Model 6517B Electrometer Reference Manual
Section 4: Basic measurements
6517B-901-01 Rev. C / August 2015
4-9
Guarding the circuit minimizes these effects by driving the inner shield of the triaxial cable at signal
potential, as shown in the following figure. Here, a unity gain amplifier with a high input impedance
and low output impedance is used. Since the center conductor (HI) and the inner shield (Guard) of
the cable are at virtually the same potential, the potential across R
L
is zero, so no current flows. Also,
with a zero potential across C
L
, there is no capacitor charging process to slow down the
measurement response.
Not shown in the figure is the outer shield of the triaxial cable, which is connected to chassis ground.
The leakage between the inner shield and the outer shield is of no consequence because that current
is supplied by the low impedance source, rather than by the signal itself.
Figure 33: Guarded voltage measurements
Current measurements
The Model 6517B can make current measurements from 10 aA to 21 mA.
