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What’s an example of where you picked the wrong Op-Amp … and why?

baldengineer

I’m working on an episode about Op Amps and measuring some basic parameters. (For those like me who would rather measure something than go cross-eyed looking at a datasheet.)

Since there are more than a handful of op amps, I can imagine there are situations where people select one or re-use one without fully evaluating all of its characteristics. So, that got me thinking, what are some cases where you picked an op-amp for an application, but it was (clearly?) the wrong one… and why?

Bonus question: does anyone still use the ua741 … for anything?

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  • I am currently trying to decode USB-PD CC communication, which is a ~1.6V signal. To sample the signal with a logic analyzer I wanted to amplify it to a high level of 3.3V.
    My first choice with a component on hand, was the Microchip MIC6271. It is rail to rail and has a 2 MHz gain bandwidth and a slew rate of 0.9V/usec. PD messages are sent at 300 KHz +-10% with rise and fall time of measured 600 nsec. Using the rise time for calculation, this signal is already beyond the limit of the opamp capabilities. 600 nsec rise time corresponds to 1.667 MHz and a gain of 2 calculates to 3.3 MHz gain-bandwidth already, leaving no headroom to the max GBW of the opamp. Also the slew rate of the signal is ~2.6V/usec measured, exceeding the opamp spec. This adds up to a amplified signal not representing the input signal at all. See image, blue channel 2 is the CC signal; yellow channel 1 is the amplified signal.


    image

    So, a different opamp with >10 MHz GBW and >3 V/usec slew rate is on order. I think a good rule of thumb is a GBW 5 times as the edge frequency.
    Unfortunately, I have nothing in the parts bin, so this route needs to wait to be explored further.

    Edit: The opamp was used in a non-inverting configuration with a gain of 2; feedback and negative to GND resistors had equal values.

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  • I am currently trying to decode USB-PD CC communication, which is a ~1.6V signal. To sample the signal with a logic analyzer I wanted to amplify it to a high level of 3.3V.
    My first choice with a component on hand, was the Microchip MIC6271. It is rail to rail and has a 2 MHz gain bandwidth and a slew rate of 0.9V/usec. PD messages are sent at 300 KHz +-10% with rise and fall time of measured 600 nsec. Using the rise time for calculation, this signal is already beyond the limit of the opamp capabilities. 600 nsec rise time corresponds to 1.667 MHz and a gain of 2 calculates to 3.3 MHz gain-bandwidth already, leaving no headroom to the max GBW of the opamp. Also the slew rate of the signal is ~2.6V/usec measured, exceeding the opamp spec. This adds up to a amplified signal not representing the input signal at all. See image, blue channel 2 is the CC signal; yellow channel 1 is the amplified signal.


    image

    So, a different opamp with >10 MHz GBW and >3 V/usec slew rate is on order. I think a good rule of thumb is a GBW 5 times as the edge frequency.
    Unfortunately, I have nothing in the parts bin, so this route needs to wait to be explored further.

    Edit: The opamp was used in a non-inverting configuration with a gain of 2; feedback and negative to GND resistors had equal values.

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  • in reply to wolfgangfriedrich

    Fun fact, slew rate / GBW limits were the driving idea behind this video.

    I saw a very similar waveform for a very similar reason. I should have known, but I didn't even consider the limitation until I saw the output.

    The other driving factor was mentioned too: Common-Mode. I have a measurement where I completely misinterpreted the datasheet and tried, for more hours than I should admit, to prove it was wrong. My friend looked at my measurements and said: "yeah, your op-amp is limited to X." He was absolutely right. (I have to go back and get the details on that one.)