SBAS856D June 2017 – May 2019 DAC8740H , DAC8741H
PRODUCTION DATA.
The A2 op amp employs negative feedback to drive the potential at both input nodes, V+ and V–, to the same voltage. 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 configuration drives the combined voltage drop across R9 and R11 to the same voltage drop across 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 / (102.4 kΩ)] * (100), translates to 4 mA, while the span, [VDAC / (25.6 kΩ)] * 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-mVPP HART signal into a 1-mA PP frequency shift keyed (FSK) signal that resides on top of the 4-mA to 20-mA analog current signal.