Section 4: Basic measurements
Model 6517B Electrometer Reference Manual
4-16
6517B-901-01 Rev. C / August 2015
Note that as C
S
increases in value, Z
S
decreases in value, thereby increasing the noise gain. Again,
at the point where Z
S
= Z
F
, the input noise is amplified by a factor of two.
The maximum value of source capacitance (C
S
) for the Model 6517B ammeter is 10,000 pF. You can,
however, usually measure at higher source capacitance values by inserting a resistor in series with
the ammeter input, but remember that any series resistance increases the voltage burden by a factor
of I
IN
* R
SERIES
. For example, the range of resistance listed in the table of minimum recommended
source resistance values (see
Source resistance
(on page 4-14)) results in voltage burden values in
the range of 1 mV to 1 V. A useful alternative to a series resistor is a series diode, or two diodes in
parallel back-to-back. The diodes can be small-signal types and should be in a light-tight enclosure.
Guarding
For current measurements, guarding is used to drastically reduce leakage currents in high impedance
test circuits. Ammeter input LO (inner shield of the triaxial cable) is used as the guard.
High impedance current measurements
Significant leakage could occur across a high impedance (
1 G
) DUT through the insulators as
shown in the following figure, where R
L1
and R
L2
represent the leakage resistance. So instead of
measuring just the current (I
R
) through R, you are also measuring the leakage current (I
L
). The current
measured by the ammeter is I
R
+ I
L
.
Figure 39: High impedance current measurements - unguarded
By connecting ammeter input LO to the metal mounting (guard) plate, as shown in the following
figure, the leakage current (I
L
) is shunted to ammeter input LO and is not measured by the ammeter.
Thus, the ammeter only measures I
R
.
Figure 40: High impedance current measurements - guarded
