Keithley 6485 - Figure I-5 Shielding a High Impedance Device

Model 6485 Picoammeter Instruction Manual Applications Guide I-9
Figure I-5
Shielding a high impedance device
The cabling in the circuit also requires shielding. Capacitive coupling between an electro-
static noise source and the signal conductors or cables can be greatly reduced by surround-
ing those conductors with a grounded metal shield, as shown in Figure I-6. With this
shield in place, the noise current generated by the electrostatic voltage source and the cou-
pling capacitance flows through the shield to ground rather that through the signal conduc-
tors.
Figure I-6
Electrostatic shielding
To summarize, error currents due to electrostatic coupling can be minimized by following
these guidelines:
A
+
-
6485
Picoammeter
Metal
Shield
R
V
HI
LO
HI
LO
V
Noise
current
Shield
Shield-to-cable
capacitance
Source-to-shield
capacitance
Electrostatic
voltage source
Ground-referenced
signal conductor
Ground