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SN74LVC1G14: Distortion in output waveform

Ahmet Inan
Intellectual 450 points
Part Number: SN74LVC1G14
Other Parts Discussed in Thread: SN74LV8154, SN74LVC1G17, LMV7219, SN74LV4040A

Tool/software:

Hi TI Technical Support Team,

I am designing a PCB which will be used to count/measure the frequency. The measured frequency will be proportional to the concentration of the parameter. The frequency range can be between 1MHz and 50MHz. I am planning to use SN74LV8154 as 32 bit counter to count the edges of the signal. My input signal is noisy sine wave.

In my initial test setup, I used SN74LVC1G14DBVR as Schmitt Trigger to convert sine to square wave, such that I can count edges of the square wave to measure the frequency. I used 16MHz crystal oscillator as reference signal. The output of SN74LVC1G14DBVR is given below.  I think when the frequency of the input sine wave will be near 50MHz, the waveform will be even worse. Could you please suggest me proper schmitt trigger  IC (or maybe another method) so that I can convert sine wave (max frequency about 50MHz) to square wave?

Thank you for your help,

Ahmet

 

  • +1
    Guru 316470 points

    LVC devices can do 50 MHz. The ringing is probably caused by the oscilloscope ground clip (use a ground spring instead), or by the line being unterminated. A short trace between the buffer and the counter would not need termination; please show the board layout.

    If the input signal does not reach logic levels, or if the switching threshold specifications of the SN74LVC1G17 are too loose for you, use a fast comparator like the LMV7219.

    The SN74LV8154 is not guaranteed to support more than 40 MHz. The SN74LV4040A supports higher frequencies, but is asynchronous.

  • 0 in reply to Clemens Ladisch
    Intellectual 450 points

    Hi Clemens,

    Thank you for your answer.

    In my test setup, the crystal was on breadboard and I connected it to schmit triger circuit (on another pcb) using jumper cables. The line was unterminated, and the distance from cristal pin to IC was not short  (~20cm cable). As you said, maybe these are reasons for ringing.

    The layout of my final design is given below. This is demo board with 4 layers. Mid layers are ground planes. This PCB is connected to another PCB that has MCU. I will produce the PCB and make the tests again.

    Regards,

    Ahmet