SBAS528D June 2013 – December 2021 DAC7760 , DAC8760
PRODUCTION DATA
The second method to implement HART is to couple the HART signal through the ISET-R pin when IOUT is operated using an external RSET resistor. The FSK signal from the modem is ac coupled into the pin through a series combination of Rin and Cin as shown in Figure 9-4.
The magnitude of the ac current output is calculated as (VHART × k) / Rin, where k is a constant that represents the gain transfer function from the ISET-R pin to the IOUT pin and depends on the selected current output range as follows: k = 60 for the 4-mA to 20-mA range, 75 for the 0-mA to 20-mA range, and 90 for the 0-mA to 24-mA range. The series input resistor and capacitor form a high-pass filter at the ISET-R pin and Cin must be selected to make sure that all signals in the HART extended-frequency band pass through unattenuated.