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 <excite_current> parameter is used only to tell the EU conversion
what the Current Source SCP channel is now set to. <excite_current>
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 <range> parameter can be used to choose a fixed A/D
range. When not specified (defaulted), the module uses auto-range.
•
The <ch_list> 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