Do op amps need one bypass capacitor or two?

Question

As with other ICs, it's standard practice to place bypass capacitors near the supply voltage pins of op amps. But I've seen conflicting opinions on how to properly bypass an op amp (here, for example). Some people suggest putting a single capacitor between the V+ and V- pins. Others suggest using two capacitors, one from V+ to ground and one from V- to ground. Which of these methods gives the best result? I'm using OPA827s as unity-gain voltage followers for audio signals, but I'd like to know whether the answer's the same for other situations as well.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

If the output load is primarily to ground, then two capacitors. If it's to either supply, then one capacitor will suffice.

The purpose of bypass capacitors is to provide a low-impedance close to the chip (bypassing any series inductance to the supply rails). Since most op-amps do not have a ground pin the internal circuitry does not care about the ground level, however when you apply a load to ground the current path is from the positive or negative supply, through the circuitry on the chip, out the output and through the load to ground. A capacitor from the positive and negative supplies will make sure that loop is physically small and thus low inductance.

^{simulate this circuit – Schematic created using CircuitLab}

The left two schematics show an op-amp driving positive and negative current into a load connected to ground, and how the bypass capacitor appears in the loop. The right hand one shows a load connected to the negative rail. The capacitor in the right hand schematic is twice as effective with the load connected this way (1uF rather than 500nF) and it saves a part.

Answer 2

One capacitor for each supply, connected to ground.

Consider that, if you have dual supplies, and each supply produces a positive spike of equal amplitude, a capacitor between the two supplies will see a constant voltage, and will attenuate neither spike.