Datasheet AD693 (Analog Devices) - 10

AD693
increasing the application voltages by adding resistance between external voltage divider; the Aux-Amp is then used as a follower Pins 14 and 3 will decrease the temperature span. to make a stiff drive for the bridge. Similar applications with An external voltage divider may also be used in conjunction higher resistance sensors can use proportionally higher voltage. with the circuit shown to produce any range of temperature Finally, to accommodate the 2 mV/V sensitivity of the bridge, spans as well as providing zero output (4 mA) for a non 0 the full-scale span of the Signal Amplifier must be reduced. temperature input. For example, measuring VX with respect to a Using the load cell in both tension and compression with 1 V of voltage 2.385 times the excitation (rather than 2 times) will excitation, therefore, dictates that the span be adjusted to 4 mV. result in zero input to the Signal Amplifier when the RTD is at By substituting in the expression, RS1 = 400 Ω/[(30 mV/S) – 1], 100°C (or 138.5 Ω). the nominal resistance required to achieve this span is found to As suggested in Table I, the temperature span may also be adjusted be 61.54 Ω. Calculate the minimum resistance required by by changing the voltage span of the Signal Amplifier. Changing the subtracting 10% from 61.54 Ω to allow for the internal resistor gain from 2 to 4, for example, will halve the temperature span to tolerance of the AD693, leaving 55.38 Ω (See “Adjusting Input about 52°C on the 4-20mA output configuration. (See section Span.”) The standard value of 54.9 Ω is used with a 20 Ω “Adjusting Input Span.”) potentiometer for full-scale adjustment. The configuration for a three-wire RTD shown in Figure 17 can If a load cell with a precalibrated sensitivity constant is to be accommodate two-wire sensors by simply joining Pins 1 and 5 used, the resultant full-scale span applied to the Signal Amplifier is of the AD693. found by multiplying that sensitivity by the excitation voltage. (In Figure 18, the excitation voltage is actually (10 kΩ/62.3 kΩ) (6.2 V) = 0.995 V).
INTERFACING LOAD CELLS AND METAL FOIL STRAIN GAGES
The availability of the on-chip Voltage Reference, Auxiliary
THERMOCOUPLE MEASUREMENTS
Amplifier and 3 mA of excitation current make it easy to adapt The AD693 can be used with several types of thermocouple the AD693 to a variety of load cells and strain gages. inputs to provide a 4-20 mA current loop output corresponding to a variety of measurement temperature ranges. Cold junction The circuit shown in Figure 18 illustrates a generalized approach in compensation (CJC) can be implemented using an AD592 or which the full flexibility of the AD693 is required to interface to a AD590 and a few external resistors as shown in Figure 19. low resistance bridge. For a high impedance transducer the bridge can be directly powered from the 6.2 V Reference. From Table II simply choose the type of thermocouple and the appropriate average reference junction temperature to select Component values in this example have been selected to match values for RCOMP and RZ. The CJC voltage is developed across the popular standard of 2 mV/V sensitivity and 350 Ω bridge RCOMP as a result of the AD592 1 µA/K output and is added to resistance. Load cells are generally made for either tension and the thermocouple loop voltage. The 50 Ω potentiometer is compression, or compression only; use of the 12 mA zero tap biased by RZ to provide the correct zero adjustment range allows for operation in the tension and compression mode. An appropriate for the divider and also translates the Kelvin scale of optional zero adjustment is provided with values selected for the AD592 to °Celsius. To calibrate the circuit, put the +2% FS adjustment range. thermocouple in an ice bath (or use a thermocouple simulator Because of the low resistance of most foil bridges, the excitation set to 0) and adjust the potentiometer for a 4 mA loop current. voltage must be low so as not to exceed the available 4 mA zero The span of the circuit in °C is determined by matching the current. About 1 V is derived from the 6.2 V Reference and an signal amplifier input voltage range to its temperature equivalent Figure 18. Utilizing the Auxiliary Amplifier to Drive a Load Cell, 12 mA ± 8 mA Output –10– REV. A