• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 319 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LF398-N: Selecting the Appropriate Hold Capacity Ch.

Nicolas Negre
Prodigy 15 points
Part Number: LF398-N


Tool/software:

Hello E2E Support Forums!

This is my first time posting, let me know how to best utilize these forums.

In my design, I am using the LF398H/NOPB (same Datasheet as the LF398-N described above) as a sample and hold amplifier to hold an analog signal for ADC processing.

In our industrial application, what would be some expected worst cases that the hold capacitor could expect?

What are some capacitor parameters that would ensure the longevity of the hold capacitor?

Additional context: This is an old design in which the previous hold capacitor (Panasonic ECQP1H104GZ) has gone obsolete, and we are looking for a drop-in replacement polypropylene capacitor. I have not been able to find an alternate component that is better in all aspects. The most favorable alternate I have found is the Vishay MKP1839410251, or possibly the Vishay MKP1837410161F, however, those PP capacitors have an insulation resistance/withstand voltage that is not better than the old part, and further analysis of the application has to be done. I do not expect help with finding an alternate component, my goal is to better understand the characteristics of the LF398H to make a better guided decision. 

Let me know if you need more details and thank you very much for your help!

Nicolas

  • 0
    TI__Mastermind 28150 points

    Hi Nicolas,

    Oh that is an older design. Using through hole components are becoming increasingly rare. 

    Your assumptions to go with a polypropylene capacitor is smart. Testing was done on this device with different types of capacitors against it's sample time to see what the absorption error would be. You can see screenshots of the tests from the datasheet below: 

    The problem is these tests don't specify the exact capacitors used, just the type of capacitors. That makes it difficult to understand what the effects would be across different pp capacitors. 

    I suspect that the insulation resistance will not play a large factor to the overall voltage sag over sample time as compared to the material of the capacitor. The LF398-N has it's own leakage paths that will drain the hold capacitor one way or another. I think think this will be more dominant than the capacitor's. 

    Best Regards,

    Robert Clifton 

  • 0 in reply to Robert Clifton56
    Prodigy 15 points

    Thank you for the information and insight.

    I can see why the insulation resistance would likely play a small part in the overall voltage sag. However, I am still a little lost on how I could quantify the insulation resistance requirement for my application. I found Section 9.2.1 of the LF398-N datasheet to most closely resemble our amplified analog signal application. 

    In this situation, how could I quantify an acceptable value for the hold capacitor's insulation resistance?

    Would it be best to evaluate the signal characteristics of the 1st amplifier or the Vcc supply voltages?

  • 0 in reply to Nicolas Negre
    TI__Mastermind 28150 points

    Hi Nicolas,

    I think the signal characteristics of the 1st amplifier would provide more information than the VCC supply voltages. The effects of the voltage sag will be apparent on the output of the device. 

    Best Regards, 

    Robert Clifton