Features Low Total Unadjusted Error 2.5V, 5V Bridge Excitation Reference 5.1V Regulator Output Low Range Drift: ±25ppm/°C Max Low Offset Drift: 0.25μV/°C High psr: 110db min High CMR: 86dB Min Wide Power Supply Range: 7.5V to 36V 14-pin DIP and SO-14 Surface Mount Applications Pressure Bridge Transmitters Strain Gauge Transmitters Temperature Bridge Transmitters Industrial Process Control
scada remote data acquisition remote sensor weighing system accelerometer description
The XTR106 is a low cost, monolithic 4-20 mA 2-wire current transmitter designed for bridge sensors. It provides full-bridge excitation (2.5V or 5V reference), instrumentation amplifier, sensor linearization, and current output circuitry. The current to power additional external input circuits can be
VREG pin.
Instrumentation amplifiers are available with a wide gain range to accommodate a variety of input signal types and sensors. The unadjusted total error of the entire current transmitter, including the linear bridge, is low enough to be used in many situations without adjusting the application. The XTR106 operates from a loop supply voltage as low as 7.5V.
A linearizer circuit provides a second-order correction to control the bridge excitation transfer function voltage. It offers up to 20:1 improved nonlinearity, even with low-cost sensors.
The XTR106 is available in 14-pin plastic dip and SO-14 surface mount packages and is specified for the –40°C to +85°C temperature range. Operation from –55°C to +125°C

functional map

application information
Basic connection diagram of the XTR106.

The loop power vps provides power to all circuits.
The output loop current is measured as the series voltage with the load resistance, and a left 0.01µf to 0.03µf supply bypass capacitor is recommended to be connected between V+ and IO. An input 0.03µf capacitor is recommended for applications where fault and/or overload conditions may saturate.
A 2.5V or 5V reference voltage can be used to excite bridge sensors. For 5V excitation, pin 14 (VREF5) should be connected to For 2.5V excitation, connect pin 13 (VREF2.5) to pin 14, the terminals of the output bridge are connected to the amplifier inputs, VIN and VIN. A 0.01µf capacitor is shown connected between the inputs, recommended for high impedance bridges (>10KΩ). Resistor rg sets the gain to the required in-amp bridge voltage at full scale, vfs.
lin polarity and rlin provide a second-order linearization correction bridge for improved linearity of 20:1. Connect to LIN Polarity (Pin 12) to determine the polarity of nonlinearity correction and should be connected to IRET or VREG. Even if linearity correction is not required, LIN polarity should be tied to VREG.
The choice of rlin according to the equation in Figure 1 depends on Klin (the linearization constant) and the nonlinearity of the bridge with respect to VFS (see the "Linearization" section). The transmission function of the entire current transmitter is: IO=4MA+VIN • (40/RG) (1) VIN in volts, VIN in ohms Among them, vin is the differential input voltage. It can be clearly seen from the transfer function that if rg is not used (rg=∞), the gain is the zero current of the xtr106 output.
A negative input voltage (VIN) will result in an output current of less than 4 mA. An increasingly negative VIN will cause the output current to be limited to around 1.6ma. If the current is sourced from the reference and/or VREG, the current limit may be increased. Refer to the typical performance curves, "underscale current vs IREF+IREG" and "underscale current vs temperature". An increase in positive input voltage (greater than full-scale input, vfs) will produce an increasingly larger output current according to the transfer function, until the output current limit is approximately 28 mA. Refer to the Typical Performance Curve, "Overscale Current vs. Temperature".
IRET is also a local base that is the reference point for VREG and the on-board voltage reference. The IRET pin allows external use of any current circuit sensed by the XTR106 and included in the output current without causing errors. The range of input voltage to the XTR106 refers to this pin.

