Front Panel Operation
2-24
Not shown in Figure 2-24 is the outer shield of the triax cable
which is connected to chassis ground. The leakage between
the inner shield and the outer shield is of no consequence
since that current is supplied by the low impedance source,
rather than by the signal itself.
2.6
Current measurements
The Model 6517A can make current measurements from
10aA to 21mA.
2.6.1 Basic measurement procedure
To achieve optimum precision for low-level current mea-
surements, input bias current and voltage burden can be min-
imized by performing the offset adjustment procedures in
paragraph 2.19.3 (OFFSET-ADJ).
NOTE
After measuring high voltage in the volts
function, it may take a number of minutes
for input current to drop to within speci-
fied limits. Input current can be verified by
placing the protection cap on the INPUT
triax connector and then connecting a
jumper between COMMON and chassis
ground. With the instrument on the 20pA
range and zero check disabled, allow the
reading to settle until the input bias current
is within specifications.
Perform the following steps to measure current:
NOTE
To ensure proper operation, always enable
zero check ("ZeroCheck" displayed) be-
fore changing functions (V, I, R, or Q).
The Z-CHK key controls zero check.
1. With zero check enabled (“ZeroCheck” displayed), se-
lect the amps (I) function. The Z-CHK key toggles zero
check between the on and off states.
NOTE
The input circuit configuration changes
with zero check enabled. See paragraph
2.13 for details.
2. To achieve optimum accuracy for low current measure-
ments, it is recommended that you zero correct the in-
strument. To do so, select the lowest measurement range
(20pA) and press REL. The REL indicator turns on and
the “ZCor” message is displayed. Correcting zero on the
lowest range will correct all ranges because of internal
scaling.
3. Select a manual measurement range that is consistent
with the expected reading, or enable auto range (see
paragraph 2.12 for detailed range information).
4. Connect the Model 6517A to the current to be mea-
sured. Figure 2-25 shows typical connections for current
measurements.
NOTE
If measuring current in a floating circuit
where significant leakage may exist be-
tween the ammeter input and circuit low,
connect the Model 6517A to the circuit as
shown in Figure 2-26. Notice that amme-
ter input LO is connected to circuit high.
Paragraph 2.6.3 (Guarding; Floating Cur-
rent Measurements) explains how this
guarding technique affects the measure-
ment. Also note that a safety shield should
be used if the input of the ammeter is float-
ing at a hazardous voltage level
(V
F
≤
30V).
5. Press Z-CHK to disable zero check and take a reading
from the display.
NOTE
To disable zero correct, enable zero check
and press REL.
Inner Shield
Triax Cable
R
S
HI
6517A
Input
LO
E
S
R
L
C
L
Source
Center
Conductor
A = I
Guard
15k
Ω
Figure 2-24
Guarded voltage measurements