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

3.4.5 Analog Rate of Change Test

The analog rate of change test shows how fast the primary variable can be changed from minimum to maximum value and back again. For this test, a special test mode is encoded in the device to cycle the output from 4mA to 20mA at a rate of about 18Hz. This test mode is enabled through switches on the transmitter board. In the transmitter, the AFE881H1 is set to the sinusoidal slew-rate mode so that the transitions from minimum to maximum approximate a sine-wave shape. The sinusoidal slew-rate mode maximizes the signal in the low-pass primary-variable band.

This signal is then filtered through a HCF_TOOL-31 filter. This filter is a 500Hz to 10kHz bandpass filter that passes all of the in-band HART signal through. The test is devised to show that when the primary variable is changed, the transition does not cause any noise large enough to trigger a HART reception. Figure 3-15 shows the test setup for the analog rate of change test.

HART Analog Rate of Change Test Setup Figure 3-15 HART Analog Rate of Change Test Setup

Figure 3-3 shows a plot of the analog rate of change test. The top trace is the primary variable transitioning back and forth from minimum to maximum. This is a transition of 4mA to 20mA. Across a 500Ω resistor, this is an 8V transition. Table 3-5 reports the resulting measurements of the analog rate of change test.

Analog Rate of Change Test Oscilloscope Results Figure 3-16 Analog Rate of Change Test Oscilloscope Results

The bottom trace shows the same signal filtered through an HCF_TOOL-31. This signal must be less than 150mVPEAK. For this test, the 270mVPP result is the equivalent to 135mVPEAK, and the device passes this test.

Table 3-5 HART Analog Rate of Change Measurement Results
TEST MEASUREMENT MAXIMUM RESULT
Analog signaling filtered signal amplitude 270mVPP 150mVPEAK Pass
Communication errors during analog signaling 100 attempts 0 errors Pass

During the test, the HART test system was also used to test active transmissions during this periodic output transition. Using the HART test system, the comtest command sends a set of consecutive commands. While the device is cycling through the minimum and maximum values, the HART test system sends 100 commands and the transmitter responds to these commands to verify there are no errors in transmission.