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LM2903B: Input impedances.

Kengo.Y
Guru 11265 points
Part Number: LM2903B
Other Parts Discussed in Thread: LM2903

Hi team,

How much is input impedance?
Please let me know if you have a specific value.

I can see the below post.
But this answer is 100kΩ or more.
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/760906/how-can-i-determine-the-input-impedance-of-comparator/2812446?tisearch=e2e-sitesearch&keymatch=lm2903%2525252520impedance#2812446

Sincerely.
Kengo.

  • +1
    Guru 317550 points

    The slope of this curve is approximately 30 GΩ.


    (Figure 6-23)

  • +1
    Guru 140200 points

    Hi Kengo,

    no, this wasn't my answer Relaxed

    The answer is: "Way way more than 100k."

    From Clemens' figure you can deduce the worst case at 25°C which is dU / dI = 2V / 0.5nA = 4G.

    Kai

  • +1
    TI__Guru 61576 points

    Hello Kengo,

    I am not a fan of "Input Impedance". This spec is more suited for unbuffered instrumentation amplifiers.

    It is better to realize that there is up to 3.5nA coming OUT of the input towards ground. The input is a bipolar PNP, so there is a definite base current.

    So any resistance between the input and ground will get a positive voltage drop. So a 100k series resistance would generate +350uV across it.

    If you simulate your input circuit network separately, use a +3.5nA current source to simulate the comparator input instead of a paralleled Gigohm resistor. The results will be much more accurate. 

    The LM2903B models already have the current source on the inputs, so an external current source is not necessary.

    For more info, please see the LM2903 family appnote:

    Application Design Guidelines for LM339, LM393, TL331 Family Comparators

    Thanks again, Clemens and Kai!