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Operation
2-30
Figure 2-22
External triggering example
6. Begin the measurement sequence by pressing the Model
7001 STEP button. The Model 7001 will close the first
channel and trigger the Model 6512 to take a reading.
When the Model 6512 completes the reading, it will
trigger the Model 7001 to go to the next channel. The
process repeats until all programmed channels have
been scanned.
2.13 Measurement considerations
The Model 6512 is a highly sensitive instrument that can
measure extremely low signal levels. At these low signal lev-
els, a number of factors can affect a measurement. Some con-
siderations when making measurements with the Model
6512 are discussed in the following paragraphs.
2.13.1 Ground loops
Ground loops that occur in multiple-instrument test set-ups
can create error signals that cause erratic or erroneous mea-
surements. The configuration shown in Figure 2-23 introduc-
es errors in two ways. Large ground currents flowing in one
of the wires will encounter small resistances, either in the
wires, or at the connecting points. This small resistance re-
sults in voltage drops that can affect the measurement. Even
if the ground loop currents are small, magnetic flux cutting
across the large loops formed by the ground leads can induce
sufficient voltages to disturb sensitive measurements.
MODEL 7001
CHANNEL
READY
EXTERNAL
TRIGGER
METER
COMPLETE
OUTPUT
EXTERNAL
TRIGGER
INPUT
MODEL 6512
Figure 2-23
Multiple ground points create a ground loop
To prevent ground loops, instruments should be connected to
ground at only a single point, as shown in Figure 2-24. Note
that only a single instrument is connected directly to power
line ground. Experimentation is the best way to determine an
acceptable arrangement. For this purpose, measuring instru-
ments should be placed on their lowest ranges. The configu-
ration that results in the lowest noise signal is the one that
should be used.
Figure 2-24
Eliminating ground loops
INSTRUMENT
A
INSTRUMENT
B
INSTRUMENT
C
TYPICAL GROUND LOOP
CAUSES CURRENT FLOW
IN A SIGNAL LEAD
SIGNAL LEADS
POWER LINE GROUND
INSTRUMENT
A
INSTRUMENT
B
INSTRUMENT
C
POWER LINE GROUND
Summary of Contents for 6512
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