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LMH6503: In case IRG is set lower than 1.7mA, what problem will happen?

Katsuhiro Miyamoto
Expert 1480 points

Part Number: LMH6503

Tool/software:

Hello support team,

My customer would like to set IRG lower than 1.7mA. What problem will happen in this case?

Background: They observed oscillation when RG=130ohm, RF=130ohm, and VG=-1V with no signal input. Increasing RF to 200ohm, the oscillation stopped. To keep a safe margin for oscillation, they are considering to set RF 300 to 400 ohms. At the same time, they would like to keep the gain at some fixed value, thus RG will need to be increased. If RG is increased in their case, there could be IRG less than 1.7mA.

Best regards,
Katsuhiro

  • +1
    TI__Mastermind 22550 points

    Hi Katsuhiro,

    The IRG spec in the datasheet can be a little confusing. The spec is the IRG Max value, meaning the max the device's input buffers can output for subsequent stages internal to the device. The minimum this max value can be is the 1.7mA. Meaning a worst-case device, it will only be able to drive 1.7mA of current through RG and will therefore have limited gain throughout the different stages of the device. The customer should have no problem with a lower IRG value, as long as they are accounting for the effects of the lower IRG condition. As for the oscillations, the values of RG and RF can be found using the formulas in the datasheet. It does highlight that below a minimum value for these resistors, stability will be a problem and to limit this issue, scaling/tuning is the best solution.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Expert 1480 points

    Hi Ignacio,

     

    IRG “max” value spec. Understood. It’s certainly confusing.

    Thank you very much for the quick and clear explanation.

     

    Best regards,

    Katsuhiro

  • 0 in reply to Katsuhiro Miyamoto
    TI__Mastermind 22550 points

    Hi Katsuhiro,

    No problem, glad to help.

    Best Regards,

    Ignacio