Model 6517B Electrometer Reference Manual
Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015
4-17
Floating current measurements
As discussed previously, guarding uses a conductor at essentially the same potential as the sensitive
input to drastically reduce leakage currents in high impedance test circuits. No current can flow when
there is a 0 V drop across a leakage resistance.
For floating current measurements, ammeter input low is used as the guard since it totally surrounds
input high (through the input triaxial cable) and it is at nearly the same potential as input high. In
reality, the ammeter drops <1 mV and is known as the voltage burden.
The following figure shows an unguarded floating current measurement in a high impedance circuit.
The goal is to measure the current (I
R
) through resistor R. However, a leakage path (R
L
) exists from
ammeter input LO to test circuit common. Since the ammeter drops < 1 mV, approximately 10 V is
dropped by R
L
. The current through R
L
is approximately 10 nA (10 V/1 G
= 10 nA). Thus, the current
that is measured by the Model 6517B is the sum of the two currents (I = I
R
+ 10 nA). Obviously, if I
R
is
a low-level current, the 10 nA leakage current corrupts the measurement.
Figure 41: Floating current measurements - unguarded
The following figure shows the guarded version of the same circuit. Notice that the only difference is
that the connections to the electrometer are reversed. Resistor R
L
now represents the leakage from
ammeter input HI to ammeter input LO, and resistor R
G
represents the leakage from ammeter input
LO (guard) to test circuit common. As previously mentioned, the ammeter drops < 1 mV. It then
follows that there is a < 1 mV drop across R
L
. Thus, the current through R
L
is < 1 pA (<
1 mV/1 G
= < 1 pA). The current that is measured by the Model 6517B is the sum of the two
currents (I = I
R
+ < 1 A). The use of guarding reduced the leakage current from 10 nA to < 1 pA. Note
that the 10 nA leakage current (I
G
) from ammeter input LO to test circuit low still exists, but it is of no
consequence since it is not measured by the Model 6517B.
Figure 42: Floating current measurements - guarded
