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SN74AHCT125: Driving Optocoupler SN74AHCT125

Thilesh De Silva
Intellectual 390 points
Part Number: SN74AHCT125
Other Parts Discussed in Thread: SN74LVC1G17
Hello and good day to all of you,
I'm Thilesh De Silva a research assistant in University of Peradeniya Sri Lanka. I'm Using SN74AHCT125N for one of my application circuits. Please find the attached document which indicates the circuit diagram of a fabricated PCB (1st attachment) and the Oscilloscope output images (2nd and 3rd attachments).
When I turned on both the Switches (SW1 and SW2) the output is indicated in the blue color graph in image 01. Since the output of the SN74 is Oscillating at the beginning, The output of the Opto-coupler is also oscillating. (600 Ohm resistor is added as a damping resistor.)
If I turned on only SW1 and turned off SW2 the output is indicated in the blue color graph in image 02. Same thing happens If I turned on SW2 and turned off SW1. In this situation the Output of the Opto-coupler is acceptable. But I need to 2 circuits to be functioning.
all the enable pins of the SN74 device are grounded and Input Capacitance of the Opto-coupler is 50pF at 1MHz Input and the forward current of the LED is 7.5mA
Please help me solve this issue and I'm really grateful to all of you
Thank you and best regards,
Thilesh De Silva
  • 0
    TI__Mastermind 44630 points
    Hi Thilesh,

    Oscillation like this usually occur do to slow or noisy inputs into the Logic device. Please see the application report below for more info on this.

    www.ti.com/.../scba004d.pdf

    I'm not sure if this is the case since I don't you aren't showing me the input signal. If CH 1 is showing the input then that is definitely too slow.

    You could also have some bouncing if it is a mechanical switch, in which you would want to have a debounce circuit to alleviate this. Unfortunately, adding this circuit will inherently slow down the input as well. I will suggest a device that potentially solve all these issues. You can use the SN74LVC1G17 which is a single channel ( I noticed you only use a single channel on the device) buffer with Schmitt trigger inputs. This device will allow you to have a slow and noisy input without oscillating at the output. It will also allow you to add a debounce circuit if necessary.
  • 0 in reply to Dylan Hubbard
    Intellectual 390 points
    Hi Dylan

    First of all thank you very much for replying me and please forgive me for the delayed reply. i saw the oscillations at the input signal when i turned on both the circuits which was not seen in the single circuit operation. i added a small ceramic capacitor, 68pF between the input pin of the SN74AHCT125N and the ground. Then the output from the two opto-couplers were fine. So I can move forward with that modification. Thanks again

    best regards,
    Thilesh De Silva