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SN74AHC14 Input voltage limits

GURU B
Prodigy 60 points
Other Parts Discussed in Thread: SN74AHC14

Hello,

I was testing a old control board and observed that, SN74AHC14 is seeing an input of +/- 7.5V. -7.5V is being clamped to -0.7. And 7.5V input is read as is. SN74AHC14 is supplied from 3.3V supply rail. 

The device is working fine but the input voltage is above the specs as per datasheet

Is there a clamping diode for -Ve input voltages ?

And What is the absolute maximum input voltage on this device before it gets damaged ?

I am modifying this circuit in new board but I am wondering how this device is not damaged in the older board.

  • 0
    TI__Mastermind 46480 points

    for AHC There is a clamp diode to gnd wich will clamp the voltage as long as the current is limited less than 20ma. There is no clamp diode to Vcc on the input.  There is usually a parasitic that will clamp the voltage around 7.5V. It is not recomended to go beyond Vcc

     HC will have clamp diodes to vcc and gnd on the inputs and can be used as a clamp as long as the current is limited.