JAJSEF2D June 2013 – December 2021 DAC7760 , DAC8760
PRODUCTION DATA
Figure 9-7 illustrates a common generic solution for realizing these desired voltage and current output spans.
The current output circuit is comprised of amplifiers A1 and A2, MOSFETs Q1 and Q1, and the three resistors RSET, RA, and RB. This two-stage current source enables the ground-referenced DAC output voltage to drive the high-side amplifier required for the current-source.
The voltage output circuit is composed of amplifier A3 and the resistor network consisting of RFB, RG1, and RG2. A3 operates as a modified summing amplifier, where the DAC controls the noninverting input and the inverting input has one path to GND and a second to VREF. This configuration allows the single-ended DAC to create both the unipolar 0-V to 5-V and 0-V to 10-V outputs and the bipolar ±5-V and ±10-V outputs by modifying the values of RG1 and RG2.
Figure 9-6 generates clean ±15-V supplies using a synchronous step-down regulator (TPS54062) and two high-voltage, ultra-low noise, linear regulators (TPS7A49 and TPS7A30). A field supply terminal is shown instead of the more common use case of a back-plane supply. The design uses two triple channel isolators (ISO7631FC) to provide galvanic isolation for the digital lines to communicate to the main controller. Note that these isolators can be driven by the internally-generated supply (DVDD) from the DACx760 to save components and cost. The DACx760 supplies up to 10 mA that meets the supply requirements of the two isolators running at up to 10 Mbps. Note that additional cost savings are possible if noncritical digital signals such as CLR and ALARM are tied to GND or left unconnected. Finally, a protection scheme with transient voltage suppressors and other components is placed on all pins which connect to the field.
The protection circuitry is designed to provide immunity to the IEC61000-4 test suite which includes system-level industrial transient tests. The protection circuit includes transient voltage suppressor (TVS) diodes, clamp-to-rail steering diodes, and pass elements in the form of resistors and ferrite beads. For more detail about selecting these components, see TIPD153.