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LF412: Change in Bandwidth

Tim Sobering
Prodigy 135 points

Part Number: LF412
Other Parts Discussed in Thread: UA741, LM2904B

I teach an electronics lab.  After problems in the lab yesterday I had an LF412 set up on my bench and measured the step response.  I obtained trise=63 ns which corresponds to a BW 5.5 MHz (unity gain, 250mVpp, probably some peaking in the response).  

I pulled a new part from this semester's stock and I get 180 ns (1.94 MHz) using the same test set up.

Minimum in the datasheet is 2.7 MHz, typ. is 3 MHz. Testing is at +/-15V

I set up a gain of +2 and repeated the measurements.  I get 4 MHz for last semester's part and 1.38 MHz for the new part (output voltage 200 mVpp).  So in effect, I see a 3x difference in bandwidth between the two parts.  Good learning experience for the students but I'm concerned with the low BW in the new part.  Purchased from Digikey.  Any insights?  

Old part is on the right in the photo.

125a9dc8-4b39-4026-b645-2d27de95db09~1.jpg

 

 

  • 0
    Prodigy 135 points

    Screenshots with scope measured risetimes.  I verified the risetime measurement with cursors.

  • 0 in reply to Tim Sobering
    TI__Expert 6710 points

    Hi Tim,

    The reason you are seeing this discrepancy is due to the design refresh that the LF412 went through recently. The details of this can be found in this PCN. The datasheet is in the process of being refreshed to incorporate any differences seen between the old and new die. 

    The new die has a higher slew rate when the input signal is larger. Can you try repeating your measurement with an input signal of 1V? I believe it should result in a higher bandwidth measurement.

    Best regards,
    Carrie

  • 0 in reply to Carrie
    Prodigy 135 points

    Thanks for the prompt response.  I wondered if there was a new PCN.  We already got surprised by the change to the uA741...It used to be easy to measure bias current in the lab...now it's a bit harder. Slight smile

    See the screenshot...The result is odd.  The initial rise is better, but there's a long "tail".  This is with the +2V/V gain.

  • 0 in reply to Tim Sobering
    TI__Expert 6710 points

    Hi Tim,

    This looks like the slew boost effect of the device. The initial 1.5V has a fast ramp rate, and then device reverts back to the small signal response you previously measured as the differential gets smaller. This will be one of the clarifications we make in the datasheet update.

    If you're wanting to "easily" measure IB in the lab, might I suggest the LM2904B as an alternative op amp? The IB for the device is ~10nA, which makes it relatively easier to measure IB than a pA IB device.

    Best regards,
    Carrie

  • 0 in reply to Carrie
    Prodigy 135 points

    Thanks Carrie.  This will make a great discussion for the Electronics II class.  For the lab class, I need parts that are more "ideal" (or at least easier on the students).  I'll check out the 2904 as 10 nA is in the "easy" range. 

    Do you know when the new datasheet will be posted and simulation models updated (741 model also)?

    Thanks for all your help and the quick responses.

  • 0 in reply to Tim Sobering
    TI__Expert 6710 points

    Hi Tim,

    No problem, let me know if you have any other questions! The LF412 datasheet should be updated in the next couple of months. Regarding the models, please see the friend request I sent you.

    Best,
    Carrie

  • 0 in reply to Carrie
    Prodigy 135 points

    Carrie...I might be looking in the wrong place, but to my knowledge I have not received a friend request.

  • 0 in reply to Tim Sobering
    TI__Expert 6710 points

    Hi Tim,

    Can you go to your profile and click on "Friends"? I believe you should see a drop down menu and "Requests to Review". My request should be under that section.

    Best,
    Carrie

  • 0 in reply to Carrie
    Prodigy 135 points

    That's where I was looking.  I can find the request but pulling the "Accept/Reject" menu down and picking "Accept friend request" does nothing.  I tried Firefox and Edge browsers.  Sorry to be a pain...  

  • 0 in reply to Tim Sobering
    TI__Expert 6710 points

    Hi Tim,

    I have sent you a direct message. Let me know if you're able to see it