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I have seen circuit diagram in analog devices application note which shows that both supply rails have a 1uF and 0.1uF capacitor for dual supply opamp but only the Vcc has them in a single supply opamp.

Why are decoupling capacitors used on both supply rails in dual supply opamp?

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    \$\begingroup\$ For a single supply opamp, the -ve supply is already ground. Where would you decouple it to? \$\endgroup\$ – Roger Rowland Aug 4 '16 at 8:21
  • \$\begingroup\$ Note that, as far as I know, there are benefits to both configurations. You sometimes see dual-supply circuits where VCC is bypassed to VEE. I think it depends a bit on what the opamp is driving. \$\endgroup\$ – pipe Aug 4 '16 at 8:51
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The supply current for both supplies are different most of the time. Only if the output of the opamp with dual supply is at ground potential and the output current is zero both supply currents are (almost) equal. If the ouput jumps positive, there will be a short increase of the positive supply current and a decoupling capacitor is needed for positive supply. If the output jumps negative, the other decoupling capacitor is needed. If the output voltage of the opamp changes dynamically, both decoupling condensators are necessary. If there are no decoupling condensators, the opamp may oscillate in some cases. Decoupling capacitors are especially important for fast opamps. Both capacitors should have short connections to the supply terminals of the opamp and to ground.

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  • \$\begingroup\$ +1. BTW, The correct term in English is "capacitor". An older term was "condenser", but it is completely obsolete except in the expression "condenser mic" sometimes found in relation with microphones. "condensator" is not an English word (interference with German, maybe?). \$\endgroup\$ – Lorenzo Donati Aug 4 '16 at 9:27
  • \$\begingroup\$ Why not use vdd to vss decoupling rather than decouple both to ground using capacitors? \$\endgroup\$ – quantum231 Aug 4 '16 at 11:18
  • \$\begingroup\$ If you use vdd to vss decoupling, what happens if the opamp needs more current from positive supply for a short time? The only capacitor from vdd to vss could not help against a drop of positive supply voltage at the opamps terminal. \$\endgroup\$ – Uwe Aug 4 '16 at 11:28
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The reason is that all supply rails have decoupling to ground, whether there is only one rail, or several.

If it's a supply rail, decouple it to ground.

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