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  #1  
Old 03-05-2009, 10:16 AM
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Default How to use Decoupling capacitors for Op amp in dual rail

Hello

I have op amp OP07 and TL084 in a board. Some 2to 30 in a board with a MCU onboard.

How to provide the decoupling capacitors for these op amps. +12 and -12 are supply rails?

We need to decouple both rails or one is needed? How the the optimum value is decided?

Thanks
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Old 03-05-2009, 10:28 AM
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Put a 100nF capacitor between the +ve rail and ground, another one between -ve rail and ground and one between +ve and -ve rails.
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Old 03-05-2009, 11:31 AM
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Put a 100nF capacitor between the +ve rail and ground, another one between -ve rail and ground and one between +ve and -ve rails.
So three non polar capacitors .1uF for each op amp. I have some 30 op amp in a PCB. So huge part count.

What is really the function of de copupling capacitors in Op amp circuits? What are the consequences in general if not provided?

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Old 03-05-2009, 12:38 PM
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Any IC consume power depending on the output load condition (drive power). So when load curent increases suddenly, a voltage dip may occure at the power pins of IC. During that time, decoupling capacitor provides the enough charge not to have voltage dip.

Depending on freqency of operation (analog, digital (in KHz, MHz)) value can be decided like 0.001µF, 0.01µF, 0.1µF etc.
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Old 03-05-2009, 03:00 PM
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Well, actually - just two 100nF nonpolarized caps per opamp.
One from Vcc to ground, and one from Vee to ground, as close to the supply pins as you can.
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Old 03-05-2009, 10:29 PM
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Well, with op amps, if you have a good low noise power supply you don't really need decoupling capacitors because they don't switch rapidly and cause noise on the pcb tracks.
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Old 02-09-2012, 09:04 PM
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Why the extra between V+ and V-? Is this in case of a fast transition between upper and lower voltage rails with a signal carrying a higher load?
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Old 02-09-2012, 09:53 PM
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Quote:
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Well, with op amps, if you have a good low noise power supply you don't really need decoupling capacitors because they don't switch rapidly and cause noise on the pcb tracks.
Not entirely true. The decoupling is not to prevent noise on the power rails, it's to stabilize the op amp. Some op amps (especially higher frequency ones) will become unstable or show deviations in their frequency response if their power rails are not properly decoupled.
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Old 02-09-2012, 09:57 PM
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Why the extra between V+ and V-? Is this in case of a fast transition between upper and lower voltage rails with a signal carrying a higher load?
If the power rails are properly decoupled to ground, then you shouldn't need the third cap. The power rail AC currents are from each rail to ground through the output load (plus or minus rail depending upon whether the output polarity is positive or negative), so the primary currents to be decoupled are from each of the rails to ground, not between rails.
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Old 02-09-2012, 11:00 PM
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Caps are damn cheap. Add then every place (even that third cap) if you are laying out a PC board. If you find you don't need them all you just don't populate some spots.

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If the power rails are properly decoupled to ground, then you shouldn't need the third cap.
I've never had cause to need that third cap either.

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Well, with op amps, if you have a good low noise power supply you don't really need decoupling capacitors because they don't switch rapidly and cause noise on the pcb tracks.
Sure, leave em out, if you don't mind extraneous oscillations and other weird artifacts.
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