Another physical layer test is to measure the receive impedance of the transmitter. The AFE882H1 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.

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 V_A and V_B and the calculated equivalent impedance for Z_M looking into the transmitter.

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

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 receive resistance is tested to be greater than 250kΩ and the equivalent receive capacitance is tested to be 3600pF. For high-impedance transmitters, the minimum equivalent resistance is 100kΩ and the maximum capacitance is 5nF.