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John Arg
John Arg
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If an IC with open-drain outputs (and hence pull-up resistors) is used, we of course put decoupling capacitors on the IC supply, but why don't we also add decoupling capacitors between the power pin of each pull-up and ground?

When an output switches high, current will flow from the supply through the wiring inductance, and through the pull-up. A decoupling capacitor here would reduce/eliminate the effect of the wiring inductance, leading to much cleaner switching, especially at higher frequencies, as well as lesser noise conducted from the switching output to the rest of the circuit. Yet I have never seen a data sheet or application note even recommend this.

Is it because the decoupling capacitor for the IC will be enough to provide for the outputs too? If this is the case, when laying out a PCB, this would mean all pull-ups have to be placed as physically close as possible to the decoupling cap right?

enter image description here

If an IC with open-drain outputs (and hence pull-up resistors) is used, we of course put decoupling capacitors on the IC supply, but why don't we also add decoupling capacitors between the power pin of each pull-up and ground?

When an output switches high, current will flow from the supply through the wiring inductance, and through the pull-up. A decoupling capacitor here would reduce/eliminate the effect of the wiring inductance, leading to much cleaner switching, especially at higher frequencies, as well as lesser noise conducted from the switching output to the rest of the circuit. Yet I have never seen a data sheet or application note even recommend this.

Is it because the decoupling capacitor for the IC will be enough to provide for the outputs too? If this is the case, when laying out a PCB, this would mean all pull-ups have to be placed as physically close as possible to the decoupling cap right?

If an IC with open-drain outputs (and hence pull-up resistors) is used, we of course put decoupling capacitors on the IC supply, but why don't we also add decoupling capacitors between the power pin of each pull-up and ground?

When an output switches high, current will flow from the supply through the wiring inductance, and through the pull-up. A decoupling capacitor here would reduce/eliminate the effect of the wiring inductance, leading to much cleaner switching, especially at higher frequencies, as well as lesser noise conducted from the switching output to the rest of the circuit. Yet I have never seen a data sheet or application note even recommend this.

Is it because the decoupling capacitor for the IC will be enough to provide for the outputs too? If this is the case, when laying out a PCB, this would mean all pull-ups have to be placed as physically close as possible to the decoupling cap right?

enter image description here

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John Arg
John Arg
  • 269
  • 2
  • 7

Why don't we use decoupling capacitors for open-drain outputs?

If an IC with open-drain outputs (and hence pull-up resistors) is used, we of course put decoupling capacitors on the IC supply, but why don't we also add decoupling capacitors between the power pin of each pull-up and ground?

When an output switches high, current will flow from the supply through the wiring inductance, and through the pull-up. A decoupling capacitor here would reduce/eliminate the effect of the wiring inductance, leading to much cleaner switching, especially at higher frequencies, as well as lesser noise conducted from the switching output to the rest of the circuit. Yet I have never seen a data sheet or application note even recommend this.

Is it because the decoupling capacitor for the IC will be enough to provide for the outputs too? If this is the case, when laying out a PCB, this would mean all pull-ups have to be placed as physically close as possible to the decoupling cap right?