Sensoray 7429 - Manual (Page 35)

T HERMISTORS
Thermistors (type 10K) have much higher resistance values than RTD's over most of
their operating range, therefore a connection identical to the RTD two-wire configura-
tion is recommended for most applications.
If you are using a thermistor to measure temperatures near the high end of it's range,
however, it may be advisable to implement a three- or four-wire hookup to prevent error
due to lead-loss effects. As with RTD's, thermistors should be connected using shielded
cable. The shield should be connected only to the 7429 channel SHIELD pin.
S TRAIN AND P RESSURE G AGES
In a typical strain/pressure gage, four wires connect the gage to the gage interface sys-
tem. Two wires supply gage excitation, while the other two wires carry the gage output
signal. The 7429 is designed to support gages requiring constant voltage excitation.
Gage input impedances may range from 120 ohms on up, although most standard gages
have nominal impedances of either 120 or 350 ohms. Like RTD's, strain and pressure
gages should use shielded cable. The shield should be connected only to the channel
SHIELD pin on the 7429.
Strain/pressure gage connections
4 TO 20 M ILLIAMP C URRENT L OOPS
Coprocessor firmware supports 4-20 mA current source inputs on any channel; how-
ever, a 250-ohm 0.01% resistor must be installed on the channel inputs as shown in the
following diagram. These resistors are available as an option for the 7429. Order Sen-
soray PN 7408R.
Sense+
Sense-
Power+
Power-
Shield