Floating Measurements - Keithley 486 Instruction Manual

SECTION 3
Front Panel Operation
To prevent ground loops, test system co-on should
be connected to earth ground at only a single point, as
shown in Figure 3-23. Note that circuit common is con-
nected to earth ground only at Instrument B (via the
ground link). The ground link for Instrument A has
been removed. Experimentation is the best way to
determine an acceptable arrangement. For this purpose,
measuring instruments should be placed on their lowest
ranges. The con guration that results in the lowest
noise signal is the one that should be used.
L
3.16.8 Floating Measurements
CAUTION
To prevent damage to the instrument, adhere
to the following precautions when eating
the input above chassis ground:
1. Do not connect ANALOG OUTPUT LO
to CHASSIS. If there is a ground link
installed at the rear panel binding posts,
remove it.
2. Make sure that any monitoring instm-
ment connected to the analog output is
eating.
The circuit connected to the input of the Model 4&36/487
in Figure 3-24 shows a typical con guration to make
eating measurements. A problem occurs when
ANALOG OUTPUT LO is connected to chassis ground.
As shown in the illustration, connecting ANALOG
OUTPUT LO to chassis ground will provide a current
path for the voltage source through the internal fusible
resistor R203. In this con guration, a voltage source
with high current capabilities could damage the resistor.
For example, if voltage source (E) is set to 1OOV and has
a lOOmA limit, current through R203 could be lOOmA
(lOOV/lkn=lOOmA) damaging the l/ZW fusible
resistor.
If the V-Source of the Model 487 is used to make eating
measurements, damage to R2.03 cannot occur because of
the 2.5mA limit. However, the measurement could be
compromised if the V-Source is forced into current limit.
Figure 3-24. Improper Method to make Floating Measurements
3-36