G-10
Applications Guide
Model 6487 Reference Manual
To summarize, error currents due to electrostatic coupling can be minimized by following
these guidelines:
•
Keep all charged objects (including people) and conductors away from sensitive
areas of the test circuit.
•
Avoid movement and vibration near the test area.
•
When measuring currents <10nA, shield the device under test by surrounding it
with a metal enclosure and connect the enclosure electrically to the test circuit
common terminal.
Shielding vs. Guarding
Shielding usually implies the use of a metallic enclosure to prevent electrostatic interfer-
ence from affecting a high impedance circuit. Guarding implies the use of an added low
impedance conductor, maintained at the same potential as the high impedance circuit,
which will intercept any interfering voltage or current. A guard does not necessarily pro-
vide shielding. (
“Leakage currents and guarding,” page G-2
.)
Making connections
To avoid measurement errors, it is critical to make proper connections from the picoam-
meter to the device under test. To make a proper connection,
always connect the high
resistance terminal of the meter to the highest resistance point of the circuit under
test.
shows a picoammeter connected to a current source that consists of a voltage
source in series with a resistor. An AC powered voltage source usually has a significant
level (often several volts) of line frequency common mode voltage.
Figure G-7
Connecting the HI terminal (picoammeter) to high resistance
+
-
6487
Picoammeter
Current Source
R
HI
LO
A
