AS1713 datasheet(10/15 Pages) AMSCO

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AS1713

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

8 Application Information

Ground Loop Interference Suppression:

In real life situations the signal source and the amplifier are often located a distance apart, but still share the same

ground arrangement with a number of other circuits. The ground system is never perfect as it has a small distributed

resistance, capacitance and inductance. Thus, the ground appears as a distributed impedance.

As a various current flow into and out of the ground system exists, a small voltage drop will inevitably occur, causing

different voltages within the ground. In Figure 23 and Figure 24, RGND denotes the ground resistance between the

input signal ground and the output signal ground. The voltage drop across RGND should ideally have no effect on indi-

vidual circuit performance.

In the single ended inverting amplifier shown in Figure 23, the amplifier sees VIN and VGND in series, so the amplifier

output is:

VOUT = - [R2/R1] [VIN+VGND](EQ 1)

Figure 23. Single Ended Inverting Amplifier

VGND is part of the output expression and is known as ground loop interference, ground bounce or even common

impedance crosstalk. In some situations this interference signal can be close to or the same value as the wanted input

signal such as in sensor applications.

A difference amplifier is a simple method used to reduce the effect of ground interference. VIN is regarded as a differ-

ential input signal, and VGND a common mode signal. From Figure 24, the amplifier output is:

VOUT = - [R2/R1] VIN

(EQ 2)

Figure 24. Difference Amplifier

The additional cost of extra matched resistors is offset by the rejection of the unwanted common mode ground interfer-

ence.

–

RGND

+ VGND -

VIN

VOUT

AS1713

VIN

RGND

+ VGND -

SENSE

OUT

REF

INN

INP

VOUT

–

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