CN0321

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 that offers a programmable current source and programmable voltage output designed to meet the requirements of industrial process control applications.

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

The AD5422 has an on-board 10 ppm/°C reference. For higher performance over temperature, this design uses an ADR02 reference. The ADR02 is a 5 V precision reference that allows for an input voltage of up to 36 V. It has a 0.05% maximum accuracy error and a 3 ppm/°C maximum temperature drift. This drift contributes approximately 0.02% error across the industrial temperature range.

The AD5422 allows for an internal or external precision, current setting resistor for the current output circuitry. This design uses the internal current sensing resistor option; however, even higher accuracy can be achieved by using a precision external 15 kΩ resistor.

By leaving the DV_CC SELECT pin of the AD5422 floating, an internal 4.5 V power supply is connected to the DV_CC pin which is used as a digital power supply for the AD5700-1 and the field side of the isolators. Alternatively, the 5 V output low dropout (LDO) regulator on the ADuM3471 can be used. The LDO provides a tighter regulated 5 V rail; however, it does not allow for dc overvoltages of greater than 20 V due to the absolute maximum ratings on the regulator input pin of the ADuM3471.

The output connector configuration for the 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 0.5% precision internal oscillator provides significant space savings in channel-to-channel isolated applications where a clock crystal would otherwise be needed per channel. The crystal would typically be bigger than the AD5700-1 IC itself; therefore, the internal oscillator results in significant space saving.

The HART modem output is attenuated by C1 and C2 and ac-coupled into the AD5422 via the CAP2 pin. Additional information can be found in the AN-1065 Application Note. Circuit Note CN-0278 describes an alternate HART coupling method using the R_SET pin that offers greater power supply rejection; however, it requires an external precision current setting resistor.

Isolated Power

The voltage output headroom required by the AD5422 is 0.8 V maximum, and the current output needs a 2.5 V headroom maximum. Therefore, a >12.5 V supply is required to output a 20 mA current through a 500 Ω load. In this design, the minimum supply voltage (overtemperature) is no lower than 13.5 V, which allows for some additional headroom.

The ADuM347x devices are quad-channel digital isolators with integrated pulse-width modulation (PWM) controllers and low impedance transformer drivers (X1 and X2). The only additional components required for an isolated dc-to-dc converter are a transformer and simple full-wave diode rectifier. The devices provide up to 2 W of regulated, isolated power when supplied from a 5.0 V or 3.3 V input, which eliminates the need for a separate isolated dc-to-dc converter.

The iCoupler^® chip-scale transformer technology is used to isolate the logic signals, and the integrated transformer driver with isolated secondary side control provides high efficiency for the isolated dc-to-dc converter. The internal oscillator frequency is adjustable from 200 kHz to 1 MHz and is determined by the R_OC value. When R_OC = 100 kΩ, the switching frequency is 500 kHz.

The ADuM3471 regulation is from the positive 15 V supply. The feedback for regulation is from the divider network (R1 and R2). The resistors are chosen such that the feedback voltage is 1.25 V when the output voltage is 15 V. The feedback voltage is compared with the ADuM3471 internal feedback setpoint voltage of 1.25 V. Regulation is achieved by varying the duty cycle of the PWM signals driving the external transformer.

The negative supply is loosely regulated and could potentially be as low as −26.4 V if unloaded. For this reason, a 25 V Zener diode was placed on the negative supply. This diode draws a small current from the supply when it is lightly loaded; however, it ensures that it clamps at around 25 V.

Another approach is to use an isolation transformer with a 4:1 turns ratio; when it is unloaded, the negative rail does not go as low. In applications that require higher compliance voltages or very low power dissipation, a different power supply design should be considered.

Input Power

The circuit in Figure 1 is powered by a 24 V supply. The ADP2441 is used to step the 24 V down to 5 V to supply all controller side circuitry.

The ADP2441 can accept up to 36 V, reliable transient protection of the supply input is also more easily achieved.

The ADP2441 also features a number of other safety/reliability functions, such as undervoltage lockout (UVLO), a precision enable feature, a power good pin, and overcurrent limit protection. It can achieve up to 90% efficiency for an input of 24 V and an output of 5 V.

Isolation

The ADuM3471 power isolation circuitry includes four fully isolated voltage channels with a 2.5 kV isolation rating. These four channels are used to isolate the four data lines (SCLK, LATCH, SDIN, and SDO) of the AD5422. Isolation of the SDO line is not essential for the operation of the circuit; however, it does allow access to diagnostic and fault features, as well as register readback.

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

Further information on C942BD62-247D-4349-9EDB-0CCABE06F048 products is available at www.analog.com/icoupler.

DC Overvoltage Protection

The circuit in Figure 1 allows for continuous +36 V and −28 V dc overvoltage protection. This means the circuit is protected in cases where a dc power supply line is accidentally connected to the output.

During an overvoltage condition, the supplies are pulled up or down via the external protection diodes. The resistance between these diodes and the output terminals limits the peak current.

The maximum/minimum voltage on the output terminals is limited by the breakdown voltage on any circuitry connected to the output or power supplies. The current and voltage outputs of the AD5422 can tolerate +48 V down to−28 V. The AV_SS input can tolerate −28 V, and the AV_DD can tolerate +48 V. The ADR02 reference can tolerate 36 V on its supply. The ADC_IP pin of the AD5700-1 is protected by a 150 kΩ resistor that limits any current, followed by a 300 pF capacitor to block any dc current. Do not expose other ICs to higher voltages during the dc overvoltage condition.

Transient Voltage Protection

The AD5422 contains ESD protection diodes that prevent damage from normal handling. The industrial control environment can, however, subject I/O circuits to much higher transients. To protect the AD5422 from excessively high voltage transients, external power diodes and a surge current limiting resistor may be required, as is shown in Figure 1.

The constraint on the resistor value in the current output path, shown in Figure 1 as 18 Ω, is that during normal operation, the output level at I_OUT must remain within its voltage compliance limit of AV_DD − 2.5 V, and the two protection diodes and resistor must have the appropriate power ratings. With 18 Ω, for a 4 mA to 20 mA output, the compliance limit at the terminal is decreased by V = I_MAX × R = 0.36 V.

The constraint on the resistor value in the voltage output path, shown in Figure 1 as 100 Ω, is that there must be 0.8 V headroom over the output voltage. The effect of this resistor can be minimized by using the +V_SENSE input. Shown in Figure 1, the +V_SENSE input is protected by a 22 Ω resistor. There is also a corresponding 22 Ω resistor on the −V_SENSE path. These two 22 Ω resistors cause some absolute gain error that may need to be calibrated out at room temperature; the reason for this error is that there is only about 70 kΩ impedance in the internal feedback circuitry of the AD5422. The advantage of sensing the voltage at the output and not at the V_OUT pin of the AD5422 is that the protection resistor for the V_OUT pin has a varying voltage across it, depending on the load current drawn. Sensing at the terminal avoids this error source.

Further protection is provided with transient voltage suppressors (TVS) or transorbs. These are available as both unidirectional and bidirectional suppressors and in a wide range of standoff and breakdown voltage ratings. Size the TVS with the lowest breakdown voltage possible while not conducting in the functional range of the current output. As previously discussed, it is recommended that all remotely connected nodes be protected.