Making Charge Measurements - Keithley 6512 Instruction Manual

Operation
Current measurement considerations
At very low levels (in the picoampere range or below), noise
currents generated in the cable or from other sources can af-
fect measurements. Currents generated by triboelectric ef-
fects are a primary cause of noise currents generated in
connecting cables. These currents are generated by charges
created at the junction between a conductor and an insulator
because of friction. Coaxial and triaxial cables are especially
prone to such noise currents, which are generated by cable
flexing. To minimize these effects, the cable should be tied
down firmly to minimize any flexing. Also, special low-noise
cable, constructed with graphite between the shield and insu-
lator, is available to minimize these effects (see paragraph
1.9). However, even with low-noise cables, several tens of
femtoamps of noise currents can be generated by cable
movement.
Voltage burden is frequently a consideration when making
current measurements. Ideally, the input voltage burden
should be zero for the instrument to have absolutely no effect
on the circuit it is measuring. If the voltage burden is too high,
its effects can degrade measurement accuracy considerably.
To see how voltage burden can upset measurement accuracy,
refer to Figure 2-10. A source, represented by E
put resistance R , is shown connected to the input of a pi-
S
coammeter. The voltage burden is represented by a constant
voltage source at the input as E
rent as seen by the meter would simply be:
However, if E has a non-zero value, the current now be-
IN
comes:
I
=
Note that the Model 6512 voltage burden is typically 1mV or
less. Additional considerations include source resistance and
capacitance, as discussed in paragraph 2.13.
2-14
with an out-
S
. If E were zero, the cur-
IN IN
E
S
I
= ------ -
R
S
E E
–
S IN
---------------------- -
R
S
SOURCE
I
R S
E S
Figure 2-10
Voltage burden considerations

2.7.6 Making charge measurements

The Model 6512 is equipped with three coulombs ranges to
resolve charges as low as 10fC (10
as 20nC (20 × 10
-9
C). When the instrument is placed in one
of the coulombs ranges, an accurately known capacitor is
placed in the feedback loop of the amplifier so that the volt-
age developed is proportional to the integral of the input cur-
rent in accordance with the formula:
1
V
= --- -
C
The voltage is scaled and displayed as charge.
After measuring high voltages in volts, or
following an overload condition in ohms,
it may take a number of minutes for the in-
put current to drop to within specified lim-
its. Input current can be verified by placing
a shielded cap on the INPUT jack and then
connecting a jumper between the COM
and chassis ground terminals. With the in-
strument on the 2pA range and zero check
disabled, allow the reading to settle until
the instrument is within specifications.
METER
E S - E IN
E IN
I =
R S
(VOLTAGE
BURDEN)
-14
C) and measure as high
∫
Q
S
∫
i dt
id t
ss = ------- -
C
NOTE