Keithley 6485 User Manual

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

Keithley 6485 Specifications

General

Model	Keithley 6485
Category	Picoammeter
Input Impedance	>10^14 Ohms
Voltage Burden	<200µV
Accuracy	0.4% of reading
Voltage Source Range	Not Applicable
Voltage Source Resolution	Not Applicable
Voltage Source Accuracy	Not Applicable
Interface	IEEE-488
Power Requirements	100V to 240V AC, 50/60Hz
Dimensions	89 mm high x 214 mm wide x 369 mm deep (3.5 in x 8.4 in x 14.5 in)