Keithley 6517A, Руководство пользователя

Страница: 78 / 343

Front Panel Operation
2-57
signal level is still within the selected range). For details on
these display messages, see paragraph 2.3.2.
For the ohms function, each measurement range has a lower
reading limit that is one decade below the selected range. For
example, the 20M
Ω
range has a lower reading limit of 2M
Ω
.
Measuring a device that is less than 2M
Ω
will cause the UN-
DERFLOW message to be displayed. See paragraphs 2.3.2
and 2.7 (Ohms Ranges) for more information.
With AUTO range selected, the instrument will automatical-
ly go to the most sensitive (optimum) range to make the mea-
surement. Note that with AUTO range selected for the ohms
function, the instrument cannot go to the 2T
Ω
, 20T
Ω
or
200T
Ω
ranges since a hazardous voltage level (400V) may
be selected by the instrument. You must select these ohms
ranges manually.
For the amps, ohms and coulombs function, you can set au-
torange limits to speed up the autoranging process. Setting
limits eliminates upper and/or lower ranges from the autor-
ange search. This speeds up the measurement process. These
limits are set from the AUTORANGE option of the appropri-
ate function configuration menu.
2.12.2 Display resolution
The Model 6517A can display readings at 3.5, 4.5, 5.5 or 6.5
digit resolution. The display resolution of a reading depends
on the selected resolution setting (fixed or auto). The default
display resolution for every function is 5.5 digits. Table 2-18
summarizes the relationship between speed (SET-BY-RSLN
setting) and the selected resolution setting. With auto resolu-
tion selected, the instrument selects the optimum resolution
for the present speed (integration period setting). See Table
2-19. See paragraphs 2.5.2 (volts), 2.6.2 (amps), 2.7.2
(ohms) and 2.8.2 (coulombs) to set display resolution and
speed.
Input
HI
Current
Input
Common
Preamp
Out
Op Amp
+
-
Zero
Check
1
Ω
(Chassis)
LO
S
To Ranging
Amplifier
S
Q1
10M
Ω
Model 6517A
Figure 2-44
“Transdiode” logarithmic current configuration
NOTE
The circuit topology of Figure 2-44 works
for positive input currents only. For bipo-
lar input signals, an external offset bias
must be applied, or use a PNP transistor
for Q1.
2.11.6 Non-decade current gains
The Model 6517A electrometer input uses internal decade
resistance feedback networks for the current ranges. In some
applications, non-decade current gains may be desirable. As
shown in Figure 2-45, an external feedback resistor, R
FB
, can
be used to serve this purpose. Limitations on the magnitude
of the feedback current require that the value of R
FB
be
greater than 10
2
Ω .
2.12 Range and resolution
The range and resolution setting (fixed or auto) for each mea-
surement function are saved when changing functions.
2.12.1 Measurement range
The measurement range affects the accuracy of the measure-
ment as well as the maximum signal that can be measured.
The measurement ranges for each function are listed in the
specifications. The maximum input signal level for voltage,
current, and charge measurements is 105% of the measure-
ment range. For example, the maximum signal level on the
2V range is 2.1V (2V
× 1.05 = 2.1V). When the integrated
(average) input level exceeds the selected range, the OVER-
FLOW message will be displayed. However, if a stray out of
range transient (such as a noise spike) occurs, the message
OUT OF LIMIT will be displayed (assuming the integrated