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SN74LVC1404: Schmitt Trigger doesn't seems working

Danny Chen1
Prodigy 20 points

Part Number: SN74LVC1404

Hello

I have connected the SN74LVC1404 with a 8MHz crystal following the instructions from the datasheet. I played around SN74LVC1404 would clear some of the jitters that I found with SN74LVCGX04, where the output do not have a schmitt trigger.

However here are the results:

The upper signal (yellow, ch1) is from OSCOUT on SN74LVC1404 and the lower signal (blue, ch2) is from Y on SN74LVC1404.

My question being why doesn't the schmitt trigger help cleaning up the signal but have a more severe jitter? Vcc is connected to 3.3V of benchtop power supply.

Thanks
Danny

  • 0
    TI__Guru* 97666 points

    Hi Danny, and welcome to the forums!

    I don't see a jitter measurement on your scope -- are you talking about the ringing? The Schmitt-trigger buffer in the '1404 is intended to square up the edges of the signal, which it does very well. It also increases the drive strength, which will cause ringing into a light load.

    As you can see on your yellow waveform, the output of the unbuffered inverter has relatively slow edges - more like ramps than vertical lines. This is to be expected from a low gain inverter (single stage), which is necessary for a crystal driver. The Schmitt-trigger inverter has much faster edges, and thus has more overshoot/undershoot.  This is caused by parasitic inductance in the system and can be smoothed out by loading the line with additional capacitance -- although it's not really necessary.