SLAAEH8 October   2024 AFE781H1 , AFE782H1 , AFE881H1 , AFE882H1 , DAC8740H , DAC8741H , DAC8742H

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
    1. 1.1 The 4-20mA Loop
    2. 1.2 The HART Protocol
      1. 1.2.1 Adding HART to the 4-20mA Loop
      2. 1.2.2 HART FSK
  5. 2AFE881H1 HART Modem
    1. 2.1 AFE881H1 HART Transmitter
    2. 2.2 Detailed Schematic
      1. 2.2.1 Input Protection
      2. 2.2.2 Startup Circuit
      3. 2.2.3 Voltage-to-Current Stage
      4. 2.2.4 Voltage-to-Current Calculation
      5. 2.2.5 HART Signal Transmission
      6. 2.2.6 HART Input Protection
      7. 2.2.7 Current Consumption
      8. 2.2.8 HART Transmitter Board
      9. 2.2.9 HART Protocol Stack
  6. 3HART Testing and Registration
    1. 3.1  HART History and the FieldComm Group
    2. 3.2  HART Testing Overview
      1. 3.2.1 HART Protocol Specifications
      2. 3.2.2 HART Protocol Test Specifications
      3. 3.2.3 Remote Transmitter Device Testing
    3. 3.3  HART Test Equipment
    4. 3.4  HART Physical Layer Testing
      1. 3.4.1 FSK Sinusoid Test
      2. 3.4.2 Carrier Start and Stop Time Tests
      3. 3.4.3 Carrier Start and Stop Transient Tests
      4. 3.4.4 Output Noise During Silence
      5. 3.4.5 Analog Rate of Change Test
      6. 3.4.6 Receive Impedance Test
      7. 3.4.7 Noise Sensitivity Test
      8. 3.4.8 Carrier Detect Test
    5. 3.5  Data Link Layer Tests
      1. 3.5.1 Data Link Layer Test Specifications
      2. 3.5.2 Data Link Layer Test Logs
    6. 3.6  Universal Command Tests
    7. 3.7  Common-Practice Command Tests
    8. 3.8  Device Specific Command Tests
    9. 3.9  HART Protocol Test Submission
    10. 3.10 HART Registration
  7. 4Other TI HART Modem Designs
  8. 5Summary
  9. 6Acknowledgments
  10. 7References

2.2.3 Voltage-to-Current Stage

Figure 2-3 shows a simplified schematic of the V-to-I stage of the transmitter. This stage sets the current of the loop based on the voltage output of the DAC.

Voltage-to-Current Converter Stage for the Transmitter Board Figure 2-6 Voltage-to-Current Converter Stage for the Transmitter Board

In this circuit, the DAC voltage is set across 100kΩ of equivalent resistance. The voltage at the opposite end of the 100kΩ is a virtual ground set by the feedback of the OPA333. The output of the OPA333 drives Q4 which sets the controls the loop current.

The current I1 flows through the 40.2kΩ resistor to set the LOOP– to a voltage below ground.

Equation 1. I1 = VOUT / 100kΩ
Equation 2. VLOOP– = I1 × 40.2kΩ

Current through the 40.2kΩ resistor is amplified 1000:1 with the 40.2Ω resistor and I2 is pulled from ground to LOOP–.

Equation 3. I2 = VLOOP– / 40.2Ω = I1 × 40.2kΩ / 40.2Ω = I1 × 1000
Equation 4. ILOOP– = I1 + I2 = 1001 × I1 = 1001 × VOUT / 100kΩ