External Transistor The external pass transistor, Q1, conducts the 4-20 mA loop current associated with the signal. The use of external transistors combines most of the power dissipation with the XTR106's precision input and reference circuits, maintaining excellent accuracy.
Since the external transistor is in the feedback loop, the characteristics are not critical. Requirements: VCEO=45V minimum, β=40 minutes, Pd=800mW. If the loop supply voltage is lower, the power consumption requirement may be lower than 36V. Figure 1 lists some possible choices for Q1.
The XTR106 can run transistors without external channels. However, the accuracy will decrease due to internal power dissipation. Operation without Q1 is not recommended for extended temperature ranges. A connection at the IRET pin and operation below 0°C may require the E (transmitter) pin without Q1 to guarantee a full 20mA full-scale output, especially when V+ is close to 7.5V. The low operating voltage (7.5V) of the XTR106 allows operation directly from a PC power supply (12V ±5%). When used with the RCV420 current loop receiver, the load resistor voltage drop is limited to 3V.
Bridge Balance This adjustment can be used to compensate the bridge and/or fine tune the offset voltage of the XTR106. The values of r1 and r2 depend on the bridge, as well as the desired trim range. This trim circuit places additional load on the VREF output. Make sure that the additional load on VREF does not affect the zero output. See typical performance curve, "underscale current vs IREF + IREG" trim circuit effective load is almost equal to r2.
An approximate value of r1 can be calculated: where rb is the resistance of the bridge. v trim is the desired ± voltage trim range (unit: V).
Make r2 equal to or less than r1.
Many bridge sensors are inherently nonlinear. With the addition of an external resistor, the parabolic nonlinearity can be compensated for a 20:1 improvement over the uncompensated bridge output. Linearity correction is done by changing the bridge.
excitation voltage. The signal-dependent change in the excitation voltage of the bridge adds a second-order term to the transfer function (including the bridge). This can be custom corrected for non-linearity of the bridge sensor.
Positive and negative bridge nonlinearity errors can be compensated by connecting the LIN polarity pins correctly. Correct the positive nonlinearity of the bridge (bend up); LIN polarity (pin 12) should be connected to the IRET (pin 6) component which causes VREF to increase with the bridge to compensate for the output of the positive bow in the bridge. To correct for negative nonlinearity (bend down), connect the LIN polarity to VREG (pin 1) as shown. This causes VREF to decrease with the bridge output. The Lin polarity pin is a high impedance node.

If linearity correction is not required, the rlin and lin polarity pins should be connected to VREG. This results in a constant reference voltage independent of the input signal. The rlin or lin polarity pins should not be left open or tied to another potential.
rlin is an external linearizing resistor and is connected between pin 11 and pin 1 (VREG) To determine the value of rlin, the bridge sensor with constant excitation voltage must be known. The linear circuit of the XTR106 can only compensate for the parabolic part of the nonlinearity of the sensor. When the maximum value deviates from the linear output occurs at the mid-scale nonlinear curve similar to that shown in the sensor loop supply voltage applied to the XTR106, V+, with a connection on IO, pin 7. The V+ range is 7.5V to 36V. The loop supply voltage vps will be different from the voltage applied to the XTR106 based on the voltage drop across the current sense resistor, rl (plus any other voltages added to the line).
If a low loop supply voltage is used, the value of rl (including loop wiring resistance) must be relatively low to ensure V+ maximum remains at 7.5V or higher 20mA loop current: Recommended for designs with V+ equal to or greater than 7.5V for loop currents up to 30mA , allowing out-of-range input conditions. In the case of a 5V sensor, V+ must be at least 8V using excitation, if the corrected bridge nonlinearity is greater than +3%.

The low operating voltage (7.5V) of the XTR106 allows operation directly from a PC power supply (12V ±5%). When used with the RCV420 current loop receiver, the load resistor voltage drop is limited to 3V.
Bridge balance bridge trimmer circuit (r1, r2). This adjustment can be used to compensate for the bridge and/or trim the offset voltage of the XTR106.
The values of r1 and r2 depend on the bridge, as well as the desired trim range. This trim circuit places additional load on the VREF output. Make sure that the additional load on VREF does not affect the zero output. See typical performance curve, "underscale current vs IREF + IREG" trim circuit effective load is almost equal to r2.
An approximate value of r1 can be calculated: where rb is the resistance of the bridge.
v trim is the desired ± voltage trim range (unit: V).
Make r2 equal to or less than r1.
Linearization Many bridge sensors are inherently nonlinear. With the addition of an external resistor, the parabolic nonlinearity can be compensated for a 20:1 improvement over the uncompensated bridge output. Linearity correction is done by changing the bridge. excitation voltage. The signal-dependent change in the excitation voltage of the bridge adds a second-order term to the transfer function (including the bridge). This can be custom corrected for non-linearity of the bridge sensor. Positive and negative bridge nonlinearity errors can be compensated by connecting the LIN polarity pins correctly. Correct the bridge for positive nonlinearity (bend up); LIN polarity (pin 12) should be connected to IRET (pin 6). This causes VREF to increase with the bridge to compensate for the output response of the forward bow in the bridge. To correct for negative nonlinearity (bend down), connect the LIN polarity to VREG (pin 1) as shown. This causes VREF to decrease with the bridge output. The Lin polarity pin is a high impedance node.
If linearity correction is not required, the rlin and lin polarity pins should be connected to VREG. This results in a constant reference voltage independent of the input signal. The rlin or lin polarity pins should not be left open or tied to another potential.
rlin is the external linearizing resistor and connects between pin 11 and pin 1 (VREG) 3b. To determine the value of rlin, a bridge sensor with constant excitation voltage must be known. The linear circuit of the XTR106 can only compensate for the parabolic part of the nonlinearity of the sensor. The midscale occurs when the maximum value deviates from the linear output.

