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SN54AHC373: Input transition speeds

Sarah Taylor
Prodigy 20 points
Part Number: SN54AHC373
Other Parts Discussed in Thread: SN54HC373, LM139, SN54AC373,

Hi there,

I am looking to understand the use of the SN54HC373 octal latch as a replacement for the AHC version.

In the two datasheets, I can see the following:

AC datasheet states: "Minimum Input Edge Rate = 125mV/ns" (8ns/V). No maximum is provided.

AHC datasheet states: "Maximum transition rise or fall rate = 20 ns/V". No minimum is provided.

I am concerned about changing an existing circuit that implements the AHC octal latch, and replacing it with the AC logic family part.

The input to this latch is coming from the LM139 comparator output (without any hysteresis implemented), whose datasheet does not specify output transition speeds clearly.

I am concerned there is a risk of damage over long term use, or latch up (or something else!) if the AC is dropped in as a replacement. Is that a valid concern?

  • 0
    TI__Guru* 96862 points

    Hello,

    From the SN54AC373 datasheet, it shows a maximum of 8 ns/V for the input transition rate at 5V:

    From the SN54AHC373 datasheet, it shows a maximum of 20 ns/V for the input transition rate at 5V:

    -

    Since you're currently using the SN54AHC373 and are planning to switch to the SN54AC373, there could be some area of concern since the AHC device has a less restrictive specification.

    Considering you have a functional circuit already, could you just measure the output transition rate of the LM139 and use that for your determination?