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Ringing in Circuit

Other Parts Discussed in Thread: OPA657, LMH6629

Set-UP: I am using an OPA657 as a transimpedance amplifier. I have a 10MHz, passive, RC filter on the front end (100pF, 160 ohms), The OPA 657 has filter caps on the +5/-5V terminals of .1uF and 6.8uF. My gain resistor is 1.6K with a feedback cap of 0.7pF.

 

Problem: If I have nothing connected to the input of the OPA I have a ringing of 145MHz, if I put a lower impedance (just wire leads or bnc leads) it drops to 85MHz, if i put in a 75 ohm into the input it has 130MHz of ringing. What am I doing that is causing this substantial ringing?

 

Notes: End game I am putting in a 20-30nS pulse, amplifiying it, into a 0-3V uC input. Right now I am just working with the first stage OPA657 as a transimpedance amplifier. I also power it externally with a dual output power supply +5/-5V, to avoid any added noise from the power lines.

  • Hi James,

    With a transimpedance configuration (Figure 3 of OPA657 datasheet), the shunt capacitance (CD) is a very important parameter that dictates the value of CF for compensation and the resulting stability or instability.

    If I understand your front-end filtering, you have a 100pF shunt on the OPA657 inverting node. Is that correct? If so, then the compensation capacitor CF should be raised higher to about 2.5pF per this expression from the LMH6629 datasheet:

    GBWP= 1.6GHz (OPA657 data)

    CIN= ~100pF

    RF= 1.6k

    What is the function of this 10MHz filter you are using? I'm thinking that you need to move this filtering elsewhere to avoid affecting input capacitance like this.

    Regards,

    Hooman

  • hi,
    I am generating Square wave from FGPA at 150MHz is the sampling frequency and DAC CLk (current output DAC). but Squarewave is having spike in the rising edge and falling edge . i hv used opamp with slew rate 2500V/µs for convert differential to single end..
    please leave me some idea, to get very good Squarewave
  • Hello,

    I recommend that you attach your schematic, scope plots, additional details (including what device you are using), so that we can help you better.

    Regards,

    Hooman