305
Chapter 7 Tutorial
Ground Loops
4
7
As shown below, the 33250A has a fixed series output impedance of 50Ω,
forming a voltage divider with the load resistance.
As a convenience, you can specify the load impedance as seen by the
function generator and thereby display the correct load voltage. If the
actual load impedance is different than the value specified, the displayed
amplitude, offset, and high/low levels will be incorrect. Variations in
source resistance are measured and taken into account during an
instrument calibration. Therefore, the accuracy of the load voltage
depends primarily on the accuracy of the load resistance as shown below.
∆ V
L
(%) ≅ x ∆ R
L
(%)
Ground Loops
Except for its remote interface connectors and trigger connector,
the 33250A 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 on the
following page 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
1MΩ in parallel 45 nF) in the path to oppose the flow of I
GND
and
thereby minimize this effect.
Agilent 33250A
50Ω
V
GEN
R
L
V
L
50
R
L
50+
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