333
Chapter 7 Tutorial
Ground Loops
4
7
As shown below, the 33220A 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
The signal-generation portion of the Agilent 33220A 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 1 MΩ in parallel with
45 nF) in the path to oppose the flow of I
GND
and thereby minimize this
effect.
Agilent 33220A
50Ω
V
GEN
R
L
V
L
L
5
+
----------------
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