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.
Question: In the figure, if the microcontroller has 3.3V I/O, wouldn't it introduce less noise to the AD5700 via its VCC pin if that pin only were routed from the AD5421 REGout pin (set to 3.3V) and the AD5421 DVdd pin supplied power to the microcontroller, the IODVdd pin on the AD5421 and the IOVcc pin (not shown in the figure) on the AD5700? I would think this would also lessen the need for the two RC filters.
There are a number of permutations in which the AD5421 regulated outputs can be used. DVDD is fixed at 3.3V but REGOUT is programmable. As you have suggested, there is no issue with driving the AD5700 from REGOUT of the AD5421 and the uC from DVDD.
However, it should be pointed out that the coupling path for noise getting to the Io loop via the regulators on the AD5421 is still present, even when the HART modem is not transmitting (silent) e.g. Noise During Silence HART test. The uC can still couple noise onto the loop via the AD5421 regulator paths, hence the need for the filter on this supply line.
In terms of the filter on the HART modem supply line, it's purpose is to minimize any noise generated by the HART modem getting onto the Io loop, again, via the AD5421 regulator path.
So, both filters are recommended in the circuit, irrespective of whether it is supplied by DVDD or Regout.
Thanks. That clearly answers my question. I will definitely leave the filters in place.
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