Datasheet

LM2876

SNAS088C –AUGUST 1995–REVISED MARCH 2013

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“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

same wire. The addition of the finite wire resistance, R

, results in a voltage difference between the two points as

shown below.

The load current I

will be much larger than input bias current I

, thus V

will follow the output voltage directly, i.e.

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

circuit may oscillate. If there were only one device to worry about then the values of R

and R

would probably be

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.

The single-point ground concept should be applied rigorously to all components and all circuits when possible.

Violations of single-point grounding are most common among printed circuit board designs, since the circuit is

surrounded by large ground areas which invite the temptation to run a device to the closest ground spot. As a

final rule, make all ground returns low resistance and low inductance by using large wire and wide traces.

Occasionally, current in the output leads (which function as antennas) can be coupled through the air to the

amplifier input, resulting in high-frequency oscillation. This normally happens when the source impedance is high

or the input leads are long. The problem can be eliminated by placing a small capacitor, C

, (on the order of 50

pF to 500 pF) across the LM2876 input terminals. Refer to the External Components Description

(1)

section

relating to component interaction with C

(1) (Figure 1 and Figure 5)

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