Ground Loops - Keithley 595 Instruction Manual

OPERATION
3.13 MEASUREMENT CONSIDERATIONS
The Model 595 is a highly sensitive instrument that can
measure extremely low signal levels. At these low signal
levels, a number of factors can affect a measurement. These
factors are discussed in the following paragraphs.
3.13.1 Ground Loops
Ground loops that occur in multiple-instrument test set-
ups can create error signals that causeerratic or erroneous
measurements. The configuration shown in Figwe 3-U. in-
troduces errors in two ways. Large ground currents flow-
ing in one of the wires will encounter small resistances,
either in the wires, or at the connecting points. These
resistances ~result in voltage drops that can affect the
measurement. Even if the ground currents are small,
_
I
POWER LINE GROUND 1
F
F&we 3-12. Eliminating Ground Loops
magnetic flux cutting aaoss the large loops formed by the
ground leads
can
induce sufficient voltages to disturb sen- 3.13.2 Electrostatic lnterfewnce
sitive measurements.
To prevent ground loops, instruments should be connected
to ground at only a single point; as shown in Figure 3-12.
Note that only a single instrument is connected directly to
power line ground. Experimentation is sometimes the best
way to determine an acceptable arrangement. For this pur-
pose, measuring instruments should be placed on their
lowest ranges. The configuration that results in the lowest
noise signal is the tine that should be used.
Electrostatic interference occurs when an elechically
charged object is brought near an uncharg+ object, thus
inducing a charge on the previously uncharged object.
Usuaily, effects of such electrostatic action are not noticeable
because low impedance levels allow the induced charge to
dissipates quickly. However, the high impedance levels
chamcteristics of many quasistatic CV measorementido not
allow these charges to decay rapidly, and erroneous or
.unstable readings may result. These erroneous or unstable
readings may be caused in the following ways:
SIGNAL
LEADS
1. DC electrostatic fields can cause undete&ed errors or
noise in the reading
2.
AC electrostatic fields can cause errors by driving the in-
put amplifier into sahzration, or through rectification can
produce DC errors.
Electrostatic interference is first recognizable when hand
or body movements near the experiment-cause fluctoations
IN A SIGNAL LEAD
L.-<----e
in the reading.
A.
- POWER LINE GROUND - I
:
Figure 3-11. Multiple Ground Points Creating a
Ground Loop
3-22