This circuit provides a complete, fully isolated, analog output channel for programmable logic controllers (PLCs) requiring standard 4mA to 20 mA HART® ^{1 -} compatible current outputs and unipolar/bipolar output voltage ranges and distributed control system (DCS) modules. It provides a flexible building block for channel-to-channel isolated PLC/DCS output modules or any other industrial application requiring fully isolated analog outputs. The circuit also provides external protection on the analog output.

The AD5422 16-bit analog-to-analog converter (DAC) is software configurable to provide all necessary current and voltage outputs.

The AD5700-1 is the industry's lowest power and smallest HART-compatible IC modem. Used in conjunction with the AD5422 , it forms a complete HART-compatible 4 mA to 20 mA solution. The AD5700-1 integrates an internal precision oscillator for additional space savings, especially in channel-to-channel isolation applications. The PLC/DCS solution must be isolated from the local system controller to protect it from ground loops while ensuring that it is not affected by external events. Traditional solutions utilize discrete ICs to provide power and digital isolation. When multi-channel isolation is required, the cost and space of discrete power solutions can become a significant disadvantage. Opto-isolator-based solutions often provide reasonable output regulation but require additional external components and thus increase board area. Power modules are often bulky and may have poor output regulation. The circuit in Figure 1 uses the ADuM347x family of isolators and power conditioning circuits, along with corresponding feedback isolation. Use an external transformer to transfer power to the other side of the isolation barrier.

The ADuM3482 provides UART signal isolation for the AD5700-1 .

The ADP2441 , is a 36 V step-down DC-DC regulator that operates on an industry-standard 24 V power supply and has wide input voltage tolerance. It steps down the voltage to 5 V to power all controller side circuits. The circuit also integrates standard external protection on the 24 V supply side and also provides DC overvoltage protection from +36 V to −28 V.

¹ HART is a registered trademark of the HART Communication Foundation.

Analog output

For industrial control modules, standard analog output voltage and current ranges include ±5 V, ±10 V, 0 V to +5 V, 0 V to +10 V, +4 mA to +20 mA, and 0 mA to +20 mA. The AD5422 is a precision, fully integrated 16-bit DAC with built-in programmable current source and programmable voltage output designed to meet the needs of industrial process control applications.

The AD5422 provides all of the output ranges listed previously, with the current and voltage output ranges available on separate pins. 10% overrange capability is provided for all voltage ranges, and 0 mA to 20 mA overrange is provided for current outputs. Analog outputs are short-circuit and open-circuit protected.

The AD5422 has an on-chip 10 ppm/°C reference. To achieve higher performance over the specified temperature range, the design uses an ADR02 voltage reference. The ADR02 is a 5 V precision voltage reference that allows input voltages up to 36 V. Its maximum accuracy error is 0.05% and its maximum temperature drift is 3 ppm/°C. This drift contributes approximately 0.02% error over the industrial temperature range.

The AD5422 supports internal or external precision current setting resistors as current output circuits. This design uses the internal current-sense resistor option; however, greater accuracy can be achieved by using a precision external 15 kΩ resistor.

By floating the DV _CC SELECT pin of the AD5422 , the internal 4.5 V supply can be connected to the DV _CC pin for the digital power supply of the AD5700-1 and the field side of the isolator. You can also use the 5 V output low dropout (LDO) regulator on the ADuM3471 . The LDO provides a more compact 5 V regulated supply rail; however, due to the absolute maximum ratings on the ADuM3471 regulator input pins, it does not tolerate dc overvoltages higher than 20 V.

The output connector configuration of EVAL-CN0321-SDPZ hardware is shown in Table 1.

HART compatibility

The AD5700-1 is used in conjunction with the AD5422 to form a complete HART-compatible 4 mA to 20 mA solution. The AD5700-1 is a 0.5% precision internal oscillator that greatly saves board space in applications with channel-to-channel isolation that would otherwise require a clock crystal for each channel. The crystal is typically larger than the AD5700-1 IC itself; therefore, the area saved by the internal oscillator is enormous.

The HART modem output is attenuated through C1 and C2 and ac coupled to the AD5422 through the CAP2 pin . See AN-1065 application note  for additional information . Circuit note CN-0278 describes an alternative HART coupling method using the R SET pin that has higher power supply rejection but requires the use of an external precision current setting resistor.

Isolated Power

The AD5422 requires a maximum of 0.8 V headroom for the voltage output and a maximum of 2.5 V headroom for the current output. Therefore, a power supply greater than 12.5 V is required to output 20 mA through a 500Ω load. In this design, the minimum supply voltage (over temperature) does not exceed 13.5 V, allowing a certain margin.

The ADuM347x are quad-channel digital isolators with an integrated pulse-width modulation (PWM) controller and low-impedance transformer drivers (X1 and X2). An isolated DC/DC converter requires only the following additional components: a transformer and a simple full-wave diode rectifier. The device can deliver up to 2 W of regulated isolated power from a 5.0 V or 3.3 V input supply, eliminating the need for an additional isolated DC-DC converter.

iCoupler® chip-scale transformer technology is used to isolate logic signals; the integrated transformer driver with isolated secondary side control function can improve the ^efficiency of isolated DC/DC converters. The internal oscillator frequency is adjustable from 200 kHz to 1 MHz, determined by the value of R OC . When R OC = 100 kΩ, the switching frequency is 500 kHz.

