FAQ: AD5421 Supply Pin Configuration

Document created by tjohnson Employee on Sep 25, 2012
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Can the either REG_OUT or DVDD outputs of the AD5421 be used to drive the IODVDD supply pin?





The AD5421 is a loop powered 4 to 20mA Iout DAC. It accepts a loop voltage up to 60V (typically 24V in a loop-powered transmitter) & outputs a current in the range 4 to 20mA. The on-chip voltage regulator (REG_OUT) provides a regulated voltage output to supply the AD5421 and the remainder of the transmitter circuitry. It is programmable  to1.8, 2.5, 3.0, 3.3, 5.0, 9 or 12 Volts. DVDD is a 3.3V digital power supply output and should be decoupled to COM with 100nF || 4.7uF capacitors.


The SPI pins (SCLK, /SYNC, SDIN, SDO, FAULT, /LDAC & SDO) are referenced to IODVDD. The RANGE[1:0], REG_SEL[2:0], ALARM_CURRENT_DIRECTION & Rint_Rext pins are referenced to DVDD. This architecture also allows a user to use a lower IODVDD interface supply if, for example, a 1.8V supply was being used for the microconverter.


Sample Application Diagram for HART Capable Smart Transmitter

In this case REG_OUT (set to output 3.3V) was used to drive the ADuCM360 microconverter supply & also the AD5700 HART Modem. The 3.3V DVDD output was used to drive IODVDD. This was the arrangement used in the HART enabled smart transmitter reference demo registered with the HART Communication Foundation.


An alternative to this arrangement would be for REG_OUT to stay connected as it is in the diagram below, but also be connected to IODVDD of the AD5421. In this case DVDD would be disconnected from IODVDD and a 4.7uF || 0.1uF cap to ground would be tied to the DVDD pin and a 0.1uF cap to ground to the IODVDD pin.