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If I want to connect the output of a logic gate to the input of another logic gate, do I have to use a current limiting resistor? I would like to connect two NANDs (CD4011) in series and then again with the clock of a flip-flop (CD4013.) If the inputs of a CMOS gate has high impedance I suppose it is safe not to use resistors, isn't it?

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  • \$\begingroup\$ what have you tried? ... did it fail? \$\endgroup\$
    – jsotola
    Jan 15, 2023 at 18:28
  • \$\begingroup\$ refer to the datasheet ... what are the output voltage levels? ... what are the allowed input voltage levels? \$\endgroup\$
    – jsotola
    Jan 15, 2023 at 18:31
  • \$\begingroup\$ I don't want to try and burn my gates... \$\endgroup\$ Jan 15, 2023 at 18:35
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    \$\begingroup\$ Output current on this old logic gates is already very low (1-2 mA is quoted in the datasheet when driven at 5V), so the gates already have more resistance in them then you would typically be adding with a resistor. \$\endgroup\$ Jan 15, 2023 at 18:38
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    \$\begingroup\$ The fact that the inputs have a high impedance means that you MUST have well defined logic voltages at ALL inputs, whether used or not. This means an input must be connected to another logic output, or to rail, or to ground. You can also use a pullup or pulldown resistor to an input that you don't use now, but may want to use later, or that you sometimes disconnect the drive to. Leaving an input, any input, floating is a great way to either burn your chip, or to introduce wierd behaviour that's difficult to fault find. And no, resistors are not normally used output to input. But they can be. \$\endgroup\$
    – Neil_UK
    Jan 15, 2023 at 18:49

2 Answers 2

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Should I use a current limiting resistor

No. As long as the two gates are powered at the same voltage, a resistor won't have any benefits, and may actually be disadvantageous as it will slow down the signal.

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    \$\begingroup\$ Such a slow down sometimes can be useful or wanted, for instance helps getting rid of noise, if there is any. Decreasing the slew-rate might also reduce the noise emitted, but if this relevant on CMOS is another question... \$\endgroup\$
    – datenheim
    Jan 15, 2023 at 18:46
  • \$\begingroup\$ @datenheim My circuit works with DC levels. That means there is no concern for EMI ? \$\endgroup\$ Jan 15, 2023 at 18:53
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    \$\begingroup\$ Correct: no concern. \$\endgroup\$ Jan 15, 2023 at 18:56
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You could limit the signal rise time by adding a resistor that would effectively form a RC circuit with the input capacitance. But with short distances and only occasional changes in signal it's unlikely you would be experiencing EMC issues and therefore would not need the series resistors. Adding one won't damage the gates, but will slow operations depending on RC time constant.

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  • \$\begingroup\$ So, excluding EMI there is no other need for resistors between gates powered from the same supply voltage ? \$\endgroup\$ Jan 15, 2023 at 18:55
  • \$\begingroup\$ Yes. Series resistors could also be used for enhancing ESD protection if the signal was exposed to ESD, but from gate to gate that's not the case. \$\endgroup\$
    – Ralph
    Jan 15, 2023 at 19:11

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