High-speed or low-power solutions for RS-485 fieldbus

Analog Devices’ iCoupler® digital isolator and RS-485 transceiver product families address two major needs in industrial applications: higher data rates and lower power modes of operation.

For high-performance motor control encoder applications, higher data rates, smaller RS-485 transceiver packages, and IEC 61000-4-2 ESD protection are typically required. The ADM3065E/ADM3066E 50 Mbps transceivers offer IEC 61000-4-2 ESD protection to ±12 kV (contact) and ±12 kV (air) in a space-saving 10-lead LFCSP package, providing an EnDat encoder with a Set of reliable solutions (please refer to AN-1397 application note for more information). Additionally, adding high-speed stable signal and power isolation to the ADM3065E/ADM3066E can be achieved with the isoPower® ADuM6401 or the isoPower ADuM6000 and iCoupler ADuM241D as described in this application note.

In battery powered systems, downhole applications (for example, mining), and process control systems that operate in a 4 mA to 20 mA loop, there is often a high demand for a low power mode of operation. The ADuM1441, a micropower digital isolator from Analog Devices, has a quiescent power consumption of less than 23 μA in shutdown mode. The ADM3483 3.3 V, 250 kbps RS-485 transceiver has extremely low quiescent power consumption, typically only 2 nA in shutdown.

Figure 1 shows a stable and reliable low-power isolated RS-485 solution for downhole applications. The ADM3483 is shared with the ADuM1441 to provide a reliable, low-power link to remote underground measurement nodes. System interface cards include an ARM® Cortex® microcontroller unit (MCU), ADuCM3027, and integrated analog front end (AFE) AD7124-4 for remote temperature and pressure measurements. Firmware updates for the system interface cards are delivered over long-distance RS-485 cables, and are updated to enable low data rate transmissions (eg, 9.6 kbps) over long distances of up to 1 km.

High-speed or low-power solutions for RS-485 fieldbus

Figure 1. Robust and reliable low-power isolated RS-485 solution for downhole applications

Isolated High Speed ​​RS-485

Using iCoupler and isoPower technology, galvanic isolation with reinforced insulation and 5 kV rms transient withstand voltage can be added to the ADM3065E. The ADuM6401 provides the required quad 5 kV rms signal isolation, operating rates up to 25 Mbps, and an integrated DC/DC converter. The ADuM6401 and the ADM3065E (shown in Figure 2) need to configure the VISO pin to 3.3 V by connecting the VSEL pin to the GNDISO pin and connecting the 5V power supply to the VDD1 pin. Operation at 3.3V ensures that the ADM3065E remains within the load capability of the ADuM6401 even at data rates of 25 Mbps.

Using the ADuM241D quad-channel digital isolator and the ADuM6000 isolated DC/DC converter, 50 Mbps data rates and ADM3065E isolation can be achieved, as shown in Figure 3. The ADuM241D is capable of data rates up to 150 Mbps and provides the precise timing required to fully support the ADM3065E operating at a 50 Mbps data rate.

However, operating at the 50 Mbps data rate requires operating the ADM3065E at 3.3 V.

If 5V operation is required, the VSEL pin on the ADuM6000 can be connected to VISO, but the maximum supported data rate will be reduced (eg, <10 Mbps). For more information, refer to the ADuM241D and ADuM6000 data sheets.

The DC/DC converters in the ADuM6401 and ADuM6000 isoPower devices provide regulated isolated power for the ADM3065E (and ADuM241D). The two isoPower devices utilize high-frequency switching elements to transfer power through a transformer. Users must follow radiation standards for printed circuit board (PCB) layout. For PCB layout recommendations, please refer to the AN-0971 application note.

High-speed or low-power solutions for RS-485 fieldbus

Figure 2. 25 Mbps RS-485 solution with signal and power isolation (simplified diagram, not all connections shown)

High-speed or low-power solutions for RS-485 fieldbus

Figure 3. 50 Mbps RS-485 solution with signal and power isolation (simplified diagram, not all connections shown)

Isolated Low Power RS-485

Figure 4 shows the combination of the ADuM1441 micropower, quad, digital isolator and the ADM3483 low power, half-duplex RS-485 transceiver.

When the ADM3483 is in shutdown (driver enable DE pin low and receiver enable pin high), the quiescent supply current is typically only 2 nA (1 μA maximum specification). As shown in Figure 4, Pin 7 and Pin 10 of the ADuM1441 are connected to GND1 and GND2, respectively. This means that when the ADuM1441 isolator is in shutdown mode with no bus communication activity, its quiescent current is less than 23 µA. Overall, the solution’s static power consumption is as low as less than 24 µA.

If pins 7 and 10 of the ADuM1441 are connected directly to VDD1 and VDD2, respectively, the static power consumption of the ADuM1441 is only 1.2 µA. This can be done with jumper connections on the PCB, the user can choose to connect pin 7 to VDD1 or GND1, and also choose to connect pin 10 to VDD2 or GND2. Adding the 1.2 μA quiescent power feature in the ADuM1441 to the ADM3483 quiescent power supply enables a fully isolated RS-485 node that consumes only 2 μA in shutdown or standby mode. To ensure proper operation of the isolator, pin 7 and pin 10 of the ADuM1441 must be connected to GND1 and GND2, respectively.

Figure 4. Low-Power, Isolated RS-485 Node

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