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SN7407: Input voltage even with pulldown?

David Daminelli
Prodigy 80 points
Part Number: SN7407
Other Parts Discussed in Thread: SN7404, SN74LS07, SN74LVC07A,

Hello,

Im having an issue with the SN7404 IC. The image bellow shows how it is designed. The R7 Resistor Array is used to pull down the 7407 input, so when the CN7 connector is disconnected, the 7407 should stay low. But, when this circuit is powered up, I am measuring 1.4V on the input pins. Is this normal? 

  • +1
    Guru 243220 points

    TTL inputs can source up to 1.6 mA when low (see IIL in the electrical characteristics table). To prevent the input from going above 0.8 V, this requires a pull-down resistor of no more than 500 Ω.

    Why did you choose a TTL device? If you use the SN74LS07, you can use up to 4 kΩ. Or just use a CMOS device with high-impedance inputs like the SN74LVC07A (which is also cheaper).

  • 0
    TI__Guru* 96591 points

    Hi David,

    Just to add to what Clemens said already, the SN7407 uses 6k resistors internally as part of the input circuit, and that's going to cause contention with your 10k resistors.

    Suffice it to say that the input isn't high impedance like what you see with a modern CMOS device. I agree with his assessment.