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XTR106UA Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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XTR106UA Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 15 page 10 ® XTR106 R 1 ≈ 5V • R B 4 • V TRIM EXTERNAL TRANSISTOR External pass transistor, Q1, conducts the majority of the signal-dependent 4-20mA loop current. Using an external transistor isolates the majority of the power dissipation from the precision input and reference circuitry of the XTR106, maintaining excellent accuracy. Since the external transistor is inside a feedback loop its characteristics are not critical. Requirements are: VCEO = 45V min, β = 40 min and P D = 800mW. Power dissipation require- ments may be lower if the loop power supply voltage is less than 36V. Some possible choices for Q1 are listed in Figure 1. The XTR106 can be operated without an external pass transistor. Accuracy, however, will be somewhat degraded due to the internal power dissipation. Operation without Q1 is not recommended for extended temperature ranges. A resistor (R = 3.3k Ω) connected between the I RET pin and the E (emitter) pin may be needed for operation below 0 °C without Q1 to guarantee the full 20mA full-scale output, especially with V+ near 7.5V. The low operating voltage (7.5V) of the XTR106 allows operation directly from personal computer power supplies (12V ±5%). When used with the RCV420 Current Loop Receiver (Figure 8), load resistor voltage drop is limited to 3V. BRIDGE BALANCE Figure 1 shows a bridge trim circuit (R1, R2). This adjust- ment can be used to compensate for the initial accuracy of the bridge and/or to trim the offset voltage of the XTR106. The values of R1 and R2 depend on the impedance of the bridge, and the trim range required. This trim circuit places an additional load on the VREF output. Be sure the additional load on VREF does not affect zero output. See the Typical Performance Curve, “Under-Scale Current vs IREF + IREG.” The effective load of the trim circuit is nearly equal to R2. An approximate value for R1 can be calculated: (3) where, RB is the resistance of the bridge. VTRIM is the desired ±voltage trim range (in V). Make R2 equal or lower in value to R1. LINEARIZATION Many bridge sensors are inherently nonlinear. With the addition of one external resistor, it is possible to compensate for parabolic nonlinearity resulting in up to 20:1 improve- ment over an uncompensated bridge output. Linearity correction is accomplished by varying the bridge excitation voltage. Signal-dependent variation of the bridge excitation voltage adds a second-order term to the overall transfer function (including the bridge). This can be tailored to correct for bridge sensor nonlinearity. Either positive or negative bridge non-linearity errors can be compensated by proper connection of the Lin Polarity pin. To correct for positive bridge nonlinearity (upward bowing), Lin Polarity (pin 12) should be connected to IRET (pin 6) as shown in Figure 3a. This causes VREF to increase with bridge output which compensates for a positive bow in the bridge response. To correct negative nonlinearity (downward bow- ing), connect Lin Polarity to VREG (pin 1) as shown in Figure 3b. This causes VREF to decrease with bridge output. The Lin Polarity pin is a high impedance node. If no linearity correction is desired, both the RLIN and Lin Polarity pins should be connected to VREG (Figure 3c). This results in a constant reference voltage independent of input signal. RLIN or Lin Polarity pins should not be left open or connected to another potential. RLIN is the external linearization resistor and is connected between pin 11 and pin 1 (VREG) as shown in Figures 3a and 3b. To determine the value of RLIN, the nonlinearity of the bridge sensor with constant excitation voltage must be known. The XTR106’s linearity circuitry can only compen- sate for the parabolic-shaped portions of a sensor’s nonlinearity. Optimum correction occurs when maximum deviation from linear output occurs at mid-scale (see Figure 4). Sensors with nonlinearity curves similar to that shown in LOOP POWER SUPPLY The voltage applied to the XTR106, V+, is measured with respect to the IO connection, pin 7. V+ can range from 7.5V to 36V. The loop supply voltage, VPS, will differ from the voltage applied to the XTR106 according to the voltage drop on the current sensing resistor, RL (plus any other voltage drop in the line). If a low loop supply voltage is used, RL (including the loop wiring resistance) must be made a relatively low value to assure that V+ remains 7.5V or greater for the maximum loop current of 20mA: (2) It is recommended to design for V+ equal or greater than 7.5V with loop currents up to 30mA to allow for out-of- range input conditions. V+ must be at least 8V if 5V sensor excitation is used and if correcting for bridge nonlinearity greater than +3%. R L max = (V +)– 7.5V 20mA –R WIRING 8 XTR106 0.01µF E I O I RET V+ 10 7 6 R Q = 3.3kΩ For operation without external transistor, connect a 3.3k Ω resistor between pin 6 and pin 8. See text for discussion of performance. FIGURE 2. Operation without External Transistor. |
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