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SECTION 3
PERFORMANCE
VERIFICATION
3.1 INTRODUCTION
This section contains information
necessary to verify that the
Model 485 performance
is within specified accuracy Model
485 specifications
may be found at the front of this manual.
Ideally, performance verification
should be performed when
the instrument
is first received to ensure that no damage or
change in calibration
has occurred
during shipment.
The
verification
procedure may also be performed whenever
in-
strument accuracy is suspect or following
calibration.
If per-
formance
on any of the ranges is outside specified limits,
calibration can be performed as described in Section 5.
NOTE
If the instrument
does not meet specifications
and it is still under warranty (less than 12 months
since date of shipment),
contact your Keithley
representative
or the factory to determine the
action to be taken.
3.2 ENVIRONMENTAL
CONDITIONS
All measurements should be made at an ambient temperature
between 18e and 28OC (65" to 82OFl with a relative humidity
less than 70%.
3.3 RECOMMENDED
TEST EQUIPMENT
Equipment for verifying the performance of the Model 485 is
listed in Table 3-I. Alternate equipment may be used as long
as the equipment
accuracy
is at least as good as the
specifications
listed in Table 3-1. Information
needed to build
the test fixture can be found in Figure 3-1.
3.4 INITIAL
CONDITIONS
Before performing the verification
procedures, make sure the
Model 485 meets the following
conditions:
1. If the instrument has been subject to temperatures below
18eC (65OF) or above 28°C (82eFj. allow sufficient time for
the instrument
to reach temperatures
within
the range.
Generally, it takes one hour to stabilize an instrument that
is 10°C (18°F) outside of this range.
2. Turn on the Model 485 and allow it to warm up one hour.
The instrument may be operated from either line power or
battery pack power, as long as the battery pack has been
fully charged as described in paragraph 2.2.3.
3.5 VERIFICATION
PROCEDURE
The following
paragraphs give the basic verification
pro-
cedure for checking accuracy.
To properly check the accuracy of the Model 485, precision
current sources are necessary. These precise currents are ob-
tained by using the DC calibrator and measured resistors in a
shielded test fixture, as shown in Figure 3-1.
Proceed as follows to check the accuracy of the Model 485:
NOTE
Record
the
following
measurements
and
calculations in Table 3-2. Use a pencil so that the
table can be reused.
1. Using the Keithley Model 191 or 192, measure and record
the resistance
of the lOk0,
100kD. IMg
and IOMD
resistors.
2. Using the Guildline Model 9520, measure and record the
resistance of the lOOMD, 1GD and 1OGB resistors.
3. Calculate and record the required DC calibrator setting for
each range as follows:
DC Calibrator Setting:
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.
8. Set the Model 485 to the appropriate range.
7. Connect the DC calibrator to the test fixture.
8. Connect the test fixture to the input of the Model 485
using a low-noise
coaxial
input
cable, such as the
Keithley Model 4801.
9. Using Table 3-2 as a guide, install the appropriate resistor
in the test fixture, set the DC calibrator to the calculated
setting and check all the ranges of the Model 485.
10. Reverse the + and - test leads at the DC calibrator and
repeat all checks with negative current.
3-1
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