Rel - Keithley 486 Instruction Manual

SECTION 3
Front Panel Operation
1. Press SHIFT and then FILTER SELECT in that order.
One of the following
messages
will be displayed to
indicate the currently selected filter(s):
FILTER
DIGUAL
FILTER
ANALOG
FILTER
DIG+AN
(Digital filter selected)
(Analog filter selected)
(Both digital and ana-
log filters selected)
2. To select a different filter, use the rotary knob, or the
4 and b keys to display the desired filter selection.
3. Enter the displayed filter selection by pressing
Sm and then FILTER SELECT. The instrument
will return to the normalmeasurement display state.
NOTES:
1. Typically, both filters should be used unless trying to
optimize speed.
2. Factory initialization selects both analog and digital
filters, and enables the filter feature.
3. The filter selection and operation (enabledidis-
abled) can be saved as the default condition (see
paragraph 3.8.6).
4. When using the digital filter in a one-shot trigger
mode, keep in mind that the first triggered conver-
sions will not represent settled readings. A number
of reading conversions are necessary to acquire a set-
tled reading for the digital filter. For this reason, the
digital filter should typically not be used when using
the one-shot trigger mode.
3.5.5 Rel
REL
El
The rel (relative) feature seTyes as a means of baseline
suppression by allowing a stored offset value to be sub-
tracted from subsequent readings. When rel is enabled,
the ins!mment takes the currently displayed reading as a
baseline value. All subsequent readings represent the
difference between the applied signal level and the
stored baseline.
A baseline level can be established for both current and
V/I ohm measurements and is "remembered" by both
functions. For example, a 1p.A baseline can be established
for current measurements and a lOOa baseline can be es-
tablished for V/I ohms. These levels will not be canceled
by switching back and forth between functions. Once a
baseline is established for a measurement function, that
stored level will be the same regardless of what range the
Mode1486/487 is on For example, if 1pA is established as
the baseline on the 2@ range, then the baseline will also
be $A on the 20@, 200@ and 2mA ranges. A relative
baseline level can be as large as full range. Selecting a
range that cannot accommodate the baseline level will
cause an overrange condition.
When a REL baseline level is established for current, the
Rel'ed currentwillbeineffectwhenV/Iohmsisselec+ed
(Model 487). This allows leakage current to be cancelled
when performing V/I ohms measurements.
NOTE
The following discussion on dynamic range is
based on a display resolution of 5-l /2 digits.
At 4-1/2d resolution, the number of counts
would be reduced by a factor of 10.
By design, the dynamic measurement range of the Model
486/487, at 5-1/2d resolution, is 439,998 counts. With rel
disabled, the displayed reading range of the instrument
is ?219,999 counts. With rel enabled, the Model 486/487
has the capability to display &439,998 counts. This in-
creased display range ensures that the dynamic measure-
ment range of the instrument is not reduced when using
a relative baseline value.
The following two examples will use the
maximum
al-
lowable rel values (+219,999 counts and -219,999 counts)
to show that the dynamic measurement range will not be
reduced. It is important to realize that the increased dis-
play range does not increase the
maximum
allowable in-
put level to the instrument. For example, on the 2mA
range, the Model 486/487 will always overrange when
more than z!z2.19999ti is connected to the input.
Example 1 -The insinunent is set to the 2mA range and
a maximum -2.19999mA is established as the rel value.
When -2.19999mA is connected to the input, the display
will read O.OiXlOOmA. When +2.19999mA is applied to the
input, the display will read +4.39998mA. Thus, the dy-
namic measurement range of the Model 486 is OmA to
+4.39998mA, which is 439,998 counts.
Example 2 -The instrument is set to the 2mA range and
a maximum +2.19999ti is established as the rel value.
When +2.19999mA is connected to the input, the display
wilI read O.OOCOOmA. When-2.19999mA is applied to the
3-7