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SN74AC04: wrong input impedance in the spice model

Clemens Ladisch
Guru 243180 points
Part Number: SN74AC04
Other Parts Discussed in Thread: SN74HC04

In the simulation, the SN74AC04's inputs sink quite a lot of current.

SN74‍AC04.cir has:

.PARAM RA = 2.4e-05 
...
RA  A  GND {RA}

Is this device really supposed to have an input impedance of 24 µΩ? (The SN74HC04 has a more plausible 240 MΩ.)

  • 0
    TI__Guru* 96591 points

    Hey Clemens,

    Thanks for bringing this to my attention. You're right - this is certainly not the intended input impedance for the AC family of logic. I looked at a few other devices in the AC family and it seems that they all have 2.4 Gohm input resistance, which I would also consider to be incorrect.

    I would recommend to use 55 Mohm, which would represent the worst-case 25C value from the datasheet.

    I'll have a talk with our modelling team and see what we can do to get those updated as soon as possible.  The way our updating system works, it might be a while before the new models make it to the web.