I am designing a small device with two MAX485 chips.

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Do they need the decoupling capacitors on the 5V power?

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    \$\begingroup\$ This really depends on the impedance and inductance of your supply to the chip; a decoupling capacitor is generally best practice, generally has low cost/complexity for assembly, and shouldn't hurt. \$\endgroup\$ – Reinstate Monica - ζ-- Jan 29 '19 at 23:41
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    \$\begingroup\$ As a general rule ALL IC's should have ample decoupling capacitors. \$\endgroup\$ – Sparky256 Jan 29 '19 at 23:46
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    \$\begingroup\$ The bypass cap are almost universally used with the purpose of minimising the power supply impedance. \$\endgroup\$ – pantarhei Jan 29 '19 at 23:49
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    \$\begingroup\$ Sometimes you can share decoupling caps if you know the feed inductance (e.g. 10nH/cm * length * 2), current rise time and tolerance for LdI/dt=V drop \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jan 30 '19 at 1:58

Yes. All almost ICs need decoupling capacitors. Devices such as these '485 drivers especially need them due to the current surges the device experiences when switching the signal states to the low value termination resistors used on the bus lines.

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    \$\begingroup\$ You could debate whether all ICs strictly need decoupling caps, but it never hurts to add some. \$\endgroup\$ – Hearth Jan 30 '19 at 0:04
  • \$\begingroup\$ @Hearth this was my question. If it does not hurt \$\endgroup\$ – P__J__ Jan 30 '19 at 0:22

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