Model 6487 Reference Manual Applications Guide G-3
Another way to reduce leakage currents is to use guarding. A guard is a conductor con-
nected to a low impedance point in the circuit that is nearly at the same potential as the
high impedance lead (the sensitive node) being guarded. Guarding can isolate the high-
impedance input lead of the picoammeter from leakage current due to voltage sources.
Guarding may also be necessary to prevent leakage current due to test fixturing.
Figure G-1 shows a high-megohm resistor supported on two insulators mounted in a metal
test fixture. This circuit is guarded by connecting the LO of the picoammeter to the metal
case. This will put the top of the insulator support post on the right at almost the same
potential as the bottom. The voltage difference is equal to the voltage burden of the
picoammeter. Since the top and the bottom of the insulator are at nearly the same poten-
tial, no significant current will flow through it and nearly all the current from the device
under test will flow through the picoammeter.
Figure G-1
Guarding to reduce leakage currents
Input bias current
An ideal picoammeter would read 0A with an open input. In practice, however, ammeters
do have some current that flows when the input is open. This current is known as the input
bias (offset) current. It should be noted that this current emanates from points within the
ammeter and flows through the internal ammeter circuitry, but it does not flow through the
DUT. The input bias current for the Model 6487 is included in the offset portion of the
accuracy specification.
6487
V-Source
+
-
6487
Picoammeter
Metal
Shield
Measured
Current
HI
LO
HI
LO
Equivalent Circuit
DUT
Test Fixture
Insulators
A
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