SBAS940 December 2018 DAC8742H
PRODUCTION DATA.
The A2 operational amplifier employs negative feedback to ensure potentials at both input nodes, V+ and V-, are equivalent. This establishes the set of KCL equations (1) – assuming no HART communication, VHART = 0 V.
A2 also drives the base of the NPN BJT, Q1, which enables current to flow from its collector through emitter pins and through the R8 resistor, while maintaining an equivalent potential drop from its input nodes to the net represented by TP4. This ensures that the combined voltage drop across R9 and R11 is equivalent to the combined drop of R10 and R12.
Using this relationship, along with current Equation 3 and Equation 4, IOUT is calculated as follows:
For a VREF value of 4.096 V, the zero-scale portion of the transfer function, [VREF/(102.4k)]*(100), translates to 4 mA, while the span, [VDAC/(25.6k)]*100, encompasses 16 mA. This final product is a system capable of sourcing 4 mA to 20 mA, which is dependent on DAC output voltage. The value of R4 is responsible for converting the 500-mV p-p HART signal into a 1-mA p-p frequency shift keyed (FSK) signal that resides on top of the 4-mA to 20-mA analog current signal.