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OPA4171: Is it Unity-gain stable? What is the best practice for unused op amp channel termination?

Part Number: OPA4171

Hello, 

I have a customer that wants to check if the OPA4171 is unity-gain stable. From looking at the datasheet and some quick “unity-gain stable” definitions, it looks like it might be as there’s only one pole in the open-loop gain and the gain hits 0 db at the first pole. Is this correct?

 

Also, is there a best practice for properly terminating unused op amp channels? Would something like the below snippet work?

Best,

Quentin Boothman

  • Hey Quentin,

    The OPA4171 is unity-gain stable, and you are close with your analysis. Your first pole is the dominant pole at low frequency, which begins your 20dB/decade dropoff, your second pole is after your 0dB crossing, which you can see with your phase starting to dropoff at the end of the Aol graph.

      

    Another good thing to look for if the device doesn't specify unity gain stability in the datasheet is look for a "minimum stable gain" in the electrical characteristics table. If they don't list one, it can help point to unity-gain stability. Another place to look is in the test conditions in the EC table or typical characteristics charts. THD+N is measured at unity gain, which you would not do if the amplifier is unstable. 

    There isn't a sure-fire constant way across all datasheets to find if something is unity-gain stable, but this is due to several decades of devices being released across many different datasheet "standards" over time. I believe most of our newer devices specify on the first page if a device is unity-gain stable.

    For the unused op amp, there are some pieces of literature on what to do, linked here. The best practice is unity gain, with the non-inverting terminal tied to a voltage value within the valid common-mode voltage range. Since OPA4171 has a rail-to-rail input structure, you can tie it to V- or gnd.

    Additionally it's recommended to tie it to ground with a resistor with a couple kilohms of resistance, to avoid damage in case of a ground bounce. 

    I see the customer has the supply voltage as 3.3V. Please note in the EC table some of the specifications have a minimum voltage of 4V, so you may see reduced performance at lower supply voltages. (some examples below) 

    Best,
    Jerry

  • Thanks Jerry, this answers my questions perfectly.