AN-2544: Complete 4 mA to 20 mA HART Solution with Additional Voltage Output Capability

Circuit Function and Benefits

The circuit shown in Figure 1 uses the AD5700, the industry’s lowest power and smallest footprint HART^®1-compliant IC modem, and the AD5422, a 16-bit current output and voltage output DAC, to form a complete HART-compatible 4 mA to 20 mA solution. The use of the OP184 in the circuit allows the I_OUT and V_OUT pins to be shorted together, thus reducing the number of screw connections required in programmable logic control (PLC) module applications. For additional space savings, the AD5700-1 offers a 0.5% precision internal oscillator.

Circuit Description

Application Note AN-1065 describes a manner in which the AD5420 I_OUT DAC can be configured for HART communication compliance. AN-1065 outlines how the AD5700 HART modem output can be attenuated and ac coupled into the AD5420 via the CAP2 pin. The same is true of the AD5422. However, if the application involves a particularly harsh environment, an alternative circuit configuration can be used which offers better power supply rejection characteristics. This alternative circuit requires the use of the external R_SET resistor and involves coupling the HART signal into the R_SET pin of the AD5420 or AD5422.

This circuit adheres to the HART physical layer specifications as defined by the HART Communication Foundation, for example, the output noise during silence and the analog rate of change specifications.

For many years, 4 mA to 20 mA communication has been used in process control instrumentation. This communication method is reliable and robust, and offers high immunity to environmental interference over long communication distances. A limitation, however, is that only 1-way communication of one process variable at a time is possible.

The development of the highway addressable remote transducer (HART) standard provided highly capable 2-way digital communication, simultaneously with the 4 mA to 20 mA analog signaling used by traditional instrumentation equipment. This allows for features such as remote calibration, fault interrogation, and transmission of additional process variables. Put simply, HART is a digital two-way communication in which a 1 mA peak-to-peak, frequency-shift-keyed (FSK) signal is modulated on top of the 4 mA to 20 mA analog current signal.

Figure 1 shows the manner in which the AD5422 can be combined with the AD5700 HART modem and a UART interface to construct a HART-capable 4 mA to 20 mA current output, typical of PLC and DCS systems. The buffer connected to the +V_SENSE pin is not necessary if the application does not require the I_OUT and V_OUT pins to be shorted. The HART_OUT signal from the AD5700 is attenuated and ac-coupled into the R_SET pin of the AD5422. If the external R_SET resistor is not being used, an alternative method of connecting the AD5422 and the AD5700 via the CAP2 pin can be found in Application Note AN-1065, as previously described. This method is only relevant to the 40-lead LFCSP package option of the AD5422 because the lower pin-count 24-lead TSSOP package does not contain a CAP2 pin.

While the method described in the current circuit note requires the use of the external R_SET resistor, in return, it provides better power supply rejection performance than the alternative application note solution. The use of either solution results in the AD5700 HART modem output modulating the 4 mA to 20 mA analog current (as shown in Figure 2) without affecting the dc level of the current. The diode protection circuitry (D1 to D4) is discussed in more detail in the Transient Voltage Protection section.

Determining the Values of the External Components

Capacitors, C1 and C2, can be used in conjunction with the digital slew rate control functionality of the part to control the slew rate of the I_OUT signal of the AD5422. In determining the absolute values of the capacitors, ensure that the FSK output from the modem is passed undistorted. Thus, the bandwidth presented to the modem output signal must pass the 1200 Hz and 2200 Hz frequencies. Figure 3 shows a circuit that achieves this requirement. In this case, C2 (shown in Figure 1) is left open-circuit.

The low-pass and high-pass filter circuitry is formed through the interaction of R_H, C_L, C_H, and C1, along with some internal circuitry in the AD5422. In calculating the values of these components, the low-pass and high-pass frequency cutoff point targets were >10 kHz and <500 Hz, respectively. Figure 4 shows a plot of the simulated frequency response, while Table 1 shows the effect on the frequency response of increasing each component while the remaining component values are kept constant.

The output of the modem is an FSK signal consisting of 1200 Hz and 2200 Hz shift frequencies. This signal must translate to a 1 mA p-p current signal. To achieve this, the signal amplitude at the R_SET pin must be attenuated. This is due to the internal current gain configuration in the AD5422 design. Assuming that the modem output amplitude is 500 mV p-p, its output must be attenuated by 500/150 = 3.33. This attenuation is achieved by means of R_H and C_L.

The measurements in this circuit note were completed using the following component values:

• C1 = 4.7 nF
• R_H = 27 kΩ
• C_L = 4.7 nF
• C_H = 8.2 nF

Figure 5 shows the individual 1200 Hz and 2200 Hz shift frequencies measured across a 500 Ω load resistor. Channel 1 shows the modulated HART signal coupled into the AD5422 output (set to output 4 mA), while Channel 2 shows the AD5700 TXD signal.

Hart Compliance

For the circuit in Figure 1 to be HART-compliant, it must meet the HART physical layer specifications. There are numerous physical layer specifications included in the HART specification documents. The two that are most important in this case are the output noise during silence and the analog rate of change.

