10 TEGAM WAY • GENEVA, OHIO 44041 440-466-6100 • FAX 440-466-6110 • www.tegam.com
4-4
Section IV – Operating Instructions
When
I
is displayed, the control allows the user to inject currents of ± 2.5 nA into the input amplifier and
its driving source. This adjustment allows the user to offset any natural instrument and/or measurement
bias currents flowing through driving impedances (causing an offset voltage). Turning the control
clockwise injects a positive current and counterclockwise injects a negative current. Holding the control
depressed for 5 seconds or longer resets the offset current to 0.
NOTE: The level of injected offset current does NOT change as ranges are changed; thus, the input offset
current need only be set once; however, the offset current returns to zero when the instrument power is
turned off.
NOTE: When nulling input bias currents, the user may find using a 100k
Ω
low-noise resistor or a low-noise
resistor value approximating the source impedance of the device connected between the HI and LO input
terminals is beneficial. When using a 100 k
Ω
resistor, the 30
μ
V range is a good choice as a starting point.
The 30
μ
V range and a 100 k
Ω
resistance shows input bias currents of ± 300 pA (full scale) and allows
nulling these input bias currents to substantially less than 1 pA. If the initial indication is off scale, change
the range to 100
μ
V and then downrange to 30
μ
V. Alternatively, the 1 M
Ω
input impedance selection may
be used.
Once the range is zeroed or a desired level of input offset current is selected, the INPUT OFFSET control
may be locked (by depressing the control until the padlock icon is displayed). This prevents further
adjustment of the range zero function or input offset current. The control’s locked/unlocked status is
shown in an adjacent area of the status LCD. A
(closed padlock) indicates locked status. Either
V
or
I
indicate unlocked status.
NOTE: with the padlock displayed, adjusting the INPUT OFFSET control allows the user to select which
digit of the OFFSET voltage responds to the OFFSET up and down pushbuttons. An underscore is displayed
under the selected digit (e.g. 03,000.00 or 00,300.00).
OUTPUT LEVEL -- Rotary Control/Pushbutton
This is a multi-function button and should be thoroughly The OUTPUT LEVEL rotary control is a rotary
encoder that allows the user to adjust the ISOLATED OUTPUT level. The range of this control allows the
user to adjust the ISOLATED OUTPUT voltage from ± 0.5 volt to ± 1.5 volt when the meter is deflected to
full scale. The OUTPUT LEVEL control also serves to set the ISOLATED OUTPUT zero voltage when in Setup
mode. The OUTPUT LEVEL control is unlocked or locked by depressing the control once. The control’s
locked/unlocked status is shown in an adjacent area of the LCD. A
(closed padlock) indicates locked
status.
OVER VOLTAGE
If a potentially harmful over voltage exists at the instrument’s input amplifier, the Over Voltage circuit is
activated. Once activated, the instrument is disconnected from the input terminals and an over voltage
status is shown on the LCD.
NOTE: The Over Voltage only operates when the instrument Range is 1 mV or below unless an extreme
over voltage occurs (> 1,300 V) on the 3 mV and above ranges—such voltages are likely to damage the
instrument.
When an over voltage condition occurs, the first step is to remove the source of input over voltage. After
the over voltage source is cleared, depressing the OPERATE/ZERO pushbutton once returns the instrument
to operation in the ZERO mode. Depressing the OPERATE /ZERO pushbutton once again returns the
instrument to the OPERATE mode.
