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

3.4.6 Receive Impedance Test

Another physical layer test is to measure the receive impedance of the transmitter. The AFE881H1 is built into a high-impedance transmitter and the receive impedance must be above a minimum level over both the primary variable and HART transmission frequencies. In this test, a 5kΩ series test resistance is used in the loop to measure the receive impedance of the transmitter. This test requires a significantly higher power supply voltage. The normal starting current of the transmitter is 4mA. This amount of current across 5kΩ is 20V and a supply of over 40V is required to operate this test. Figure 3-17 shows a block diagram of the test setup for measuring the receive impedance of the transmitter.

HART Receive Impedance Test Setup Figure 3-17 HART Receive Impedance Test Setup

A signal generator inputs a sine wave of different frequencies into the loop. The oscilloscope measures the voltage from the signal generator, the voltage across the test resistor, and the voltage dropped across the transmitter. From these three measured values on the oscilloscope, and using the known 5kΩ resistance of the test resistor, the equivalent impedance of the transmitter is calculated. Over frequency, an equivalent resistance and capacitance (Rx and Cx) can be calculated and plotted.

Table 3-6 tabulates the measured values for VA and VB and the calculated equivalent impedance for ZM looking into the transmitter.

Table 3-6 Measured and Calculated Results from the Receive Impedance Test
FREQUENCY VA (V) VB (V) ZM (Ω, calculated)
200Hz 0.018 1 277778
500Hz 0.025 1 200000
950Hz 0.05 0.99 99000
1.6kHz 0.09 0.98 54444
2.5kHz 0.13 0.96 36923
5kHz 0.26 0.95 18289
10kHz 0.48 0.87 9063
20kHz 0.73 0.65 4452
50kHz 0.93 0.33 1774

The impedance is then plotted versus frequency in Figure 3-18.

Receive Impedance Test Results Plotted Over Frequency Figure 3-18 Receive Impedance Test Results Plotted Over Frequency

For this test, signals in a frequency range from 200Hz to 50kHz are measured and calculated. The equivalent resistance and capacitance for the transmitter receive impedance are 278kΩ and 1800pF. For high-impedance transmitters, the minimum equivalent resistance is 100kΩ and the maximum capacitance is 5nF.