The ADuM3471 regulates from the 15 V positive supply. Regulatory feedback comes from the voltage divider network (R1 and R2). The resistor is selected according to the following requirement: When the output voltage is 15 V, the feedback voltage is 1.25 V. The feedback voltage is compared to the ADuM3471 internal feedback setpoint voltage of 1.25 V. Regulation is achieved by changing the duty cycle of the PWM signal driving an external transformer.

The negative supply is less tightly regulated and may be as low as −26.4 V at no load. So place a 25V Zener diode at the negative supply. At light load, the diode draws a small current from the supply, but ensures that it clamps to around 25 V.

Another approach is to use an isolation transformer with a 4:1 turns ratio; when it is unloaded, the negative supply rail will not go too low. In applications requiring higher compliance voltages or very low power consumption, other power supply designs should be considered.

input power

The circuit in Figure 1 operates from a 24 V power supply. The ADP2441 is used to step down 24 V to 5 V to power all controller side circuits.

The ADP2441 accepts voltages up to 36 V and makes it easier to achieve reliable power input transient protection.

The ADP2441 also has various other safety/reliability features such as undervoltage lockout (UVLO), precision enable feature, power good pin, and overcurrent limit protection. For 24 V input and 5 V output, it can achieve up to 90% efficiency.

isolation

The ADuM3471 power isolation circuit includes four fully isolated voltage channels with an isolation rating of 2.5 kV. These four channels are used to isolate the four data lines of the AD5422 (SCLK, LATCH, SDIN, and SDO). SDO line isolation is not necessary for circuit operation but allows access to diagnostic and fault features, as well as register readback.

The ADuM3482 is a 3.75 kV quad-channel digital isolator available in a small 20-lead SSOP package (7.2 mm × 7.8 mm). The ADuM3482 core operates from a voltage range of 3.0 V to 5.5 V, while the I/O supply range is from 1.8 V to 5.5 V. These devices can be used to interface directly with 1.8 V logic devices. This isolator is used to isolate the UART signal of the AD5700-1 HART modem.

For more information about i products, please visit www.analog.com/icoupler .

DC overvoltage protection

The circuit in Figure 1 allows overvoltage protection for continuous DC voltages +36 V and −28 V. This means that the circuit is protected in the event that the DC power line is accidentally connected to the output.

During overvoltage conditions, the power supply is pulled high or low through external protection diodes. The resistance between these diodes and the output limits the peak current.

The maximum/minimum voltage at the output is limited by the breakdown voltage of any connected output or power circuit. The current and voltage outputs of the AD5422 are tolerant from +48 V to a minimum of −28 V. The AV SS input is tolerant to −28 V and the AV DD is tolerant to +48 V. The ADR02 reference input is 36 V tolerant. The ADC_IP pin of the AD5700-1 is protected by a 150 kΩ resistor that limits all current, followed by a 300 pF DC blocking capacitor. Do not expose other ICs to higher voltages during DC overvoltage conditions.

Transient voltage protection

The AD5422 has built-in ESD protection diodes to prevent damage caused by normal operation. However, industrial control environments subject I/O circuits to much higher transients. To prevent excessive transient voltages from affecting the AD5422 , external power diodes and inrush current limiting resistors may be required, as shown in Figure 1.

The constraint on the resistance value in the current output path (18 Ω in Figure 1) is that during normal operation, the I OUT output level must remain within its compliance voltage limit, which is AV DD − 2.5 V, and both protection Diodes and resistors must have appropriate power ratings. At 18 Ω, the voltage limit on the pin is reduced by V = IMAX × R = 0.36 V for a 4 mA to 20 mA output.

The constraints on the resistor value in the voltage output path (100 Ω in Figure 1) are: There must be 0.8 V margin over the entire output voltage range. Resistive effects are minimized by the +V SENSE input. As shown in Figure 1, the +V SENSE input is protected by a 22 Ω resistor. Accordingly, there is a 22 Ω resistor in the −V SENSE path. These two 22 Ω resistors cause an absolute gain error that may require calibration at room temperature; this error occurs because the AD5422 's internal feedback circuit impedance is only 70 kΩ. The advantage of sensing the voltage at the output of the AD5422 (rather than the V OUT pin) is that the V OUT pin has a variable voltage across the protection resistor, depending on the load current being drawn. Detecting at the output avoids this source of error.

Further protection can be achieved with transient voltage suppressors (TVS) or transient absorbers. These components include unidirectional and bidirectional suppressors, available in a wide variety of isolation and breakdown voltage ratings. TVS should be calibrated with the lowest breakdown voltage as much as possible, and at the same time, it should not conduct within the normal output range of current. As mentioned before, it is recommended to secure all remotely connected nodes.