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OPA855: OPA855

David Eisemann
Prodigy 120 points

Part Number: OPA855
Other Parts Discussed in Thread: LMH32401

We are using the LMH32401 and are interested in converting the differential output to single-end and driving a 50 ohm load. At the same time we want to cancel the common mode output voltage. The circuit needs to be D.C. coupled. We ran a test with a OPA855 configured as a differential amplifier and using feedback and input resistors of 1k ohms, so the differential gain was 1. The circuit appeared to be stable. The datasheet says a gain of 7V/V for stability. Does 7V/V also apply when configured as a differential amplifier?

Thank you for the help,

David

  • 0
    Guru 140200 points

    Hi David,

    like this?

    david_opa855_3.TSC

    When a minimum gain for stability is specified in a datasheet, this is always the noise gain. And the noise gain of this circuit would be 2V/V:

    Kai

  • +1 in reply to kai klaas69
    Prodigy 120 points

    Kai,

    Yes, the circuit is as you show. Even though the circuit appears to be running stably, should we increase the gain to ensure that the noise gain is > or = to 7V/V?

    Thanks, David

  • 0 in reply to David Eisemann
    Guru 140200 points

    Hi David,

    this is an often asked question: Why is my circuit not showing stability issues, allthough I violate the minimum gain recommendation?

    Because in this frequency range parasitic impedances of board and components can have a considerably impact on the phase margin. Assume these parasitic parallel capacitances of resistances, for instance. These capacitances can come from the resistor itself and/or from the solder pads sizes:

    david_opa855_4.TSC

    A phase margin of 43.6° can mean a stable performance. But if you alter a bit the parasitic capacitances you may end up with this:

    And this circuit would no longer be stable.

    But with a higher gain, the circuit would again have sufficient phase margin:

    So, when using a too low gain, the risk of instability increases considerably. This must be kept in mind, when violating the minimum gain recommendation Relaxed

    Kai