LTC2983
29
2983fc
For more information www.linear.com/LTC2983
APPLICATIONS INFORMATION
(3) Sensor Configuration
The sensor configuration field is used to define various
RTD properties. Configuration bits B20 and B21 determine
if the RTD is a 2, 3, or 4 wire type (see Table 28).
The simplest configuration is the 2-wire configuration.
While this setup is simple, parasitic errors due to IR drops
in the leads result in systematic temperature errors. The
3-wire configuration cancels RTD lead resistance errors
(if the lines are equal resistance) by applying two matched
current sources to the RTD, one per lead. Mismatches in
the two current sources are removed through transparent
background calibration. 4-wire RTDs remove unbalanced
RTD lead resistance by measuring directly across the
sensor using a high impedance Kelvin sensing. 4-wire
measurements with Kelvin R
SENSE
are useful in applica-
tions where sense resistor wiring parasitics can lead to
errors; this is especially useful for low resistance PT-10
type RTDs. In this case, both the RTD and sense resistor
have Kelvin sensing connections.
The next sensor configuration bits (B18 and B19) deter-
mine the excitation current mode. These bits are used to
enable R
SENSE
sharing, where one sense resistor is used
for multiple 2-, 3-, and/or 4-wire RTDS. In this case, the
RTD ground connection is internal and each RTD points
to the same R
SENSE
channel.
Table 28. RTD Sensor Configuration Selection
(3) SENSE
CONFIGURATION
MEASUREMENT MODE BENEFITS
NUMBER
OF WIRES
EXCITATION
MODE
NUMBER
OF WIRES
GROUND
CONNECTION
CURRENT
SOURCE
ROTATION
SENSE
RESISTOR
SHARING
RTDs
POSSIBLE
PER
DEVICE
CANCELS RTD
MATCHED
LEAD
RESISTANCE
CANCELS RTD
MISMATCH
LEAD
RESISTANCE
CANCELS
PARASITIC
THERMOCOUPLE
EFFECTS
CANCELS
R
SENSE
LEAD
RESISTANCE
B21 B20 B19 B18
0 0 0 0 2-Wire External No No 5
0 0 0 1 2-Wire Internal No Yes 9
0 1 0 0 3-Wire External No No 5 •
0 1 0 1 3-Wire Internal No Yes 9 •
0 1 1 X Reserved
1 0 0 0 4-Wire External No No 4 • •
1 0 0 1 4-Wire Internal No Yes
6 • •
1 0 1 0 4-Wire Internal Yes Yes 6 • • •
1 0 1 1 Reserved
1 1 0 0 4-Wire,
Kelvin
R
SENSE
External No No 4 • • •
1 1 0 1 4-Wire,
Kelvin
R
SENSE
Internal No Yes 5 • • •
1 1 1 0 4-Wire,
Kelvin
R
SENSE
Internal Yes Yes 5 • • • •
1 1 1 1 Reserved
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