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TLV2333: Question about Input Bias Current vs Input Differential Voltage

David Kim2
Expert 2520 points
Part Number: TLV2333
Other Parts Discussed in Thread: OPA397, OPA396, OPA392, OPA391, OPA320, OPA196, OPA191

Hi

My customer is using TLV2333.

The input differential voltage is 2V, but the graph below is expressed only up to 800mV.
I want to know the input bias current in 2V.

Thanks

  • 0
    Guru 140200 points

    Hi David,

    have you seen figure 20 in section 8.3.5 of datasheet?

    Keep in mind that with a chopper OPAmp the differential input voltage must always be zero, or by other words, the feedback loop must always stay in regulation. Allowing differential input voltages different from zero can result in weird behaviour of OPAmp. So, don't use it as a comparator, example given.

    Kai

  • 0
    TI__Guru 50991 points

    Hi David, 

    I want to know the input bias current in 2V.

    As Kai pointed out, the paragraph below explains the issues if the TLV2333 is not operating in a linear region. 

    If you have additional questions, please let us know. 

    Best,

    Raymond

  • 0
    TI__Guru 61340 points

    Therefore, to answer your question directly, with differential voltage of 2V the input current (not IB) may be simply calculated to be: Iin=(2V-0.7V)/20kohm =~65uA.

    Of course, under such condition the op amp does not work linearly and its output will be at one of its rails (depends on polarity of 2V input).

  • 0 in reply to Marek Lis
    Expert 2520 points

    Hi Marek

    Thank you for your support.

    -Additional Questions-

    1.The increase in the input current is not a problem in the use of the product.

       If this is the case, can the op amp die?

    2. It is used only as a comparator, so there is no need for work linearly .

       Then there's no problem, right?

    3. What is the value of normal operating range as below?

    Thanks

  • 0 in reply to David Kim2
    TI__Guru 61340 points

    David,

    Please see my inline answers:

    1.The increase in the input current is not a problem in the use of the product.

       If this is the case, can the op amp die?  As long as the input differential voltage is less than 200V (10mA*20kohm), the part will NOT be damaged. 

    2. It is used only as a comparator, so there is no need for work linearly .

       Then there's no problem, right?       There may be a big problem - since TLV2333 has back-to-back parasitic diodes between the input terminals, when the input differential voltage between the inputs is greater than few hundreds millivolts, one of the input diodes will forward bias and disturb (drag) the threshold voltage resulting in a grossly erroneous tripping point  (depending on input signal transient speed the error may be in hundreds of mV).

    3. What is the value of normal operating range as below?

    The value of the normal operating range of Input Differential Voltage is defined as the input voltage offset (max +/-15uV) for as long as the feedback controls the diff voltage - you cannot force it. For example, shorting the two inputs together would result in 0V differential voltage BUT this would NOT be a normal operating range because the output would then try to be driven to max Vout = Vos*AOL =+/-15uV*(3.16e6) = +/-47V (where Vos is an internal offset and AOL is an open-loop gain with a typical value of 130dB (3.16e6); this means that with inputs shorted the output would most likely collapse on one of its rails.

    All in all, don't use TLV2333 as a comparator - use instead one of the comparators shown below: 

    https://www.ti.com/amplifier-circuit/comparators/products.html#p1typ=0.001;0.01&sort=p2max;asc

    But if you insist on using an op amp because of the need for very low offset,  consider OPA391, OPA392, OPA396, OPA397, OPA320, OPA191 or OPA196, which do NOT have back-to-back diodes between the input terminals.