LM3886

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SNAS091C – MAY 1999 – REVISED MARCH 2013

LAYOUT, GROUND LOOPS AND STABILITY

The LM3886 is designed to be stable when operated at a closed-loop gain of 10 or greater, but as with any other

high-current amplifier, the LM3886 can be made to oscillate under certain conditions. These usually involve

printed circuit board layout or output/input coupling.

When designing a layout, it is important to return the load ground, the output compensation ground, and the low

level (feedback and input) grounds to the circuit board common ground point through separate paths. Otherwise,

large currents flowing along a ground conductor will generate voltages on the conductor which can effectively act

as signals at the input, resulting in high frequency oscillation or excessive distortion. It is advisable to keep the

output compensation components and the 0.1

μF supply decoupling capacitors as close as possible to the

LM3886 to reduce the effects of PCB trace resistance and inductance. For the same reason, the ground return

paths should be as short as possible.

In general, with fast, high-current circuitry, all sorts of problems can arise from improper grounding which again

can be avoided by returning all grounds separately to a common point. Without isolating the ground signals and

returning the grounds to a common point, ground loops may occur.

“Ground Loop” is the term used to describe situations occurring in ground systems where a difference in potential

exists between two ground points. Ideally a ground is a ground, but unfortunately, in order for this to be true,

ground conductors with zero resistance are necessary. Since real world ground leads possess finite resistance,

currents running through them will cause finite voltage drops to exist. If two ground return lines tie into the same

path at different points there will be a voltage drop between them. The first figure below shows a common ground

example where the positive input ground and the load ground are returned to the supply ground point via the

shown below.

in phase. Therefore the voltage appearing at the non-inverting input is effectively positive feedback and the

small enough to be ignored; however, several devices normally comprise a total system. Any ground return of a

separate device, whose output is in phase, can feedback in a similar manner and cause instabilities. Out of

phase ground loops also are troublesome, causing unexpected gain and phase errors.

The solution to most ground loop problems is to always use a single-point ground system, although this is

sometimes impractical. The third figure below is an example of a single-point ground system.

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