EVAL-AD5422EBZ

Circuit Note CN-0278 Circuits from the Lab™ reference circuits are engineered and tested for quick and easy system integration to help solve today’s analog, mixed-signal, and RF design challenges. For more information and/or support, visit www.analog.com/CN0278. Devices Connected/Referenced AD5700, Low Power HART Modem AD5700-1 AD5422 16-Bit Current and Voltage Output DAC Complete 4 mA to 20 mA HART Solution with Additional Voltage Output Capability EVALUATION AND DESIGN SUPPORT CIRCUIT FUNCTION AND BENEFITS Circuit Evaluation Boards AD5422 Circuit Evaluation Board (EVAL-AD5422EBZ, LFCSP version) AD5700-1/AD5700 Evaluation Board (EVAL-AD5700-1EBZ) Design and Integration Files Schematics, Layout Files, Bill of Materials 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 IOUT and VOUT 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. 10µF *NC 10µF 2.7V TO 5.5V *C2 10kΩ DVCC CAP2 D4 C1 4.7nF 0.1µF CAP1 AVDD REFIN FAULT REFOUT 0.1µF CLEAR DIGITAL INTERFACE D2 18Ω IOUT LATCH D1 SCLK SDIN AD5422 SDO UART INTERFACE 10.8V TO 26.4V 0.1µF *OP184 10kΩ D3 AVSS 4mA TO 20mA CURRENT LOOP 500Ω RL +VSENSE 0V TO –26.4V VOUT AVSS 10µF –VSENSE RSET GND 0.1µF 0.1µF 15kΩ VCC TXD HART_OUT RH 27kΩ *OP184 WAS USED FOR THESE MEASUREMENTS BUT AN ALTERNATIVES SUCH AS THE OP1177 COULD ALSO BE USED FOR THIS PURPOSE. CH 8.2nF CL 4.7nF RXD RTS REF 1µF AD5700 1.2MΩ 300pF 150kΩ ADC_IP AGND DGND 1.2MΩ 150pF 10803-001 CD Figure 1. AD5422 HART-Enabled Circuit Simplified Schematic 1 HART is a registered trademark of the HART Communication Foundation. Rev. 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All rights reserved. CN-0278 Circuit Note 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. CIRCUIT DESCRIPTION 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 +VSENSE pin is not necessary if the application does not require the IOUT and VOUT pins to be shorted. The HART_OUT signal from the AD5700 is attenuated and ac-coupled into the RSET pin of the AD5422. If the external RSET 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 pincount 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 RSET 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. "1" = MARK 1.2kHz "0" = SPACE 2.2kHz START TXD STOP HART_OUT 8-BIT DATA + PARITY Figure 2. AD5700/AD5700-1 Sample Modulator Waveform Rev. A | Page 2 of 10 10803-002 Application Note AN-1065 describes a manner in which the AD5420 IOUT 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 RSET resistor and involves coupling the HART signal into the RSET pin of the AD5420 or AD5422. The CN-0270 describes this solution for the AD5420, typical of line-powered transmitter applications. The current circuit note is relevant to the AD5422, which, unlike the AD5420, offers both a voltage and a current output pin, and so is particularly useful in PLC/distributed control system (DCS) applications. The AD5422 is available in both 40-lead LFCSP and 24-lead TSSOP packages and the relevance of this, to the circuit characteristics, is examined in the Circuit Description section. Circuit Note CN-0278 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 IOUT 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. C1 CAP2 AVDD AD5422 R3 BOOST R2 FAULT OP184 RSET +VSENSE –VSENSE GND CH RSET AVSS CL RL CCOMP RH VHART Figure 3. AD5422 and AD5700 HART Modem Connection RH ↓ No change ↓ 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 RSET 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 RH and CL. MEASURE CH1 MAX 280mV CH1 MIN –288mV 1 The low-pass and high-pass filter circuitry is formed through the interaction of RH, CL, CH, 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