Guarding - Keithley 263 Instruction Manual

OPERATION
PASSIVE (V/R)
CURRENTSOURCE
MEIER
r____--___-__~
I
I I
I
L -----__ -__-__
---__-__-____
Figure 3-24. Burden Voltage Considerations
Table 3-5. Output Resistance of Passive Sources picoammeters is part of their normal calibration
routine. As the input voltage is adjusted to near
7
zero, the burden voltage becomes near zero. Thus,
Range
AMPS V/R / COUL V/R
Output Resistance (RJ
I I
2Oti
2mA
200 &A
20 fiA
2
ALA
200 nA
20 nA
2 nA
200 pA
20 pA
2p.4
Figure 3-25 shows the three conductors in a triax cable.
Source low is normally connected to the inner shield of
The additional error is added to the basic accuracy
the cable. In this unguarded configuration, leakage cur-
specification. The offset specification is not affected. If the
rent will flow through the insulator (represented by Q and
burden voltage is less than lOO,V, the basic accuracy
C,) separating source Hl from source LO. Whether or not
specification is not degraded.
this leakage current causes a problem depends on the
source and the load. For the following examples, assume
Rr is lOOG% This is the rated insulator leakage resistance
NOTE
of the supplied triax cable.
Adjusting the burden voltage of electrometers and
-
-
2opc
w
200nC
20nC
2nC
zoopc
2opc
1 kfl
10 kR
10 kR
100 khl
1MQ
lOM0
1OOMQ
1GR
10GO
lOOGO
100GR
at the time of meter calibration, burden voltage
can be adjusted to be better than its specification.
3.11.3 Guarding
Guarding uses a conductor at essentially the same poten-
tial seen at output high. The guard is supplied by a low-
impedance voltage source and is used to surround out-
put high. Maintaining the guard conductor at the same
potential as output high results in drastically reduced cable
leakage currents.
3-27