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SN74ACT244: PWM Generator

Shibin Varghese
Prodigy 185 points
Part Number: SN74ACT244
Other Parts Discussed in Thread: DAC80508

Hi Team,
I would like to generate a sine signal that swings between 0V and +5V.
The frequency is around 6.8KHz. Can I generate such a signal keeping the nOE pin low and providing PWM at the input pins.
My controller providing the PWM is a 3.3 V logic.
Also, If I need to bypass the SN74ACT244 with another signal generator like DAC or something, do I need to break the connection at the output or just change the state of the nOE pin to logic high?.
My doubt is does the buffer will have any effect on my system if I bypass the SN74ACT244 keeping the nOE pin high. Or do I need to connect the output of the SN74ACT244 to my system through a zero-ohm resistor and remove the resistor when bypassing the SN74ACT244?.

  • +1
    TI__Mastermind 46095 points

    Hey Shibin,

    This device is a digital buffer which means the output is going to be a square wave. Of course you can filter out the higher frequencies and it will become sinusoidal, is this what you are looking for? 

    When the device is disabled, it shouldn't load down signals that may be connected to the same node. The data sheet provides the Ioz spec that indicates the low leakage.

  • 0 in reply to Dylan Hubbard
    Prodigy 185 points

    Hi Dylan
    Thank you for your reply.
    I am using DAC80508 to generate a sine wave.
    I'll use only one of the devices at a time. I'll bypass any one of the signal generators at a time. The signal generated from these devices is fed to a common circuit.
    When the DAC is working (SN74ACT244 is disabled by setting the nOE pin high), the loading effect on the signal due to the SN74ACT244 will be very low like in the order of (Ioz=2.5uA).
    Will the DAC create any loading problem on my signal when my SN74ACT244 is working?.

  • +1 in reply to Shibin Varghese
    TI__Guru* 97046 points

    As long as the DAC is in a high-impedance state then the SN74ACT244 won't have any issues. If you're not sure about how the DAC works, I'd recommend opening a new post under the DAC part # to get the correct experts to help you out (note that they probably won't respond until Monday). Dylan and I are experts on the logic portfolio, including the SN74ACT244.

  • 0 in reply to Emrys Maier
    Prodigy 185 points

    Hi Emrys Maler,
    Thank you for your reply.
    The output of the DAC is something like this.

    Can I expect such an output signal from the SN74ACT244 for different PWM inputs?.
    A Low Pass Filter will be used at the output to get a neat sine wave.

  • 0 in reply to Shibin Varghese
    TI__Guru* 97046 points

    Hi Shibin,

    The SN74ACT244 is a digital buffer - it will output a digital signal (high or low) based on the input -- if you input PWM, you will get out PWM.

    A low-pass filter can only do so much for a square wave. Here's an example I put together for a 1 MHz square wave being put into a 1 MHz corner frequency low-pass filter (first order):

    While it does eliminate a great deal of the higher-order harmonics, it's definitely not a sine wave.