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XTR105UG4 Datasheet(PDF) 10 Page - Texas Instruments |
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XTR105UG4 Datasheet(HTML) 10 Page - Texas Instruments |
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10 / 23 page  10 SBOS061B www.ti.com XTR105 In 3-wire RTD connections, an additional resistor, RLIN2, is required. As with the 2-wire RTD application, RLIN1 provides positive feedback for linearization. RLIN2 provides an offset canceling current to compensate for wiring resistance en- countered 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 that 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 3-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 com- mon-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 See Table II for 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 measurement 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; ensured maximum values were used in the calculations and all errors were assumed to be positive (additive). The XTR105 achieves performance that 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. Remotely Located RTDs with 3-Wire Connection. Resistance in this line causes a small common-mode voltage which is rejected by the 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 0.01 µF R CM = 1000Ω 0.01 µF 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 the XTR105. |
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Similar Description - XTR105UG4 |
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