2-, 3- and 4-Wire Bridges
It is not always necessary to locate the two resistors of the right-side half-bridge at the sensors. For negligible voltage drop in the bridge drive lines, an accurate half-bridge voltage is duplicated on the instrumentation system circuit-board (as Vbr /2) and provided for the AIN- inputs, usually through configuration jumpers on the board. This half-bridge voltage is measured through a dedicated channel and used as an offset for bridge-based sensors. By using the on-board half-bridge, only one sensor output line (AIN+) and the two bridge supply lines need to be run to each sensor bridge.
For full-bridge sensors, both AIN+ and AIN– are run from the sensor, and the bridge voltage is measured on the acquisition board. For negligible voltage drops in the bridge wires, these wiring schemes are satisfactory.
For non-negligible voltage drop in the bridge supply wires, 4-wire sensing is required. Four-wire or Kelvin sensing is the most accurate and uses separate pairs of bridge drive and sense lines.
RTD Temperature Sensors
RTDs (resistance-temperature devices) are based on the repeatable temperature coefficients (TCs) of metals such as platinum. RTDs are somewhat nonlinear and require correction. Standard RTD curves specify resistance as a function of temperature, such as the PT100 (DIN 43760) curve for platinum RTDs. A TC of resistance for two points, at 0oC and 100oC is designated as α:
For the PT100 curve, α = 3.850x10- 3/oC. But α is not constant over the full temperature range. The general RTD equation is:
where R0 is the resistance at 0oC (100 Ω or 1 kΩ) Solving for T,
From -100oC to +800oC (the workable range for suitably-encapsulated RTDs), 100 Ω RTD resistance varies by about x 6.48, from 60.25 Ω to 390.26 Ω, with a positive TC.
Typical 1 kΩ thin-film RTDs are the Sensing Devices, Inc. (SDI) GR2141 and the Minco S251PF12 (or as thermal ribbons, S17624PF440B). The SDI Pt100/15P has an R0 of 100 Ω and S251PF12 of 1 kΩ.
Unlike load cells, RTD bridges use only one sensor, as shown below, and are suited for single-ended bridge circuits, as shown. AGND is the analog ground, a separate ground connection at the measurement system to the system ground.
Thermocouples are formed when two dissimilar metals are joined, as in a spot-weld. A small voltage will occur across the two metals that varies with junction temperature. K-type (chromel-alumel) or J-type (iron-constantan) thermocouples are common and useful for measuring temperatures too high for RTDs and thermistors.
K-type thermocouples are not as sensitive as J-type but have a higher temperature range. Each connection made to thermocouple wire is another thermocouple sensor. Using copper wire, the copper-chromel and copper-alumel connections form two additional thermocouples. These undesired thermocouples are called reference-junction or cold-junction thermocouples. Their effects must be nulled out by some means of compensation.
By running the thermocouple wire to the instrumentation board connector, the reference junctions will be near the thermocouple processing circuitry and will be at about the same temperature. The cold-junction compensation circuitry measures this temperature and compensates the thermocouple circuit output.
A separate temperature sensor could be used to measure ambient temperature near the cold junctions, and compensation done in the computer.
Thermocouple integrated circuits (ICs) that amplify and cold-junction compensate thermocouple voltages are the Analog Devices Inc. AD595, for K (chromel-alumel) thermocouples, and AD594 output for J (iron-constantan) thermocouples. Their outputs are
To extend the measurement range to the high end of 1250oC (K-type) and 750o (J-type), the output voltage may need to be divided, say, by 3 to accommodate a typical 4.1 V fs range of the ADC.