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XTR105U Datasheet(PDF) 10 Page - Burr-Brown (TI) |
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XTR105U Datasheet(HTML) 10 Page - Burr-Brown (TI) |
10 / 15 page 10 ® A typical two-wire RTD application with linearization is shown in Figure 1. Resistor RLIN1 provides positive feed- back and controls linearity correction. RLIN1 is chosen ac- cording to the desired temperature range. An equation is given in Figure 1. In three-wire RTD connections, an additional resistor, RLIN2, is required. As with the two-wire RTD application, RLIN1 provides positive feedback for linearization. RLIN2 provides an offset canceling current to compensate for wiring resis- tance encountered in remotely located RTDs. RLIN1 and RLIN2 are chosen such that their currents are equal. This makes the voltage drop in the wiring resistance to the RTD a common- mode signal which is rejected by the XTR105. The nearest standard 1% resistor values for RLIN1 and RLIN2 should be adequate for most applications. Table I provides the 1% resistor values for a three-wire Pt100 RTD connection. If no linearity correction is desired, the VLIN pin should be left open. With no linearization, RG = 2500 • VFS, where VFS = full-scale input range. RTDs The text and figures thus far have assumed a Pt100 RTD. With higher resistance RTDs, the temperature range and input voltage variation should be evaluated to ensure proper common-mode biasing of the inputs. As mentioned earlier, RCM can be adjusted to provide an additional voltage drop to bias the inputs of the XTR105 within their common-mode input range. ERROR ANALYSIS Table II shows how to calculate the effect various error sources have on circuit accuracy. A sample error calculation for a typical RTD measurement circuit (Pt100 RTD, 200 °C mea- surement span) is provided. The results reveal the XTR105’s excellent accuracy, in this case 1.1% unadjusted. Adjusting resistors RG and RZ for gain and offset errors improves circuit accuracy to 0.32%. Note that these are worst case errors; guaranteed maximum values were used in the calculations and all errors were assumed to be positive (additive). The XTR105 achieves performance which is difficult to obtain with discrete circuitry and requires less space. OPEN-CIRCUIT PROTECTION The optional transistor Q2 in Figure 3 provides predictable behavior with open-circuit RTD connections. It assures that if any one of the three RTD connections is broken, the XTR105’s output current will go to either its high current limit ( ≈27mA) or low current limit (≈2.2mA). This is easily detected as an out-of-range condition. FIGURE 3. Three-Wire Connection for Remotely Located RTDs. Resistance in this line causes a small common-mode voltage which is rejected by XTR105 . OPEN RTD TERMINAL I O 1 2 3 ≈ 2.2mA ≈27mA ≈2.2mA RTD (R LINE2)(RLINE1) R Z (1) R LIN2 (1) R LIN1 (1) (R LINE3) 2 1 3 0.01µF R CM = 1000Ω 0.01µF Q 2 (2) 2N2222 NOTES: (1) See Table I for resistor equations and 1% values. (2) Q 2 optional. Provides predictable output current if any one RTD connection is broken: 13 4 3 2 R G XTR105 7 6 (1) R G R G V IN – V IN + V LIN I R1 I R2 V REG V+ I RET I O E B 8 9 Q 1 I O I O 14 11 12 1 10 EQUAL line resistances here creates a small common-mode voltage which is rejected by XTR105. |
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