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.

TLV3491: input impedance

Part Number: TLV3491
Other Parts Discussed in Thread: TLC271, TLV4062

like to know the input impedance on TLV3491

Cannot find on datasheet when for example TLC271 datasheet provide info (typ 10power12 ohm)


  • The input bias current (actually, leakage current) is at most 10 pA at 25 °C, and increases at higher temperatures (see figure 4). You can derive the input impedance from that: RI = VS / IB.

  • Hi Jaques,

    an input bias current of 1pA at 2.5V looks like having an "input resistance" of about 2.5TOhm. But strictly speaking, the TLV3491 -as being a CMOS chip- actually shows an infinitely high input resistance and the ultra low input bias current of 1pA (typical) is only the leakage current of ESD protection cells at the input. So it would not make sense to say that the input resistance is 2.5TOhm. But it makes sense to give a specification on the input bias current and to show how it changes with ambient temperature. And if I'm right with what I'm saying, the leakage current should freeze at very low temperatures which figure 4 exactly demonstrates: The leakage current goes all the way down to 0pA.

    Keep in mind that the input capacitances of TLV3491 play a way more dominant role on the impedance, especially at high frequencies. 2pF gives an impedance of 800kOhm at 100kHz. Compare this impedance with 2.5TOhm.


  • Dear Clemens,

    you are totally out of your league: 10pA multiply by 1Mohm equals 10 micro Volts - this is not the subject.

    I have changed the resistor values an order lower and it seems working.

    I am still rather surprised since the TLV40x used in same supply Under-Voltage Detection R2 is chosen as 1 Mohm (which is precisely why you buy such a component --to draw as little as possible current) RE datasheet TLV40x para 8.2.1 Under-Voltage Detection.

    I should work with same  1Mohm value R2 with  the TLV4062

    I have recheck the circuit for possible dirty remains on the surface but it was clean (it has been cleaned with isopropylic alcohol)

    Hence the question is still there: with the body marking I gave you, have you found any report of QA issues in production?



  • Dear Kai,

    Thanks taking time helping

    the TLV4062 is used in my project for Under-Voltage Detection i.e. working in pure DC.

    I am rather upset loosing time on such trivial circuit.

    Anyhow have a good day.


  • Jacques

    Sorry for any issues you have experienced to this point.  Clemens and Kai have pointed you on the right path to determining input impedance.  It really comes down to input bias current versus temperature and at the common mode voltage of your circuit.  Do you need any additional support?


  • Hi Chuck,

    Thanks taking the time on my problem.

    In fact as usual one looks for complicated explanation on a simple matter.

    Pins short circuited by flux....

    It illustrates you can draw on ressources for..nothing

    I would I have appreciated my question to be answered in first place: I would have gained time. The question was any QA Pb in production? answer No on my lot number. Consequence something on my board was wrong: the order of magnitude was astounding.

    OK here we are.



  • Hi Jacques,

    we had assumed that you already had the board properly cleaned, or course.


  • Yes, it is a first to me

    have nice WE

  • Better! Two weeks holidays from tomorrow on Relaxed

    Passez un bon moment!


  • Hello Jacques,

    Flux can turn conductive when it is burned. This happens commonly during hand soldering as the temperatures are not as controlled.

    If the flux is dry, crusty and blackened, it has carbonized and has turned into a parallel resistor that can be in the low kilo-Ohms.

    So what you have seen is not unusual..especially with the fine pith and close pin spacing making things worse.