Measurement Considerations; Path Isolation; Figure 2-33 Voltage Attenuation By Path Isolation Resistance - Keithley 7074 Instruction Manual

SECTION2
Operation
mediately after the WAIT statement that defines the set-
tling time.
PROGRAM
10 REMOTE 718
20 FOR1.65T072
30 FORJ-iTOi2
40 OUTPUT 718 ; "C';CHR$(Il;J;'X
50 WAIT1
60 REM USER'S MEASUREMENT
CODE
70 OUTPUT718 :'N";CHR$(I);J;'X
60 NEXTJ
90 NEXT
100 END
COMMENTS
Put 707 in remote.
Loop for A-H ASCII values.
Loop for all 12 inputs.
Close row CHR$(I), input J.
Wait one second senling time.
Insert meawement code here.
Open row CHR$(I), input J.
Loop back for neti channel.
Loop back lor "ext bank.
2.9 MEASUREMENT CONSIDERATIONS
Many measurements made with the Model 7074 are sub-
ject to various types of noise that can affect measurement
accuracy. The following paragraphs discuss possible
noise sources that might affect these measurements and
ways to minimize their effects.
2.9.1 Path Isolation
The path isolation is simply the equivalent impedance
between any two test paths in a measurement system.
Ideally, the path isolation should be infinite, but the ac-
tual resistance and distributed capacitance of cables and
connectors results in less than infinite path isolation val-
ues for these devices.
Path isolation resistance forms a signal path that is in par-
allel with the equivalent resistance of the DUT, as shown
in Figure Z-32. For low-to-medium device resistance val-
ues, path isolation resistance is seldom a consideration;
however, it can seriously degrade measurement accu-
racy when testing high-impedance devices. The voltage
measured across such a device, for example, can be sub-
stantially attenuated by the voltage divider action of the
device source resistance and path isolation resistance, as
shown in Figure 2-33. Also, leakage currents can be gen-
erated through these resistances by voltage sources in the
system.
Any differential isolation capacitance affects dc measure-
ment settling time as well as ac measurement accuracy.
Thus, it is often important that such capacitance be kept
----
r----i r _
7 r-z--
-l
I I
II 1 I I
I
L----J
DUT
Multiplexer
Cd
M@%lJre
Instrument
RDur = Source Resistance of DUT
Eour = Source Resistance of DUT
RpnTH = Path Isolation Resistance
KIN = Input Resistance of Measuring Instrument
E
E DUT RPATH
OUT =
ROUT + RPATH
Figure 2-33. Voltage Attenuation by Path Isolation Re
sistance
as low as possible. Although the distributed capacitance
of the multiplexer card is generally fixed by design, there
is one area where you do have control over the capaci-
tance in your test system: the connecting cables. To mini-
mize capacitance, keep all cables as short as possible.
The effects of path resistance and capacitance can be
minimized by using guarding whenever possible. Para-
graph 2.9.7 discusses guarding in more detail.
2-34