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Schmitt triggers on SN74AHC1G00 inputs

Vyacheslav Prokopiy
Expert 1305 points
Other Parts Discussed in Thread: SN74AHC1G00, SN74AHC132, SN74LVC1G98

Hello!

On SN74AHC1G00  product page http://www.ti.com/product/SN74AHC1G00/description

specified, that 

" Features ....... Schmitt Trigger Action at All Inputs Makes the Circuit Tolerant for Slower Input Rise and Fall Time"

 But in comparison table we can see next statement: "Schmitt Trigger  - No"

 What is the truth?

Thank you.

  • 0
    TI__Guru* 96681 points

    Hi Vyacheslav,

    The '00' logic function is a standard dual input NAND gate -- the '132' function is a dual input NAND gate with ST inputs.

    The SN74AHC1G00 does _not_ have Schmitt Trigger inputs.  The SN74AHC132 does.

    The easiest way to tell if a part has real ST inputs is to check the datasheet and look for V_IH and V_IL specs vs V+ and V- specs -- ST parts will have V+ and V- specified for any ST inputs, and normal inputs will only have V_IH and V_IL.

    Can you tell me a bit about your requirements?  I might be able to find a better part to match your needs.

  • 0
    TI__Guru 56990 points
    Hi Vyacheslav,

    Usually all the CMOS have a built in hysteresis for noise immunity . Compared to the AUP / LVC family , the AHC are little more tolerant on the slower signal as seen from the recommended conditions being 100ns/V for the input transition time . The description says "ST action " which is different from ST inputs with Vthreshold in the specs . Also , true ST devices says them in the device description .
  • 0
    Guru 263190 points

    The SN74AHC132 has four gates. If you want to save space, or need a logic gate for which a Schmitt trigger version does not happen to exist, consider using one of the configurable multi-function gates ('57, '58, '97, '98, '99), available in the AUP and LVC families; all of them have Schmitt trigger inputs. In your case, you could use the SN74LVC1G98.

  • 0 in reply to ShreyasRao
    Expert 1305 points
    Hello, Shreyas. Thank you for answer.
    And if it goes not about true triggers, but about "ST action" what are the threshholds for switching hysteresis in this case?
    What are V_IH and V_IL for this "ST action"? Which limits can I take to my project calculation?
  • 0 in reply to Vyacheslav Prokopiy
    Guru 263190 points

    See section 2 (p. 17) of the AHC Designer's Guide:

    Transistors Q1 and Q2 are chosen to have the same transfer characteristics to ensure that the switching threshold of a circuit of this kind is at 50% of VCC.
    […]
    The switching thresholds on the positive and negative edges at the input differ by about 200 mV and the input circuit has the hysteresis characteristic that is typical of Schmitt-trigger circuits.

    Please note that these values are typical, and that there are no guaranteed limits. If you want to rely on anything, you need to use 'real' Schmitt-trigger devices, where those limits are guaranteed:

    Hysteresis in the input circuit is intended only to process reliably signals that have a slew rate of <10 ns/V. With a signal swing of 5 V, this corresponds to rise and fall times of about 50 ns. If signals with considerably longer rise and fall times are processed, the specially developed Schmitt triggers, such as the SN74AHC(T)14, should be used. These components have a considerably larger hysteresis of about 800 mV at VCC = 5 V.
  • 0 in reply to Clemens Ladisch
    TI__Guru* 96681 points

    Thanks for your post, Clemens!