,

    Alex

  • 0 in reply to Alex Cimaroli
    TI__Genius 13935 points
    Hi Alex,

    For input voltages close to 0V you will need to provide a negative power supply for the OPA659. Otherwise you will need to find a negative rail iput amplifier. Can you run on +-6V supplies?

    If you don't have a controlled impedance source the best way to maintain signal fidelity is to have a short path between the signal source and the amplifier. That is the reason I suggested placing the OPA659 close to the panels. That way you are sampling the signal very close to the source, then you can buffer it with the amplifier and send it along a transmission line to the next element.


    Regards,
    Loren
  • 0 in reply to Loren Siebert 1
    Prodigy 250 points

    Yeah, we can use a split power supply of +/- 6V.  Thanks for you help Loren.

    -Alex