Operation
2-12
the preamplifier acts as a unity-gain amplifier with low out-
put impedance.
WARNING
Hazardous voltage (up to 300V) may be
present at the PREAMP OUT terminal,
depending on the input signal.
2.7.5 Making current measurements
The Model 6512 can resolve currents as low as 0.1fA
(10
-16
A), and measure as high as 20mA in 11 ranges. The
Model 6512 exhibits low input voltage burden and extremely
low input offset current. The low voltage burden is achieved
because the Model 6512 measures current as a feedback type
picoammeter, rather than the shunt method used by many
DMMs.
INPUT
V,
Ω
GUARD
TRIAX CABLE
SAFETY SHIELD
MEASURED
DEVICE
MODEL 6512
TO A/D
CONVERTER
S
2V ANALOG
OUTPUT
INPUT AMPLIFIER
A. CONNECTIONS
B. EQUIVALENT CIRCUIT (VOLTS MODE SHOWN)
GUARD
HI
LO
HI
GND
GUARD
LO
WARNING: USE SAFETY SHIELD FOR SIGNALS
ABOVE 30V (VOLTS) AND OHMS
HI
LO
COM
GND
100
Ω
S
HI
GUARD
PREAMP OUT
LO
SAFETY SHIELD
V,
Ω
GUARD
SWITCH
RANGING AMPLIFIER
OFF
ON
COM
Figure 2-8
Guarded input connections
CAUTION
Safe operation and good measurement
practice dictates the use of an external
resistor when necessary to limit currents
to less than 30mA.
NOTE
After measuring high voltage in volts, or
following an overload condition in ohms,
it may take a number of minutes for input
current to drop to within specified limits.
Input current can be verified by placing a
shielded cap on the INPUT jack and then
connecting a jumper between the COM
and chassis ground terminals. With the in-
strument on the 2pA range and zero check
disabled, allow the reading to settle until
the instrument is within specifications.
Summary of Contents for 6512
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