VXI VT1422A - Manual (Page 113)

Programming the VT1422A for Data Acquisition and Control 111 Chapter 4
Linking Resistance
Measurements
To link channels to the resistance EU conversion send the
[SENSe:]FUNCtion:RESistance
<excite_current>,[<range>,](@<ch_list>) command.
Resistance measurements assume that there is at least one Current Source
SCP installed (eight current sources per SCP). See Figure 4-5.
•
The parameter is used only to tell the EU conversion
what the Current Source SCP channel is now set to.
is specified in amps dc and the choices for the VT1505A SCP are
30e-6 (or MIN) and 488e-6 (or MAX). Select 488 µA for measuring
resistances of less than 8,000
Ω. Select 30 µA for resistances of
8,000
Ω and above.
•
The optional parameter can be used to choose a fixed A/D
range. When not specified (defaulted), the module uses auto-range.
•
The parameter specifies which channel(s) to link to the
resistance EU conversion. These channels will sense the voltage across
the unknown resistance. Each can be a Current Source SCP channel
(a two-wire resistance measurement) or a sense channel separate from
the Current Source SCP channel (a four-wire resistance measurement).
See Figure 4-5 for diagrams of these measurement connections.
* Because of the 150 ohm resistor in series with each of the
current source outputs, Two-Wire resistance and temperature
measurements will have a 300 ohm offset.
** The current source Hi terminal is the negative voltage node.
The current source LO terminal is the positive voltage node.
Two-Wire Measurement
(not recommended **)
Current Source SCP
Field Wiring
Sens
e
C
h
annel
*150 ohm 5%
*150 ohm 5%
R
Four-Wire Measurement
Current Source SCP
Field Wiring
HI
LO
HI
LO
R
Any Sense SCP
Sens
e
C
h
annel
HI
LO
Figure 4-5. Resistance Measurement Sensing