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AD5421 Datasheet(PDF) 4 Page - Analog Devices |
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AD5421 Datasheet(HTML) 4 Page - Analog Devices |
4 / 8 page CN-0267 Circuit Note Rev. B | Page 4 of 8 The ADuCM361 on-chip DAC with an external transistor can be used to control the 4 mA to 20 mA loop, refer to CN-0300 for details. The AD5421 can be connected via the protection directly to the loop. Alternatively, a depletion mode N-channel MOSFET can be connected between the AD5421 and the loop power supply, as shown in Figure 2. The use of the additional MOSEFT in this configuration keeps the voltage drop across the AD5421 at approximately 12 V, lowers the power dissipated in the AD5421 package, and therefore improves the 4 mA to 20 mA analog output accuracy. It also increases the maximum voltage allowed in the loop to the level of the MOSFET rating. The additional MOSFET has no effect on the HART communication. Figure 2. MOSEFT Connected to the AD5421 Loop Power Supply The AD5700 is used with a 3.8664 MHz crystal in this circuit, which is the configuration achieving the lowest power consumption. Alternatively, the AD5700-1, with an integrated 0.5 %precision internal oscillator, can be used. The internal oscillator increases the modem power supply current by 225 µA maximum, compared to the crystal oscillator, but because no external crystal is needed, this option provides both cost savings and reduced board area requirements. For the applications that are not loop powered, the AD5410, AD5420, AD5422, or AD5755 are good choices for the 4 mA to 20 mA DAC. CIRCUIT EVALUATION AND TEST Circuit Hardware The circuit shown in Figure 1 is built on the DEMO-AD5700D2Z printed circuit board shown in Figure 3. Figure 3. DEMO-AD5700D2Z Printed Circuit Board (Pressure Sensor Not Included) The DEMO-AD5700D2Z circuit board includes some additional features for easy system evaluation. The 0.1 inch-pitch connector footprints allow optional primary and secondary sensor connections. There are test points for HART RTS and CD that may be needed for HART compliance tests. A connector on the edge of the DEMO-AD5700D2Z makes the ADuCM360 single wire and UART download/debug signals accessible allowing easy software development, code download, and in-circuit debugging and emulation. The connector, with a small header extender included with the DEMO-AD5700D2Z board, is compatible with all Analog Devices, Inc., Cortex-M3 based development tools, such as the EVAL-ADuCM360QSPZ evaluation kit (the evaluation kit is not included with the DEMO-AD5700D2Z board). These features are not shown in the simplified diagram in Figure 1; however, they can be seen in the complete circuit schematic in the CN-0267 Design Support Package. The design support package also includes a full field instrument C-code example, which enables complete verification and evaluation of all hardware blocks and features of the circuit, and a limited verification of the HART interface functionality. For detailed information about HART interface specifications and resources, contact the Hart Communication Foundation. HART Compliance The DEMO-AD5700D2Z has been verified to be compliant with HART FSK Physical Layer Specification (HCF_SPEC-054, Revision 8.1), using methods and equipment specified in the HART Physical Layer Test Specification (HCF_TEST-2, Revision 2.2). The board was submitted to the Hart Communication Foundation and was successfully registered. The registered circuit can be found on the HART Communication Foundation (HFC) web site in the product catalog as DEMO- AD5700D2Z. The results of two of the tests involved the output noise during silence and the analog rate of change. 200 kΩ DN2540 BSP129 LOOP– COM AD5421 REGIN DRIVE 20MΩ 1MΩ 4700pF L L TVS 40V LOW LEAKAGE 4.7V LOW LEAKAGE TO HART INPUT FILTER LOOP+ LOOP– VLOOP |
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