Power-Up; Zero Check; Current Measurements; Relativemode - Keithley 485 Instruction Manual

2.7.1 Power-Up
NOTE
The software revision level of the Model 485 can
be disolaved uoon oower-uo bv runnino the
diagnostic program. See .Section 5,
Maintenance, for more information.
Turn on the Model 485 by pressing in the ON/OFF switch.
The following will occur automatically:
1. Reset- All zeroes will be briefly displayed before going into
the measurement mode.
2. RAM Test-If this test fails the Model 485 will lock up with
zeroes displayed.
3. NVRAM Test-This test determines if the calibration con-
stants are properly read from NVRAM. If this test fails the
display will show the error message "cErr".
Refer to Table 2-1 for more information pertaining to error
messages.
CAUTION
During power up and power down, using
AC line or battery pack, a current surge
15mA, 5V maximum) can appear at the IN-
PUT of the Modal 455. Protect any circuitry
that would be damaged by these levels by
making circuit connections only after turn-
ing on the Model 485. Disconnect these cir-
cuits before turning off the Model 485.
2.7.2 Zero Check
The instrument should be properly zeroed (after one hour
warm-up) before making any current measurements. Only
the 2nA current range needs to be zeroed. To check or adjust
zero, proceed as follows:
1. Select the 2nA range.
2. Turn off REL and LOG if they are on.
3. Depress the ZERO CHECK pushbutton. Momentary or
locking action can be achieved depending on how far the
button is depressed. To adjust zero, place the
pushbutton
in to the lock position.
4. Adjust zero level trimpot via access hole on front panel for
a zero indication on the digital display.
5. Release the ZERO CHECK button.
In normal operation, the equivalent input resistance of the
485 picoammeter is equal to or less than the maximum
burden voltage (2OOfiV) divided by the maximum range cur-
rent. This does not hold while in zero check or overload con-
ditions because the feedback to the input node is broken.
Values of input impedance for normal operation and zero
check/overload are shown in Table 23.
NOTES
1. When zero is properly adjusted, the input voltage burden
for an on-range reading will be less than or equal to 2OOpV.
2. The REL feature can be used instead of the ZERO pot to
zero the display. However, REL then cannot be used to
cancel a measurement offset or establish a reference level.
Table 2-3. Equivalent Input Resistance With Zero
Check Enabled
:urrent Maximum Effective Impedance In
Range
Input Resistance Zero Check/Overload
2 nA 1OOkD 9MGll230pF
20 nA
IOkD 9MDjj230pF
200 nA IkD 5MCll230pF
2::: 100 10 D a 900k08230pF 100kGll0.01~F
200 pA lOkDUO.OlpF
2mA 0.: IY Ik9ljO.Ol~F
2.7.3 Current Measurements
1. Zero the instrument as described in paragraph 2.7.2.
2. Select a current range that is appropriate for the expected
input level or select autorange.
3. Connect the source to the INPUT connector on the front
panel. Low-noise cable and connector accessories de-~
scribed in Section 1 should be used as required. Measure-
ment considerations are discussed in paragraph 2.8. A
summary of range, accuracy and input overload informa-
tion is given in Table 2-2.
4. Read the input current on the digital display. Overrange is
indicated by an "OL" message.
2.7.4 Relative Mode
When the relative mode is selected with an on-scale reading
on the display the following occurs:
1. The REL annunciator is displayed.
2. The next reading is stored.
3. The stored reading is then algebraically subtracted from all
subsequent readings and displayed.
A REL level can be established for the linear measurement
mode or the logarithm measurement mode and is effective
only in that mode. Changing modes will not affect a REL level
already established. However, if another REL level is set fin
the other measurement mode) the previous REL level will be
cancelled. For example, place the Model 485 in the logarithm
mode (LOG annunciator on) and select the 2pA range. With
an open input, note the approximate -10.0 reading on the
display. Press the REL button and note that the REL annun-
ciator is on and that display is zeroed. Select the linear
measurement mode by pressing the LOG button (LOG an-
nunciator off). Note that the REL annunciator is off, in-
dicating that there is not a REL level established for the linear
measurement mode. Switch back to the LOG mode and note
2-4