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SN74AC14: INTERNAL CLAMPING DIODE ?

TEK4EDU Italy
Prodigy 40 points
Part Number: SN74AC14
Other Parts Discussed in Thread: CD74AC14, SN74ACT14, , LM393

Dears
I want use SN74AC14 or similar CD74AC14 or SN74ACT14 to convert analog signal to digital signal.

To do this, I need to know if they have or not internal clamping diodes t?
If yes, clear, I must use an external in series resistor (i.e. 10k) before input pin to limit the currens
So I can fed a input signal with amplitude more than the +5V (suppose it is my power supply) and obtain the conversion.

I'm sure the 74HC14D have internal clamping diodes but I'm nor sure for these ICs.
I want use AC instead of HC because more performing.

Thanks
Gigi

  • 0
    Guru 316510 points
    The presence of a diode is implied by the allowed input voltage range going only to Vcc, and a specified limit input clamp current for the case Vi > Vcc. You can find this in the AC or HC families, but not in AHC, ALVC, LVC, or LV-A families.

    Why don't you just use a comparator?
  • 0 in reply to Clemens Ladisch
    Prodigy 40 points
    Dear Clemens. I remember old application with schmitt trigger. And also because I suppose schmitt trigger introduce less jitter then comparator.
    I must verify deepen..... Do you have some doc or link reference about your info? Thanks .
  • 0 in reply to TEK4EDU Italy
    Guru 316510 points
    The most general implementation of a Schmitt trigger is with a comparator (with positive feedback).

    The threshold of logic Schmitt triggers is not specified very precisely.
  • 0 in reply to Clemens Ladisch
    Prodigy 40 points
    Yes Clements about "comparator (with positive feedback)" but I'm not sure about the reached performance and the circuit complexity of a comparator scheme.
    About threshold: true but this is not a problem for me.
    I'm testing in this daies, a circuit using CD74AC14E DIP14 (6 Schmitt-Triggered Inverters) with 10k input. I use the other inverters to buffer or invert other digital signals and obtain same phase between them. This scheme is very simple and need only 1 resistor. Furthermore CD74AC14E is very cheap. It operate correctly from 1Hz to 4-5MHz (using a vulgar breadboard and many air wires!!!): I suppose to improve the performance with a more serius pcb and SMD technology. No visible jitter and I can drive directly 100Ohm load (50Ohm in series as out impedance and 50Ohm ext load).
    If I use comparator IC as LM393, I need more resistors, I must buffer the output to drive 100Ohm load and its switching time is around 1 microSec instead of about 10ns of CD74AC14 (100MHz).
    Probably better performance I can obtain using a high performance OpAmp as LM6181 (100MHz, also it drives low out load ): its performance are similar at CD74AC14 but it is very expensive (10 times CD74AC14) and only 2 device instead of 6.
    So I think there are many waies to reach same target with different initial considerations.
    Anyway your comments and suggestions were very interesting for me to reason with my brain.
    Thanks again and have a nice day
    Gigi