Table of Contents
Advertisement
that the REL level is still there. Again, go back to the linear
measurement mode (LOG off) and set a REL level of lfi.
the
REL annunciator
will turn on. Switch back to the LOG mode
and note that the REL annunciator
is off, indicating that the
REL level for the LOG mode is cancelled.
Once a REL level is established for a measurement mode, that
stored level will be the same regardless of what range the
Model 485 is on. For example, if + IMA is established es the
AEL level on the 20/~A range, + IpA will also be the REL level
on the 2mA range.
It is important
to note that the use of REL reduces the
dynamic range of measurements by that level. For instance,
assume that the REL level is + IpA and the Model 485 is
manually
set to the 26
range. The maximum
positive
displayed
reading
(before
overranging),
would
be
+0.9999&
This is because the A/D
converter would be
seeing
1.9999fl
(maximum)
from
the input.
Thus,
the
dynamic range of measurement is -1.9999fi
to +0.9999fi
(2.9998fi)
as compared
to
the
normal
-1.9999fi
to
+ I .9999/rA (3.9998$1.
The dynamic range of measurement
has been reduced by l&
The effects on dynamic range can
be reduced by selecting a higher range or using autorange.
2.7.6 LOG Mode
The Model 485 can make logarithm
measurements
refer-
enced to a 1A current level or to other current levels with the
use of the REL feature.
The basic procedure for using the LOG mode is to select auto
range and press the LOG button (LOG annunciator
on). To
get out of the LOG mode, press the LOG button again. The
LOG annunciator
will turn off.
Because the logarithm of a negative number is undefined, the
logarithm
is taken with the absolute value of the measured
current.
NOTE
LOG measurements
must always be made on
the lowest Dossible ranae (without overranging).
Readings dn high ranges will not allow Optimum
calculations
of the logarithm.
When in doubt,
use Auto Range.
1. Log Measurements
with IA reference
currant: The
LOG measurement
mode displays the absolute value of
the logarithm (base 101 of the input current referenced to
IA. The following
equation illustrates this relationship:
LOG Reading = Log
I
Applied Current
IA
Using this reference &rent,
the LOG riading span is from
-13.0 (fO.lpA)
to -2.699 (1tI.9999mAl.
The following
examples
compute
the expected
LOG
readings for various current levels applied to the Model
485:
A. InA
LOG reading = LDG
+$-
I
I
LOG reading = -9.000
B. 0.5&A
0.5fi
LOG reading = LOG -
I
I
LOG reading = -6.301
C. -1mA
LOG reading = LOG I+1
LOG reading = -3.000
To make LOG measurements reference to lA, proceed as
follows:
A. Zero the instrument,
if required,
as described
in
paragraph 2.7.2.
B. Select auto range on the Model 486.
C. Connect the source to the Model 485.
D. Press the LOG button.
E. Take the LOG reading from the display.
2. Log measurements
using other reference
currents:
LOG measurements,
referenced to other current levels,
can be read directly from the display of the Model 485 by
utilizing the REL feature and an accurate current source.
To make LOG measurements referenced to another cur-
rent level, proceed as follows:
A. Z.&o the instrument,
if required,
as described
in
paragraph 2.7.2.
B. Set the Model 485 to auto range.
C. Input the reference current to the Model 485.
D. With the Model 485 in the LOG mode, press the REL
button.
The REL annunciator
will turn on and the
display will zero.
E. Remove the reference current source from the Model
485.
F. The
Model
485
is now
set
up to
make
LOG
measurements referenced to that current level. Simply
input the current to be measured and take the reading
from the display.
2.7.6 Inverting
Analog Output
Two banana jacks, accessible from the rear panel, permit
monitoring
or recording of the output from the current-to-
voltage converter within the Model 485. Since the Model 465
is a feedback picoammeter, as shown in Figure 2-2, the input
current is forced through
the feedback resistor and an in-
2-5
Advertisement
Table of Contents