Output Noise During Noise

When a HART device is not transmitting (silent), it should not couple noise onto the network in the HART extended frequency band. Excessive noise may interfere with reception of HART signals by the device itself or other devices on the network.

The voltage noise measured across a 500 Ω load must contain no more than 2.2 mV rms of combined broadband and correlated noise in the extended frequency band. This noise was measured by connecting the HCF_TOOL-31 filter (available from the HART Communication Foundation) across the 500 Ω load and by connecting the output of the filter to a true rms meter (see Figure 6). An oscilloscope was also used to examine the output waveform peak-to-peak voltage.

The AD5422 output current was set to 4 mA, 12 mA, and 20 mA. Results with the band-pass filter in place were very similar for all three output current values, while the wide bandwidth noise increased slightly as the current output value increased. The rms values measured, with and without the HCF_TOOL-31 band-pass filter in the case of 4 mA output current, were 143 µV rms and 1.4 mV rms, respectively. Both of these values are well within the required specifications of 2.2 mV rms (with HART filter) and 138 mV rms (broadband noise without HART filter). For 12 mA output current, the rms values measured, with and without the HCF_TOOL-31 band-pass filter were 158 µV rms and 2.1 mV rms, respectively, again, both well within HART protocol specifications.

Figure 7 and Figure 8 show the oscilloscope plots for 4 mA and 12 mA output current, respectively. Note that the filter has a pass-band gain of 10. Channel 1 and Channel 2 on each plot show the input and output of the filter, respectively.

Analog Rate of Change

This specification ensures that when a device regulates current, the maximum rate of change of analog current does not interfere with HART communications. Step changes in current disrupt HART signaling. The same test circuit shown in Figure 6 was used. For this test, the AD5422 was programmed to output a cyclic waveform, switching from 4 mA to 20 mA with no delay at either value, to ensure the maximum rate of change. To meet the HART specifications, the waveform at the output of the filter must not exhibit a peak voltage greater than 150 mV. Meeting this requirement ensures that the maximum bandwidth of the analog signaling is within the specified dc to 25 Hz frequency band.

The normal time for the output of the AD5422 to change from 4 mA to 20 mA is about 10 µs. This is obviously too fast and can cause major disruption to a HART network. To reduce the rate of change, the AD5422 employs two features: connecting capacitors at the CAP1 and CAP2 pins, and an internal linear digital slew rate control function (refer to the AD5422 data sheet for details). For faster slew rates, a nonlinear digital ramp can be implemented on the controller/FPGA communicating with the AD5422.

It requires very large capacitor values at CAP1 and CAP2 to reduce the bandwidth below 25 Hz. The optimum solution is to use a combination of the external capacitors and the digital slew rate control function of the AD5422. The two capacitors, C1 and C2, have the effect of reducing the rate of change of the analog signal; however, not sufficiently enough to meet the specification. Enabling the slew rate control feature offers the flexibility to set the rate of change.

Figure 9 shows the output of the AD5422 and the output of the HART filter. The peak voltage at the output of the filter is within specification at 82 mV. The slew rate settings are SR clock = 3 and SR step = 2, setting the transition time from 4 mA to 20 mA at approximately 120 ms. C1 is 4.7 nF and C2 is unconnected. If this rate of change is too slow, the slew time can be reduced. With this circuit configuration of C1 = 4.7 nF and C2 unconnected, it was found that setting up an 80 ms slew time (SR clock = 1, SR step = 2) gave an analog rate of change result inside the HART specification. However, reducing the slew time further, to 60 ms (SR clock = 0, SR step = 2), pushed the result just outside of the 150 mV specification. The capacitor connected from CAP1 to AVDD can be used to counteract the effect of the increased peak voltage at the output of the filter due to faster slew times. However, care must be taken when choosing this value because it has an effect on the low-pass filter frequency cutoff discussed in the Determining the Values of the External Components section.

Figure 10 shows the results of changing the slew rate control settings to SR clock = 5 and SR step = 2, while leaving the C1 capacitor value unchanged at 4.7 nF. This results in a transition time of approximately 240 ms. The peak amplitude at the output of the filter can be reduced further by increasing the value of C1, configuring a slower slew rate, or a combination of both.

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 shown in Figure 1. The constraint on the resistor value, 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. There is also a 10 kΩ resistor shown at the positive input of the OP184 buffer. This protects the amplifier by limiting the current during a transient event. Further protection can be 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. It is recommended that all remotely connected nodes be protected.

In many process control applications, it is necessary to provide an isolation barrier between the controller and the unit being controlled to protect and isolate the controlling circuitry from any hazardous common-mode voltages that may occur.

The iCoupler family of products from Analog Devices, Inc., provides voltage isolation in excess of 2.5 kV. Further information on iCoupler products is available at www.analog.com/icouplers. To reduce the number of isolators required, nonessential signals, such as CLEAR, can be connected to GND; FAULT and SDO can be left unconnected, reducing the isolation requirements to only three signals. However, note that either FAULT or SDO are required to provide access to the fault detection features of the AD5422.