Example: Two-Wire Resistance Measurements; Example: Four-Wire Resistance Measurements - Agilent Technologies VXI bus E1476A User Manual

Example: Two-Wire
Resistance
Measurements
Example: Four-Wire
Resistance
Measurements
Chapter 2
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This example illustrates three-wire switching and a method for making
accurate 2-wire resistance measurements. The E1476A analog bus
contains current-limiting protection resistors that can introduce
measurement error if the measuring instrument does not have a suitable
high impedance. This example describes a method to make measurements
that avoids the protection resistors.
Making two-wire measurements from the analog bus requires the internal
bus protection resistors which will introduce about 100 Ω per lead to the
resistance measurement. Use Channel 00 as the common terminals to
make two-wire measurements from channels 01 through 31 on bank A.
Close tree relays VSA and VSB (channels 90 and 91) to connect bank B
channels (32 through 63) to channel 00 through the bank A bus. You can
now make two-wire measurements from 63 channels using channel 00 as
the common. No current-limiting resistors are in the path. See Figure 2-3
for a circuit diagram. A typical program sequence is:
&/26 #    
&/26 #FF
0DNH PHDVXUHPHQW
23(1 #FF
To make four-wire resistance measurements when switching (not
scanning), you must use a channel from bank A for the voltage sense and
close the voltage sense bank A tree relay (VSA). Also, you must use a
channel from bank B for the current source and you must close the current
source relay (CS) on bank B.
The VSA and CS relays connect the device under test to the analog bus.
Typically you pair the first channel of bank A with the first channel of bank
B (e.g. channels 00 and 32, 01 and 33, 02 and 34, etc.). See Figure 2-4 for
a circuit diagram. A typical program sequence follows.
NOTE
Using the SCAN command automatically pairs channels 00 and 32, 01 and
33, 02 and 34, etc. for four-wire measurements. See "Scanning
Applications".
&/26 #      !Channels 100 and 132 are a 4-wire pair
0DNH PHDVXUHPHQW LQ IRXUZLUH PRGH
23(1 # 
!Closes all paths through channel 00
!"cc" = channel to be measured (01 to 63)
!Open channel cc after measurement is made
!Open measurement channels except tree
!relays
Switchbox Applications 43