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SN74HC08: Redesign SN74HC08D to SN74AHC08D

Wouter Kragt
Intellectual 1820 points
Part Number: SN74HC08

Hello team,

In a redesign a HC logic needs to be replaced by AHC logic.

Namely, the SN74HC08D has to be replaced by SN74AHC08D.

For this, I have the following concerns:

  • Are all advanced high-speed CMOS devices using the same ESD input protective circuits (incl. the current limit resistor, please see the attached figure)?
  •  Are there any technical risks we should take into account (e.g. EMI) ?

  •  Is there anything else where you should be aware of in such a redesign?

 

 

Thank you.

 

Kind regards,

Wouter Kragt

  • 0
    TI__Guru* 97046 points

    Hi Wouter,

    Wouter Kragt said:

    In a redesign a HC logic needs to be replaced by AHC logic.

    Namely, the SN74HC08D has to be replaced by SN74AHC08D.

    Why does the HC part need to be replaced by the AHC part? What issue are they solving?

    --

    The HC family has both positive and negative clamp diodes, while the AHC family only has the negative clamp diodes - ie V_in can exceed V_cc for AHC devices, but not HC devices. If the HC device was working in a socket, I would expect the AHC to also work in 99.9% of cases.

    I would never count on that internal resistor for anything - those are built on silicon and thus have a large range of variation. They are intended as ballast for the ESD discharge circuitry only (ie reducing current for 2000V transients).

    We always recommend adding an external resistor if the clamp diode is going to be activated (ie if V_in < -0.5V). Usually this can be a very small value (100 ohms or less), and often is zero because most systems don't have such significant undershoot.

    The AHC devices do have stronger output drivers than the HC family devices (8mA vs 4mA at 4.5V). Being twice as strong, this will result in outputs that rise twice as fast and a higher bandwidth on the output signal. It also means the AHC parts have less delay than HC family - typically not an issue, but if their system had close timings originally, the change in part can cause issues (HC part is 10ns typical / 25ns max, while AHC part is 7ns max).