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Input Impedance of SN74LVC1G175

ytan
Intellectual 325 points
Other Parts Discussed in Thread: SN74LVC1G175

Hi, 

What is the input impedance of the CLR\ pin on SN74LVC1G175?

 

Can I drive this pin directly from the output of and AND gate?

 

I am having problem where the output Q will go low before the CLR\ pin drops to 0.3Vcc. 

Thanks!

  • 0
    TI__Mastermind 46480 points

    These are CMOS inputs so they would be high impedance. greater than 1Mohm.

    It can be driven by and AND gate.

  • 0 in reply to Chris Cockrill
    Prodigy 130 points

    Hi Chris,

    I am using this part as described in my design. I have the output of an AND gate directly connected to the /CLR input. What I noticed happening is the output of the flip-flop going low with very small changes on the /CLR input. The datasheet says that the low input at 5V Vcc should be 0.3 x Vcc (1.5V). I monitored the /CLR input and flip-flop output with a scope and I found that the output would go low before I could even notice a drop on the /CLR input. I decided to add some series resistance between the AND gate output and the /CLR input to see if it would fix the problem. With 1K series resistance the output would go low when /CLR dropped to about 4.0V. With 10K series resistance the output would go low when /CLR dropped to about 2.5V. This is still not 1.5V as the datasheet says, but it was improving.

    I am not understanding the reason for this behavior. I just recently noticed in the datasheet that there is a dt/dV limit of 10ns/V on any input. Could this be the cause of the problem? With a 1K series resistance I noticed a dt/dV of about 8ns/V, and with a 10K resistance I noticed a dt/dV of about 25ns/V. Would exceeding the dt/dV rating of the part cause the output to change prematurely?

    Thanks for any help you can provide.

     

    Matt

  • 0 in reply to Matthew Graney
    TI__Mastermind 46480 points

    Hi Matthew

       I dont have any answers here. The part was tested to datasheet Vih and Vil so it should work here.  Slow rise and fall on the inputs can cause problems i really dont think 25ns/V would cause any problems at romm temp. Could any of the other inputs be causing the problem. 

  • 0 in reply to Chris Cockrill
    Prodigy 130 points

    The other inputs are not changing state. I was able to fix the problem by adding 10K series resistance and a 10nF cap to GND on the /CLR input pin. I am not sure why this fixed it, but it seems that the /CLR input is extremely sensitive to noise. It is still triggering at 2.5V, not 1.5V like the datasheet says. Maybe some of these parts are out of spec?