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LF398-N: Input offset problem

Ian Bennett50
Prodigy 20 points
Part Number: LF398-N

I have been using the TI LF398-N in one of our products for many years with no problems, however our most recent production batch do not work correctly due to excessive input offset in all three LF398-Ns used in the circuit, normally there is less than 3mV difference between input and output, now there is about 15mV. The standard offset adjustment circuit described in the datasheet no longer fully zeros the offset, I changed resistor values to achieve zero offset, but then the offset varies with signal voltage. Swapping the ICs with some old ones from stock solves the problem, but I have no more in stock. I tried changing the LF398-Ns for the -AN version, but these seem to have the same problem, Is this a known issue? Is it just a bad batch of ICs that I got from RS components? I would appreciate any help or advice.

  • 0
    TI__Prodigy 575 points

    Hello Ian,

    The LF398-N has a maximum input offset voltage of 7 mV when used in the following conditions as specified in the datasheet:

    (-Vs + 3.5V) < Vin < (+Vs - 3.5V)

    +Vs = +15V

    -Vs = -15V

    Ambient Temperature = 25°C

    Ch = 0.01 µF

    RL = 10 kΩ

    Logic High = 2.5V

    Logic Low = 0V

    Can you test your LF398-N devices under the above conditions and measure their offset and let us know what your results are?

    Additionally, can you comment on whether or not there have been any changes to your circuit other than switching to newer stock of the LF398-N? If you could post a schematic or at least a partial schematic as it relates to the LF398-N, I'd like to have our applications team review as there are no known reported issues with the LF398-N and I would like to ensure this isn't an applications issue before reaching out to our quality team for comment. 

    Thank you in advance for the above information.

    Christopher Stemple

    Product Marketing - Precision Amplifiers 

  • 0 in reply to Christopher Stemple
    Prodigy 20 points

    Hello Christopher

    I have recently found the cause of the problem, two other ICs, an HEF4060BP counter and HEF4555BP decoder have also been changed to TI equivalent devices due to obsolenscence, this resulted in tiny glitches on the logic input of the LF398 which caused offset between input and output. The old Phillips LF398 is not affected by the glitches, which incorrectly led me to think the TI LF398 was the cause of the problem. To resolve the issue in the short term have bought old stock Phillips and NXP counter and decoder ICs.

    Kind regards

    Ian Bennett

  • 0 in reply to Ian Bennett50
    TI__Prodigy 575 points

    Ian,

    Glad to hear the LF398-N  wasn't the root cause, but if we can provide any applications support to help you deal with integrating the new TI pieces into your circuit, please don't hesitate as we would definitely like to see your design fully functional with all of the TI components you've mentioned. 

    For tracking purposes and to ensure maximum visibility, I would recommend creating a new post on E2E regarding the TI replacements for the HEF4060BP and HEF4555BP as well as their respective TI part numbers.

    Christopher Stemple

    Product Marketing - Precision Amplifiers