Keithley 199 Instruction Manual page 30

baseline value. The following two examples will use the
maximum allowable zero values (303,000 counts and
-303,000 cotints) to show that djkamic measurement
range will not be reduced. It is important to note that the
inaased display range does not increase the maximum
allowable input level to the instrument. For example, on
the 3V range, the Model 199 will always over-range when
more than i3.03V is connected to the input.
Example l-The instrument is set to the y DC range+
a maximum -3.03OOOV is established as the zero value.
When -3.03OOCV is connected to the input of the Model
199, the display will read 0.0000W. When +3.03oooV is con-
nected to the inpui, the display will read +6.06OOOV. Thus,
the dynamic measurement range ot the Model~le is OV
to 6.06V, which is 606,000 counts.
Example Z-The instrument is still set to the 3V DC range,
but a maximum +3.03OOOV is the zero level. When
+3.03OCKiV is connected to the input of the Model 199, the
display will read 0.01IO00V. When -3.03OGOV is connected
to the input, the display will read -6.06OOOV. Thus the
dynamic measurement range of the instrument is -6.06V
to OV, which is still 606,000 counts.
Zero Correction-The Model 199 must be properly zeroed
when using the 3COmV DC or~the 3003 range in order to
achieve rated accuracy specifications. This procedure
should be performed whenever the ambient temperature
changes. To use ZERO for zero correction, perform the
following steps:
1. Disable zero, if presently enabled, by pressing fie
ZERO button. The ZERO indicator will turn off.
2. Select the 3OOmV DC or the 30011 range.
3. Connect the test leads to the input of the Model 199
and short them together. If four-wire resistance
measurements are to be made, connect and short all
four leads together. Allow any thermals to stabilize.
Note: At 5Yxiigit resolution, low level measurement
techniques need to be employed. Use Kelvin test leads
or shielded test leads.~See paragraph 2.6.5 for low level
measurement considerations.
4. Press the ZERO button. The display will read zero.
5. Remove the short and connect the test leads to the sig-
nal or resistance to be measured.
Note: Test lead resistance (Z-wire) is also compensated
for when zeroing the 309Q range with the above procedure.
Baseline Levels--Baseline v&es can be established by ap
plying baseline levels to the instrument. To establish a
baseline level by applying a level to the Model 199, per-
form the following steps:
1. Disable zero, if presently enabled, by pressing the
ZERO button. The ZERO indicator will turn off.
2. Select a function and range that is appropriate for the
anticipated measurement.
3. Connect the desired baseline level to the input of the
Model 199 and note that level on the display.
4. Press the ZERO button. The display will zero and the
ZERO indicator will be enabled. The previously
displayed reading will be the stored baseline.
WARNING
With ZERO enabled, a hazardous voltage
baseline level (i4OV or more), not displayed,
may be present on the input terminals. If not
sun? what is applied to the input, assume that
a hazardous voltage is present.
5. Disconnect the stored signal from the input and con-
nect the signal to be measured in its place. Subsequent
readings wiU be the difference between the stored value
and the applied signal.
Notes:
1. Disabling zero cancels the zero baseline value on that
selected fun@ion. Baselines established on other func-
tions are not affected.
2 To store a new baseline on a selected function, zero
must first be disabled and then enabled again. The new
value wiIl be stored with the first triggered conversion.
3. Setting the range lower than the suppressed value will
overrange the display; the instrument ti display the
overflow message under these conditions.
4. When the ZERO button is pressed to enable zero, the
ZERO indicator light will blink until an on scale reading
is available to use as a zero level. In the one-shot trig-
ger mode, the unit must be triggered to store the zero
value.