Keithley 6487 Reference Manual

Model 6487 Reference Manual Applications Guide G-9
Figure G-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 (Figure G-6). With this shield in place,
the noise current generated by the electrostatic voltage source and the coupling capaci-
tance flows through the shield to ground rather than through the signal conductors.
Figure G-6
Electrostatic shielding
A
+
-
6487
V-Source
6487
Picoammeter
Measured
Current
(I)
Metal
Shield
HI
LO
HI
LO
High Impedance
DUT
Equivalent Circuit
V
Noise
current
Shield
Shield-to-cable
capacitance
Source-to-shield
capacitance
Electrostatic
voltage source
Ground-referenced
signal conductor
Ground

Keithley 6487 Specifications

General

Type	Picoammeter/Voltage Source
Current Measurement Range	20 fA to 20 mA
Interfaces	GPIB, RS-232
Voltage Burden	< 200 µV
Input Impedance	>10^14 Ω
Power	100 V / 120 V / 220 V / 240 V
Basic Current Measurement Accuracy	0.4%