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SN74AHC1G125: High impedance

Longchang Chen
Intellectual 1350 points
Part Number: SN74AHC1G125

Hi

 Customer is using SN74AHC1G125, when OE pull-up to 3.3V  by resistor , according to true table, now output Pin4 should be high impedance status,  then  customer pull-up pin4 by 10k resistor to 3.3V, but some chips it is not 3.3V at Pin4, it is 1.5V, 2.5V, etc , so if customer want to pull-up pin4 to 3.3V when OE is high, how to do it?  how many ohm is between input and output when OE is high?

  • 0
    Guru 278840 points
    So I guess VCC is 3.3 V?

    When the output is disabled, the voltage of pin 4 is determined only by the pull-up resistor. If the measured voltage is lower, then there must be some current flowing somewhere; either some other part of the circuit is pulling the line down, or the device is broken, or the measurement was done incorrectly.
  • 0 in reply to Clemens Ladisch
    TI__Mastermind 24805 points

    Hello,

    I agree with Clemens, if you are trying to pull up the line to 3.3V, then a 10k should be fine, as the leakage into the output pin is very low, so the 10k pullup should drive the line high.

    As Clemens mentioned, is there something else in the system drawing current? Is the 3.3V driving the input of something else? 

    Best,
    Michael

  • 0 in reply to Michael J Schultis
    TI__Mastermind 24805 points

    Follow Up Question: How do I determine the size of the pullup resistor?

    Hello,

    This is dependent upon a few things: the output loading conditions, the output leakage from the 3-state device, and the acceptable VOH level desired.

    Here are some equations to summarize:

    Choose your desired VOH, and based on the Iout, you can determine what your pullup is.

    Best,
    Michael