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TLV3011: Comparator's Input Resistance

Shihab
Expert 1225 points
Part Number: TLV3011


Hi,

I have an RC network (R=1Meg, C=0.1uF) connected to the 4th pin of TLV3011. Pin 3 connected to 3V. Power supply is 3.3V.

Ideally, when an input of 3.3V is applied to the input of RC, the time required to reach 3V across capacitor is ~250mS.

But as per testing, the delay between Input pulse going high to comparator output going low is measured as ~285mS.

I think, to get this value the PCB should have a parasitic resistance of ~28 Meg ohm from pin-4 to ground.

Can you tell me if this parasitic resistance is caused due to the input impedance of the comparator?

Thanks,

Shihab.

  • 0
    TI__Mastermind 26460 points
    Shihab
    i don't think the discrepancy between your calculations and your measurements are due to parasitic resistance. i am guessing that it's more likely due to your components not being exactly the values you are using in your calculations. Another source of error could be the offset of the comparator or your voltages not being exactly correct. Parasitic impedance only starts being a factor when you have very large resistor values. AT 1M , I don't think that's contributing enough error to alter your measurement. One way to verify is to change your voltage slightly or use different resistor and capacitors of the same value and see if the measurement changes.
    Chuck
  • 0
    TI__Guru 63136 points

    Hello Shihab,

    Have you measured the exact value of the capacitor??

    Standard capacitors can have tolerances as wide as +100/-20%.

    Your timing error is only 12%, well within the common 20% tolerance for general purpose capacitors.

    You will need to select a better grade of capacitor, and getting <10% tolerance gets expensive.

    You may need to add some trimming to the resistor to fine-tune the timing per-unit, since it is easier to trim the resistor than the cap.


  • 0 in reply to Paul Grohe
    Expert 1225 points

    Hello Paul,

    The capacitor I am using is C0G type having +/- 5% initial tolerance and +/-30ppm/degC. The resistor is +/-1%.

    I have measured the capacitance after removing it from the PCB and the capacitance is 0.1uF.

    Thanks,

    Shihab.