Front Panel Operation
2-5
The UNDERFLOW condition can usually be resolved by se-
lecting a lower ohms range or by using AUTO range. Para-
graph 2.7 (Ohms Ranges) covers range limits and explains
how to select the optimum range for ohms measurements.
OUT OF LIMITS — This message indicates that a momen-
tary or transient out-of-range condition appeared at the input,
even though the integrated (or average) signal was within the
full scale range of the A/D converter. It usually indicates that
there is too much noise on the input signal for a valid mea-
surement.
Generally, the OUT OF LIMITS condition can be eliminated
by better shielding of the signal source or by using other
noise reduction methods. Another solution is to select the
next higher range (or lower R range) to keep the transients
less than full scale.
The 2nA, 20nA, and 200nA ranges (and the R measurements
that use these current ranges) are particularly susceptible to
this condition because of the combination of speed and sen-
sitivity.
A sine wave signal riding on a dc bias level is used to dem-
onstrate an OUT OF LIMIT reading. Figure 2-A shows a
sine wave riding on a 20nA bias level. If this signal is mea-
sured on the 200nA range at normal speed, it would simply
read 20nA (which is the dc average). If however, you use the
20nA range, the positive peaks of the sine wave will be
clipped as shown in Figure 2-B. Clipping occurs at 110% of
full range (22nA on the 20nA range). Because of clipping,
the measurement of the input signal is significantly less than
20nA. To avoid these bad readings, the Model 6517A dis-
plays the OUT OF LIMITS message instead of the inaccu-
rate reading.
Note that the positive peaks of the input signal (which exceed
full scale) will not cause an OVERFLOW condition on the
20nA range since the average reading over the 16.67ms inte-
gration period is less than full scale.
The A/D hardware limit detection circuit can be disabled,
which in turn disables the OUT OF LIMITS message. How-
ever, the presence of OUT OF LIMIT readings may result in
measurements that are slightly, severely, or totally inaccu-
rate. Paragraph 2.19.7 explains how to disable the OUT OF
LIMIT message.
When both OVERFLOW and OUT OF LIMITS conditions
occur, the OVERFLOW message will be displayed.
2.3.3 Status and error messages
During Model 6517A operation and programming, you will
encounter a number of front panel messages. Typical messag-
es are either of status or error variety, as listed in Table 2-5.
The most recent status or error messages can be momentarily
displayed. Just enter a configuration menu or the main menu,
and press the PREV range key. (The display is blank if no
message is queued.)
Figure 2-A
Input signal
0
0
5
10
15
20
25
30
time 16.67ms
DCA Reading on 200 nA range = 20nA
nA
Figure 2-B
Measurement on 20nA range
0
0
5
10
15
20
25
30
time 16.67ms
DCA Reading = <20nA
22
nA
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