SBOA575 September   2023 INA823 , OPA2387 , XTR115 , XTR116

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2Theory of Operation
    1. 2.1 Wheatstone Bridge Sensor
    2. 2.2 2-Amp INA
    3. 2.3 4-20-mA Current Loop Transmitter Interface
  6. 3Simulation
  7. 4PCB Design
  8. 5Verification and Measured Performance
  9. 6Summary
  10. 7Reference
  11.   Appendix

2 Theory of Operation

Factory automation and control systems often require the status of several processes to be monitored and communicated to a control station to ensure proper operation. It is common practice for many sensors to be dispersed on a factory floor to convert process variables such as pressure, temperature and weight into electrical signals that can be transmitted to a central location. 4-20-mA current loop transmitters are ideal for this application because they allow remote processes to be monitored with only two wires: power for the sensor or surrounding circuitry and an output current corresponding to the sensor variable of interest. A Wheatstone bridge amplifier to 4-20-mA current loop transmitter circuit is proposed as a high accuracy factory automation signal chain solution.

The circuit can be divided into three sections:

  1. Wheatstone bridge sensor
  2. Discrete 2-amp instrumentation amplifier (INA)
  3. 4-20-mA current loop transmitter (XTR) interface

Figure 2-1 shows a simplified version of the full circuit. The reference voltages and local ground (IRET) of TI's XTR116 are used to power all bridge and sensor circuitry. All current consumed by the XTR powered circuitry must return through the current loop (IRET); therefore, the monitoring circuitry must consume less than 4-mA to avoid interference with the 4-20-mA output current of interest. To account for variation in device, temperature, and supply it is good practice to limit the current consumption to 3.5-mA. The XTR116 is powered by the two-wire power supply (VLOOP).

The signal chain begins with a small differential voltage developed between the two outputs of the Wheatstone bridge corresponding to the variable of interest. These voltages are fed into the discrete 2-amp INA which can amplify their difference and the output voltage can be converted into a current through a resistor (RIN). The current loop transmitter then takes the current at the input (IIN) and returns the current multiplied by 100 at the output (IOUT). The following sections provide more detail on each portion of the circuit.

GUID-20230817-SS0I-R0GP-JP2Z-HX55M71WT1MV-low.svgFigure 2-1 Simplified Bridge Amplification to 4-20-mA Circuit