Decoupling Dual Supply Op Amps - Page 1
Forum section: electronics / beginners

author: tcatman
author_rank: Newbie
timestamp: May 23, 2023, 03:35:09 pm
content: I have done extensive Google searches and Electronics Forum perusals, plus read numerous tech pdf's on the topic of successfully Decoupling the right way op amps dual or single powered. But the one thing that has not been answered in this quest for knowledge is. ---As far as Decoupling dual supply powered op amps, I see how it is effectively achieved when both power rails ± are equally same voltages at opposing polarities, ie; +9V and -9V, then the Centre Vref is going to be 0 GND, thus the decoupling caps are taken from each power rail to 0V Ground placed as close as possible to the power pins on the op amps.But what happens and how do you successfully decouple with caps off both rails, when you have something like a +18v rail and a -9V rail, and the Centre Vref point is then +4.5 Volt, how is it then decoupled. Are the caps still connected to 0V Ground, or are they then meant to be connected from each power rail to the Vref +4.5 Volt? Thanks for any further understanding to this dilemma.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 23, 2023, 04:53:19 pm
content: For an asymmetric dual supply, like +18 and -5 V one can still decouple towards a common ground. The Decoupling capacitors are for an superimposed AC part and this does not care about the DC part.For many OP-amps the negative supply is more important as it is used as the internal reference point for the compensation. So a stable negative supply can be more important than a stable positive supply.In sensitive circuits it also make sense to have additional series resistors (a few ohms) or ferrites in the supplies, so that the capacitorts are only for the OP amp and not directly connected to the main supply.

author: barshatriplee
author_rank: Regular Contributor
timestamp: May 24, 2023, 12:18:46 am
content: The capacitors should be placed as close as possible to the power pins of the op amp. This arrangement allows the capacitors to bypass any noise present on the power rails to the common ground.

author: David Hess
author_rank: Super Contributor
timestamp: May 24, 2023, 04:24:48 am
content: The asymmetry in supply voltages does not matter for decoupling, however it is not as simple as across the supplies, or from the supplies to ground, or as close to the operational amplifier pins as possible.See Analog Devices application note 202 for what is really going on and where decoupling connections should be made. Optimally this usually comes down to directly across the device, and from one or the other supplies at the device to the common return for the load. In practice this later connection is often overlooked and instead two decoupling capacitors go from the supply pins to common, but in some applications the common return for the load may be important.

author: kumarboopathi
author_rank: Newbie
timestamp: May 26, 2023, 01:49:03 am
content: Decoupling op amps in a dual power supply configuration can be a bit confusing when the power rails have different voltage levels.

author: Simon
author_rank: Unknown
timestamp: May 26, 2023, 06:38:21 pm
content: Quote from: kumarboopathi on May 26, 2023, 01:49:03 amnot really, as suggested, the same applies.