Figure 4, but not fully peaking at the mesoscale is greatly improved. Sensor nonlinear curves with "S-shape" (equivalent positive and negative nonlinearity) cannot be improved using the XTR106's correction circuit. The value of rlin is chosen according to Equation 4 shown in Figure 3. rlin depends on the linearization factor, Klin, 2.5V reference voltage and 5V reference voltage are different. The sensor's nonlinear term b (relative to full scale) is positive or negative depending on the direction of the bow.
Use a 5V reference voltage. Sensor nonlinearity is +5%/-2.5% can be corrected with 2.5V excitation. The trimming circuit shown in Figure 3d can be used for unknown bridge nonlinearities.
Gain is used to correct for bridge nonlinearity. The correction value for the gain is calculated from Equation 5 given in Figure 3 to calculate the resistance.

When using linearity correction, care should be taken to ensure that the sensor's output common-mode voltage remains within the 1.1V to 3.5V allowed by the XTR106. Equation 6 in Figure 3 can be used to calculate the new excitation voltage for the XTR106. If not, the common mode voltage at the output of the bridge is only half of this value using a common mode resistor. Exceeding the common mode range may produce unpredictable results. For high precision applications (error <1%), a two-step calibration process can be used. First, the nonlinearity is measured with an initial gain resistor for the sensor bridge's sum rlin = 0 (rlin pin is directly connected to vreg). Using the resulting sensor nonlinearity, the values of b, rg, and rlin are calculated using equations 4 and 5 in Figure 3. A second calibration measurement is then made to adjust rg to account for offsets and mismatches in linearization.
Underscale Current Total current drawn from VREF and VREG Voltage source and temperature both affect the XTR106's underscale current value (see Typical Performance Curves, "Underscale Current vs IREF + IREG). This should be a factor when choosing bridge resistors. and excitation voltage, especially for operation over a wide temperature range (see Typical Performance Curves, "Underscale Current vs. Temperature").

The two available excitation voltages for the low-impedance bridge XTR106 (2.5V and allow the use of various bridge values. Bridge impedances as low as 1kΩ can be used without any additional circuitry. Low-impedance bridges can be used to increase series resistance by adding Limit excitation of xtr106 current less than or equal to 2.5mA (Figure 5). Resistance should be increased

reverse voltage protection
The low compliance rating (7.5V) of the XTR106 allows various voltage protection methods to be used without affecting the operating range. Figure 6 shows a circuit in which the diode bridge works even if the voltage connections are reversed. This bridge causes the diode drop (about 1.4V) to lose the loop supply voltage. This will produce about 9V suitable for most applications. The diodes can be connected to the loop supply voltage and the + pin to prevent reverse output with only 0.7V of loop supply loss to connect the line voltage.
Overvoltage surge protection Remote connections to current transmitters are sometimes subject to voltage surges. It is prudent to limit the maximum surge voltage applied to the XTR106 to be as low as possible. Various Zener diodes and surge clamp diodes are designed for this purpose. Choose a clamp diode voltage rating of as low A as possible to provide the best protection. For example, a 36V protection diode will ensure that the transmitter operates normally at normal loop voltages and will still provide the appropriate level of protection against voltage surges. Characterization tests Three production batches of XTR106 did not damage loop supply voltages up to 65V.

Most surge protection zener diodes have diode characteristics in the forward direction, which will cause current flow if the loop connections are reversed. If surge protection diodes are used, series diodes or diode bridges should be used to prevent reverse connection.
Long wire lengths of RFI current loops can cause RFI. RF can pass through the sensitive XTR106's input circuitry causing errors. Typically displayed as erratic output current, depending on the location of the loop power supply or input wiring.
If the bridge sensor is located at a remote location, interference may enter at the input terminals. For integrated transmitter assemblies with short connections to the sensor, interference is more likely to come from the current loop connections.

Bypass capacitors at the input reduce or eliminate interference at that input. Connect these bypass capacitors to the IRET terminals as shown in Figure 6. Although the DC voltage at the IRET terminal is not equal to 0V (at the loop supply, VPS) this circuit point can be considered the "ground" of the transmitter.
A 0.01µF capacitor connected between V+ and IO can help minimize output glitch.