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LMH6626: Oscillating When Input Left Open and Sometimes When Not

JAMES ECK
Prodigy 71 points
Part Number: LMH6626

With the following schematic and layout I am seeing oscillation around 80 MHz, particularly when the SMA connector at BB_IN does not have any thing connected to it (an input signal or termination usually stops the oscillation, but not always). I've fiddled with the exact values quite a bit to no avail. What can I do to make this amplifier unconditionally stable?

  • 0
    TI__Genius 15975 points
    Hi James,

    In your current circuit, not connecting the SMA connector at BB_IN will configure the 1st stage LMH6626 in a closed loop gain (Av) of 1. The LMH6626 is only stable for |Av| > 10, which is why you see the circuit oscillating without anything connector at the BB_IN SMA connector. Connecting an input signal or termination with source impedance of 50-ohms configures the 1st stage LMH6626 in Av = - (1k/100) = -10, which is marginally stable condition. So, that could possibly explain as to why you see oscillation sometimes by connecting an input signal or termination.

    If you can, try a direct short to GND on the BB_IN SMA connector and I believe you should not see any oscillation in your circuit because the closed loop gain is now 20 in the circuit. Is there a reason as to why you are using 1st stage LMH6626 with inverting input instead of the non-inverting input? I think you could make your problem easier by connecting your input through the non-inverting input side of the LMH6626 where you don't have to deal with the open input termination issue.

    Also, do you have the feedback caps 1.8pF placed or DNP in the current board?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Prodigy 71 points

    Thanks! I wasn't taking into account the source impedance. I think I'll increase my front end resistors to make that less of a factor.

    I'll definitely look at using the non-inverting input. I inherited this design and will have to ask around if there was any particular logic to using the inverting on both.

    The caps are not populated, but they are an option for changes to the board that we have played with.