XTR106: 4-20mA Current Transmitter with Bridge Excitation And ...

APPLICATIONS INFORMATIONFigure 1 shows the basic connection diagram for the XTR106.The loop power supply, V PS , provides power for all circuitry.Output loop current is measured as a voltage across the seriesload resistor, R L . A 0.01µF to 0.03µF supply bypass capacitorconnected between V+ and I O is recommended. For applicationswhere fault and/or overload conditions might saturatethe inputs, a 0.03µF capacitor is recommended.A 2.5V or 5V reference is available to excite a bridge sensor.For 5V excitation, pin 14 (V REF 5) should be connected to thebridge as shown in Figure 1. For 2.5V excitation, connectpin 13 (V REF 2.5) to pin 14 as shown in Figure 3b. The outputterminals of the bridge are connected to the instrumentationamplifier inputs, V IN and V IN . A 0.01µF capacitor is shown+ –connected between the inputs and is recommended for highimpedance bridges (> 10kΩ). The resistor R G sets the gainof the instrumentation amplifier as required by the full-scalebridge voltage, V FS .Lin Polarity and R LIN provide second-order linearizationcorrection to the bridge, achieving up to a 20:1 improvementin linearity. Connections to Lin Polarity (pin 12) determinethe polarity of nonlinearity correction and should be connectedeither to I RET or V REG . Lin Polarity should be connectedto V REG even if linearity correction is not desired.R LIN is chosen according to the equation in Figure 1 and isdependent on K LIN (linearization constant) and the bridge’snonlinearity relative to V FS (see “Linearization” section).The transfer function for the complete current transmitter is:I O = 4mA + V IN • (40/R G ) (1)V IN in Volts, R G in Ohmswhere V IN is the differential input voltage. As evident fromthe transfer function, if no R G is used (R G = ∞), the gain iszero and the output is simply the XTR106’s zero current.A negative input voltage, V IN , will cause the output currentto be less than 4mA. Increasingly negative V IN will cause theoutput current to limit at approximately 1.6mA. If current isbeing sourced from the reference and/or V REG , the currentlimit value may increase. Refer to the Typical PerformanceCurves, “Under-Scale Current vs I REF + I REG ” and “Under-Scale Current vs Temperature.”Increasingly positive input voltage (greater than the fullscaleinput, V FS ) will produce increasing output currentaccording to the transfer function, up to the output currentlimit of approximately 28mA. Refer to the Typical PerformanceCurve, “Over-Scale Current vs Temperature.”The I RET pin is the return path for all current from thereferences and V REG . I RET also serves as a local ground andis the reference point for V REG and the on-board voltagereferences. The I RET pin allows any current used in externalcircuitry to be sensed by the XTR106 and to be included inthe output current without causing error. The input voltagerange of the XTR106 is referred to this pin.For 2.5V excitation, connectpin 13 to pin 14V REF 55V REF 2.5V REGR LIN(3)141311+ 1V INR LIN 10 VREGV+7.5V to 36VPossible choices for Q 1 (see text).TYPE2N4922TIP29CTIP31CPACKAGETO-225TO-220TO-220R 2(5)R 1(5)BridgeSensor5V+ R B –C IN0.01µF (2)4(4)R G32R GR GXTR106Lin (1)–V PolarityINI RET12BI OE798Q 1C OUT0.01µFI O4-20 mAV OR L+V PS–6I = 4mA + V IN • (40O)V (1)R GREGorNOTES:(1) Connect Lin Polarity (pin 12) to I RET (pin 6) to correct for positivebridge nonlinearity or connect to V REG (pin 1) for negative bridgenonlinearity. The R LIN pin and Lin Polarity pin must be connected toV REG if linearity correction is not desired. Refer to “Linearization”section and Figure 3.(2) Recommended for bridge impedances > 10kΩ4B( 3) R LIN = K LIN •(K LIN in Ω)1 – 2B(4) R G = (V FS /400µA) •1 + 2B1 – 2B(V FS in V)where K LIN = 9.905kΩ for 2.5V referenceK LIN = 6.645kΩ for 5V referenceB is the bridge nonlinearity relative to V FSV FS is the full-scale input voltage(5) R 1 and R 2 form bridge trim circuit to compensate for the initialaccuracy of the bridge. See “Bridge Balance” text.FIGURE 1. Basic Bridge Measurement Circuit with Linearization.XTR106 9SBOS092Awww.ti.com