Keithley 6487, Reference Manual, Page: 310 / 338

G-4 Applications Guide Model 6487 Reference Manual
Voltage burden
The input resistance of the picoammeter causes a small voltage drop across the input ter-
minals. This voltage is known as the voltage burden. If the voltage burden is large in rela-
tion to the voltage of the measured circuit, then significant measurement errors will occur.
Refer to Figure G-2 to see how voltage burden affects current measurements. Assume V
S
is 5mV and R
S
is 5k
Ω
to configure a 1
μ
A current source (5mV/5k
Ω
= 1µA). An ideal
picoammeter with zero voltage burden would allow 1
μ
A to flow and measure it
accurately.
In practice, however, every picoammeter has a voltage burden. If the voltage burden (V
B
)
is 1mV, the current that flows will be:
The 1mV voltage burden caused a 20% reduction in actual current. Percent error in a mea-
sured reading (I
M
) due to voltage burden can be calculated as follows:
The voltage burden of the Model 6487 depends on the selected range (see specifications).
Voltage burden may be reduced by performing the voltage offset correction procedure.
Voltage offset correction procedure
klqb
To maintain specified operation, any time there is a substantial change in the
ambient temperature, the voltage offset procedure should be performed and
saved.
1. Press the MENU key to display the following:
CAL: VOFFSET
2. Press ENTER. The instrument will prompt as follows:
INPUT CAP
3. It is not necessary to disconnect signal cables from the Model 6487, but it is recom-
mended that signal currents be reduced to zero if possible. During the calibration,
the input impedance will be 3.5M
Ω
, therefore continued signal currents will
present a corresponding voltage to the DUT.
4. Press ENTER to complete offset voltage calibration.
5. Press EXIT to return to normal display.
klqb
Like the other calibration procedures, this calibration is not permanently stored
until CAL:UNLOCK and CAL:SAVE have been performed. Saving the results of
this calibration at a temperature other than that at which the Model 6487 will be
used will adversely affect measurement results.
I
M
V
S
V
B
–
R
S
--------------------
5mV 1mV –
5k
Ω
------------------------------- 0.8
μ
A = = =
I
M
% error
100%
V
S
/V
B
( )
---------------------- =