AS1713
Data Sheet - Application Information
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
R1
R2
–
+
+
VIN
+
VOUT
-
RGND
+ VGND -
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
R1
INN
R2
SENSE
AS1713
–
EN
+
VIN
OUT
REF
+
R3
R4
INP
+
VOUT
-
RGND
+ VGND -
The additional cost of extra matched resistors is offset by the rejection of the unwanted common mode ground interfer-
ence.
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