Keithley 6512, Instruction Manual

Page: 45 / 156

Operation
2-25
2.10 Using zero correct and
baseline suppression
The Model 6512 has zero correction and baseline suppres-
sion modes that allow the cancellation of any internal offsets,
or the storage of a baseline value that can be subtracted from
subsequent readings.
2.10.1 Zero correct and zero check
The ZERO CORRECT and ZERO CHECK buttons work to-
gether to cancel any internal offsets that might reduce accu-
racy. Note that the specifications listed for the instrument at
the front of this manual assume that the instrument has been
zeroed; use the following procedure to zero the instrument.
Note that the instrument should be zero corrected on the
range to be used, or on the lowest range of the function being
used.
Proceed as follows to zero correct the instrument:
1. With the zero correct mode off, press the ZERO
CHECK button. Be sure the ZERO CHECK light is on.
In this mode, the input signal is disconnected from the
input amplifier, and the input circuit is configured. as
shown in Figure 2-19. The internal preamplifier is con-
figured to measure its own offset when zero check is en-
abled.
2. Press the ZERO CORRECT button to zero the instru-
ment. Note that if zero check is not enabled, the zeroing
process will not take place. The previously stored zero
parameter will be used instead.
3. To take readings, press ZERO CHECK to disable the
zero check mode.
4. Readings can now be taken in the normal manner. Note
that the instrument will remain zeroed even if the instru-
ment is moved uprange.
5. For maximum accuracy, the zero correction process
should be repeated every 24 hours when the ambient
temperature changes by more than 1°C, or when the
function is changed.
Figure 2-19
Equivalent input impedance with zero check enabled
NOTES:
1. Leave zero check enabled when connecting or discon-
necting input signals (except for the coulombs func-
tion), or when changing functions.
2. Zero will automatically be scaled when the instrument is
moved uprange.
3. Do not move the instrument down range after zero cor-
rection. Re-zero the instrument after moving down range.
2.10.2 Using suppression
The suppression mode allows a stored offset value to be sub-
tracted from subsequent readings. When the SUPPRESS
button is pressed, the instrument will trigger a conversion
and internally store the displayed value as a baseline. The
SUPPRESS LED will illuminate to indicate that the suppres-
sion mode is enabled. All subsequent readings will be the
difference between the suppressed value and the actual sig-
nal level.
CIN = 20pF
10M
Ω
INPUT
CIN
ZF
CIN = 20pF
INPUT
CIN
VOLTAGE
AMPS
CIN = 20pF
10M
Ω
INPUT
CIN
1000pF
COULOMBS
10M
Ω
INPUT
CIN
ZF
IN 900k
Ω
OHMS CIN = 20pF
10M
Ω
100
Ω
(mA)
100k
Ω
|| 1000pF (
µ
A)
100M
Ω
|| 220pF (nA)
100G
Ω
|| 5pF (pA)
ZF =
100k
Ω
|| 1000pF (ALL k
Ω
, 2M
Ω
)
100M
Ω
|| 22pF (20M
Ω
, 200M
Ω
,
ALL G
Ω
)
ZF =