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THS4631 in a digital storage oscilloscope application

Karen
Intellectual 940 points
Other Parts Discussed in Thread: THS4631

I am considering using THS4631 to sense an analog signal for a DSO (digital storage oscilloscope) application.  The input signal will range from 0-500V with input frequency of 0-15MHz with the high amplitude signals (100V-500V) limited to 1MHz. This signal is input at the non-inverting terminal of the THS4631 through a 1M and 30pf parallel network. This network is between the CRO probe and the non-inverting terminal of THS4631, as the circuit will limit the input current flowing through the non-inverting terminal.  Is it necessary to add protection diode at the non-inverting terminal?  I would prefer not to do this as the reverse current will add noise into the circuit.

  • 0
    TI__Intellectual 940 points

    The THS4631 has internal protection diodes so no external diodes should be necessary.

    For an added measure of safety, I would recommend adding a series resistor at the non-inverting input. The size of the resistor would be determined by how much noise can be tolerated in your design.

  • 0 in reply to Karen
    Prodigy 20 points

    Hi, Karen

     

    For an added measure of safety, I would recommend adding a series resistor at the non-inverting input.

    ==>What does mean?

    What is the  value of series resistor to reach measure of safety?

     

  • 0 in reply to Andy Lu74602
    TI__Genius 17905 points

    Andy,

    The first two posts in this thread may not be relevant to your application.

    Sometimes a series resistor is recommended to limit the continuous current that can flow through the ESD diode structures at the amplifier inputs. ESD diodes can typically tolerate up to 30mA continuous current. For example, assuming a positive supply of Vcc+ = 15V and that the input signal to the non-inverting input can go to 20V, a series resistor value of around 170ohms would limit the current through the Vcc+ ESD diode to 30mA [ (20V - 15V)/30mA ~= 170ohms].

    If you have further questions, can you please make a new post with the details of your application and your concerns?

  • 0 in reply to Kris F
    Prodigy 20 points

    Hi, Karen

     

    Thanks for your kindly reply soon.

    I'm sorry I was unclear.

    My project is same as DSO.

    And THS4631D is used by this project.

    And our SPEC is input 40Vpp,

    So the resistor is (40-5)/30mA  ~= 1K2 ohms.

     

    Andy