18
\$\begingroup\$

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.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
  • \$\begingroup\$ Your good question does not have a definative answer . \$\endgroup\$ – Autistic Jan 30 '16 at 7:35
18
\$\begingroup\$

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.

schematic

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.

\$\endgroup\$
  • 1
    \$\begingroup\$ Note, though, that this only applies if the signal is referenced to V- rather than ground. \$\endgroup\$ – WhatRoughBeast Aug 28 '15 at 19:36
  • \$\begingroup\$ Or V+, of course. \$\endgroup\$ – Spehro Pefhany Aug 28 '15 at 19:39
11
\$\begingroup\$

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.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.