feature

Isolated, full-duplex RS-485 /RS-422 transceiver; ±8 kV ESD protection on RS-485 input/output pins; 16 Mbps data rate; ANSI TIA/EIA-485-A-1998 and ISO compliant 8482 : 1987 (E); suitable for 5V or 3V operation (VDD1); high common mode transient immunity: >25kv/μs; receiver with open circuit, fault protection design; 32 nodes on bus; thermal shutdown protection ; Safety and regulatory approvals; UL listed: 5000 V rms isolation voltage; 1 minute VDE certificate of conformity according to UL 1577; German Industrial Standard EN 60747-5-2 (VDE 0884-10 Part 2): 2003-01; Germany Industrial Standard EN 60950 (VDE 0805): 2001-12; European Industrial Standard EN 60950:2000; VIORM=848V peak; Operating temperature range: -40°C to +105°C; Wide body, 16 lead SOIC package.

application

Isolated RS-485/RS-422 interface; industrial field network; INTERBUS company; multipoint data transmission system.

General Instructions

The ADM2490E is an isolated data transceiver with ±8KV ESD protection for high-speed full-duplex communication on multipoint transmission lines. It is designed for balanced transmission lines and meets the requirements of ANSI TIA/EIA-485-A-1998 and ISO 8482:1987(E). Using Analog Devices, Inc., iCoupler® technology, this device combines a 2-channel isolator, a three-state differential line driver, and a differential input receiver into a single package.

The differential transmitter output and receiver input have electrostatic discharge circuits that provide ±8 kV protection using the Human Body Model (HBM). The logic side of the device can be powered with a 5V or 3V supply, while the bus side requires an isolated 5V supply. The device features current limiting and thermal shutdown to prevent output shorts and bus contention situations that could lead to excessive power dissipation.

The ADM2490E is available in a wide-body, 16-lead SOIC package and operates over the temperature range of -40°C to +105°C.

Specification

All voltages are relative to their respective grounds; 2.7±V = 5.5 volts, 4.5 volts volts less than 5.5 volts. Unless otherwise stated, all min/max specifications apply over the entire recommended operating range. All typical specifications are T=25°C, V=V=5.0V unless otherwise noted.

VDE 0884-10 Insulation Characteristics

This isolator is only suitable for basic electrical isolation within the safety limit data. Maintenance of safety data must be ensured by protective circuits. An asterisk (*) on the package indicates VDE 0884-10 approval for 848 V peak working voltage.

Absolute Maximum Ratings

TA = 25°C unless otherwise noted. Each voltage relative to its stress is greater than the voltages listed under Absolute Maximum Ratings.

test circuit

Switching Characteristics

Typical performance characteristics

Circuit Description

Electrical isolation

In the ADM2490E, electrical isolation is implemented on the logical side of the interface. Therefore, this section has two main sections: the digital isolation section and the transceiver section (see Figure 21). The driver input signal applied to the TxD pin and referenced to logic ground (GND) is coupled through the isolation barrier to appear on the transceiver section referenced to isolated ground (GND). Similarly, the receiver input (referenced to isolated ground in the transceiver section) is coupled across the isolation barrier to appear at the RxD pin referenced to logic ground.

Optocoupler Technology

Digital signals are transmitted through the isolation barrier through ICouper technology. This technique uses chip-scale transformer windings to magnetically couple digital signals from one side of the isolation barrier to the other. The digital input is encoded into a waveform capable of exciting the main transformer windings. At the secondary winding, the induced waveform is decoded into the originally transmitted binary value.

Positive and negative logic transitions at the input cause narrow pulses (~1ns) to be sent through the transformer to the decoder. The decoder is bistable, so it can be set or reset with a pulse, indicating input logic transitions. In the absence of logic transitions at the input for more than ~1 μs, a periodic set of refresh pulses indicating the correct input state is sent to ensure DC correctness at the output. If the decoder receives no internal pulses longer than 5 μs, the input side is assumed to be depleted or functional, in which case the output is forced to the default state (see Table 12).

truth table

The truth tables in this section use the abbreviations shown in Table 10.

