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5.4.2 Calibration Storage Enable
The CAL LOCK switch of the Model 495 must be in the calibra-
tion storage enable position in order to store calibration con-
stants into NVRAM. If the switch is not placed in the enabled po
sition, calibration will be lost when the instrument is turned off.
The access hole for the CAL LOCK switch is located to the left of
the ON/OFF power switch. CAL LOCK is a two position (IN and
OUT) pushbutton switch. To enable calibration storage, insert a
small-bladed screwdriver into the access hole in the front panel
and set the pushbutton switch to the innermost position.
5.4.3 Environmental
Conditions
Calibration should be performed under laboratory conditions
having an ambient temperature
of 23 f3OC and a relative
humidity of less than 70%. With the instrument
on, allow it
to warm up for one hour. If the instrument
has been sub-
jected to temperatures
outside
this range, or to higher
humidity, allow at least one additional hour for the instrument
to stabilize before beginning the calibration procedure.
NOTE
Calibration can be stopped at any time and only
selected ranges can be calibrated if needed.
5.4.4 Calibration
Procedure
Perform the following
steps to calibrate the Model 485:
NOTE
Record
the
following
measurements
and
calculations in Table 5-l. Use a pencil so that the
table can be used again.
1. Using the Keithley Model 181 or 192, measure and record
the resistance
of the lOkC, lOOkg,
1MC and
1OMD
resistors.
2. Using the Guildline Model 9520, measure and record the
resistance of the lOOMg, lG9 and 1OGg resistors.
3. Calculate and record the required DC calibrator setting for
each range as follows (Refer to Table 5-l):
V = I (Required Current) x R (Measured Resistor)
4. Depress ZERO CHECK and select the 2nA range.
5. With an open input, adjust the ZERO pot for .OOOO* 1
count on the display.
NOTE
If Q104,
U105,
R113, RI14
or R115 were
changed,
the instrument
may not zero. See
paragraph 5.7 for instructions
on balancing the
input amplifier offset.
8. Connect the DC calibrator to the test fixture. Make sure
that the calibrator is in standby.
7. Connect the test fixture to the input of the Model 485 us-
ing a low-noise coaxial input cable (such as the Keithley
Model 4801).
8. Press in the REL and LOG pushbuttons
simultaneously
until the message "CAL"
is displayed. Release the but-
tons. The unit is now in the calibration mode as indicated
by the "CAL"
annunciator.
9. With the Model 485 in the calibration mods, perform the
following
procedure using Table 5-1 as a guide.
A. With
the DC calibrator
in standby,
install the ap-
propriate resistor in the the test fixture.
8. Set the Model 485 to the appropriate range.
C. Set the DC calibrator to output the calculated voltage
level needed to generate the required current.
D. Adjust
the display accordingly
with the use of the
STOlCLR and RCL buttons. The STO/CLR button in-
crements the displayed reading and the RCL button
decrements the displayed reading.
5.4.5 Calibration Storage
To store the calibration constants, simultaneously press REL
and LOG until the message "stof is displayed. The instrument
will return to normal operation.
If however, the message "out" is displayed, the CAL LOCK
switch is in the disable (OUT) position. The CALannunciator on
the display will blink indicating that calibration is temporary and
will be lost when the instrument is turned off.
NOTE
If calibration was inadvertently performed with CAL
LOCK in the disable (OUT) position, calibration
constants can be stored by performing the follow-
ing steps:
1. Place the CAL LOCK switch to the enable (IN) posi-
tion (see paragraph 5.4.2).
2. Simultaneously press REL and LOG until "CAL" is dis-
played. This places the instrument back into the cali-
bration mode.
3. Simultaneously press REL and LOG until "star" is dis-
played.
52
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