Keysight 33210A - Ground Loops

322
Chapter 7 Tutorial
Ground Loops
7
Ground Loops
The signal-generation portion of the Keysight 33210A is isolated from
chassis (earth) ground. This isolation helps to eliminate ground loops in
your system and also allows you to reference the output signal to
voltages other than ground. The illustration below shows the function
generator connected to a load through a coaxial cable. Any difference in
ground potentials (V
GND
) will tend to make current I
GND
flow in the
shield of the cable, thus causing a
voltage drop due to the shield's
impedance (Z
Shield
). The resulting voltage drop (I
GND
x Z
Shield
) appears
as an error in the load voltage. However, since the instrument is isolated,
there is a high series impedance (typically 1 MΩ in parallel with 45 nF)
in the path to oppose the flow of I
GND
and thereby minimize this effect.
Ground Loop Effects
At frequencies above a few kilohertz, a coaxial cable’s shield becomes
inductive, rather than resistive, and the cable acts as a transformer.
When this happens, it tends to force the shield and center-conductor
currents to be equal but opposite. For any voltage drop in the shield due
to I
GND
, there is a similar drop in the center conductor. This is known as
Keysight 33210A
50Ω
V
GEN
R
L
V
L
= V
OUT
– (I
GND
x Z
Shiel
d
)
V
OUT
Z
Shield
1 MΩ
I
GND
V
GND
45 nF
33210A users guide.book Page 322 Wednesday, July 16, 2008 11:16 AM