Thermal shutdown

The ADM2490E includes thermal shutdown circuitry to prevent excessive power dissipation of the part during fault conditions. Shorting the driver output to a low impedance source can result in high driver current. In this case, the thermal sensing circuit detects the increase in mold temperature and disables the driver output. This circuit is designed to disable the driver output when the die temperature reaches 150°C. The drives were re-enabled at 140°C as the device cooled.

Fail-safe receiver input

The receiver input includes a failsafe function that ensures the RxD pin is logic high when the A and B inputs are floating or open circuit.

Magnetic field immunity

The limit on the magnetic field immunity of the i-coupler is set by the condition that the induced voltage in the receiving coil of the transformer is large enough to incorrectly set or reset the decoder. The following analysis defines the conditions under which this may occur. Check the 3V operating condition of the ADM2490E as it represents the most susceptible operating mode.

The pulse amplitude at the transformer output is greater than 1V. The decoder's sensing threshold is about 0.5V, creating a 0.5V margin in which the induced voltage can be tolerated.

The voltage induced by the receiver coil is given by:

Where: β is the magnetic flux density (Gauss); N is the number of turns of the receiving coil; rn is the radius (cm) of the nth turn of the receiving coil. Given the geometry of the receiver coil and the requirement to induce a voltage margin of at most 0.5 V at the decoder, the maximum allowable magnetic field can be determined using Figure 22.

Taking into account the geometry of the receiver coil and the applied requirements, in the decoder, where the induced voltage is at most 50% of the 0.5 V edge, the maximum allowable magnetic field can be determined using Figure 22.

For example, at a magnetic field frequency of 1 MHz, a maximum allowable magnetic field of 0.2 kGauss induces a voltage of 0.25 volts on the receiving coil. This is about 50% of the sensing threshold and will not cause false output transitions. Similarly, if such an event occurs during a transmit pulse and is a worst-case polarity, it reduces the receive pulse from >1.0v to 0.75v, still well above the decoder's 0.5v sensing threshold.

Figure 23 shows flux density values for more familiar quantities, such as the maximum allowable current at a given distance from the ADM2490E transformer.

Under the combined effect of strong magnetic fields and high frequencies, any loops formed by PCB traces can generate an error voltage large enough to trigger the thresholds of subsequent circuits. Care should be taken to avoid this possibility when laying out such traces.

application information

Isolated power circuit

The ADM2490E requires a power supply with isolation capability up to about 5 millivolts (this current depends on the data rate used and termination resistors) supplied between the V and GND pins. A transformer driver circuit with a center-tapped transformer and LDO can be used to generate an isolated 5 V supply, as shown in Figure 25. A center-tapped transformer provides electrical isolation from the 5 V supply. The primary winding of the transformer is excited by a pair of square waves that are 180° out of phase with each other. A rectified signal is generated from the secondary winding using a pair of Schottky diodes and a smoothing capacitor. The ADP3330 linear voltage regulator provides regulated power for the bus-side circuitry (V) of the ADM2490E.

printed circuit board layout

The ADM2490E isolated RS-485 transceiver does not require external interface circuitry for logic interfaces. Power supply bypassing is required at the input and output power pins (see Figure 24). Bypass capacitors can be conveniently connected between pins 1 and 2 of V and between pins 15 and 16 of V. The capacitor value should be between 0.01µF and 0.1µF. The total lead length between the ends of the isolated VCC barrier capacitor and the input supply pins should not exceed 20 mm. Also consider bypassing between pin 1 and pin 8 and between pin 9 and pin 16, unless the ground pair on each package side is connected near the package.

In applications involving high common mode transients, care should be taken to ensure that plate coupling on the isolation barrier is minimized. Additionally, the board layout should be designed so that any coupling that occurs will have the same effect on all pins on a given component side. Failure to ensure this could cause voltage differences between pins to exceed the absolute maximum ratings of the device, resulting in latch-up or permanent damage.

typical application

The ADM2490E transceiver is designed for point-to-point transmission lines. Figure 26 shows a full-duplex point-to-point application. To reduce reflections, terminate the line with a terminating resistor at the receiver. The value of the terminating resistor should be equal to the characteristic impedance of the cable.