Loop-Powered a 4 mA to 20 mA DAC AD421 FEATURES FUNCTIONAL BLOCK DIAGRAM 4 mA to 20 mA Current Output ® HART Compatible REF IN REF OUT1 REF OUT2 16-Bit Resolution and Monotonicity LV (+2.5V) (+1.25V) (+2.5V) V CC �0.01% Integral Nonlinearity 75k� 5 V or 3 V Regulator Output 112.5k� 2.5 V and 1.25 V Precision Reference AD421 134k� 750�A Quiescent Current max Programmable Alarm Current Capability DRIVE BANDGAP REFERENCE Flexible High Speed Serial Interface COMP 121k� 16-Lead SOIC and PDIP Packages BOOST LOCAL OSCILLATOR DATA INPUT SHIFT REGISTER CLOCK SWITCHED 16-BIT CURRENT DAC LATCH SIGMA- SOURCES LATCH 40� DELTA DAC AND GENERAL DESCRIPTION FILTERING 80k� LOOP POWER-ON The AD421 is a complete, loop-powered, digital to 4 mA to RTN RESET 20 mA converter, designed to meet the needs of smart trans- mitter manufacturers in the Industrial Control industry. It pro- COM C1 C2 C3 vides a high precision, fully integrated, low cost solution in a compact 16-lead package. The AD421 is ideal for extending the PRODUCT HIGHLIGHTS resolution of smart 4 mA to 20 mA transmitters at very low cost. 1. The AD421 is a single chip, high performance, low cost The AD421 includes a selectable regulator that is used to power solution for generating 4 mA to 20 mA signals for smart itself and other devices in the transmitter. This regulator pro- industrial control transmitters. vides either a +5 V, +3.3 V or +3 V regulated output voltage. 2. The AD421’s regulated supply voltage can be used to power The part also contains +1.25 V and +2.5 V precision references. any additional circuits in the transmitter. The regulated The AD421 thus eliminates the need for a discrete regulator output value is pin selectable as either +3 V, +3.3 V or +5 V. and voltage reference. The only external components required are a number of passive components and a pass transistor to 3. The AD421’s on-chip references can provide a precision span large loop voltages. reference voltage to other devices in the system. This refer- ence voltage can be either +1.25 V or +2.5 V. The AD421 can be used with standard HART FSK protocol communication circuitry without any degradation in specified 4. The AD421 is fully compatible with standard HART cir- performance. The high speed serial interface is capable of oper- cuitry or other similar FSK protocols. ating at 10 Mbps and allows for simple connection to com- 5. With the addition of a single discrete transistor, the AD421 monly-used microprocessors and microcontrollers via a standard can be operated from V + 2 V min to a maximum of the CC three-wire serial interface. breakdown voltage of the pass transistor. The sigma-delta architecture of the DAC guarantees 16-bit 6. The AD421 converts the digital data to current with 16-bit monotonicity while the integral nonlinearity for the AD421 is resolution and monotonicity. Full-scale settling time to ±0.01%. The part provides a zero scale 4 mA output current ±0.1% typically occurs within 8 ms. with ±0.1% offset error and a 20 mA full-scale output current 7. The AD421 features a programmable alarm current capabil- with ±0.2% gain error. ity that allows the transmitter to send out of range currents to The AD421 is available in a 16-lead, 0.3 inch-wide, plastic DIP indicate a transducer fault. and in a 16-lead, 0.3 inch-wide, SOIC package. The part is speci- fied over the industrial temperature range of –40°C to +85°C. HART is a registered trademark of the HART Communication Foundation. REV. C Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. use, nor for any infringements of patents or other rights of third parties Tel: 781/329-4700 World Wide Web Site: http://www.analog.com which may result from its use. No license is granted by implication or Fax: 781/326-8703 © Analog Devices, Inc., 2000 otherwise under any patent or patent rights of Analog Devices. 1 (Using DN25D as pass transistor as per Figure 3; AD421–LOOP-POWERED SPECIFICATIONS REF IN = REF OUT2; T = T to T unless otherwise noted) A MIN MAX 2 Parameter B Versions Units Conditions/Comments OUTPUT CHARACTERISTICS 3 Current Loop Voltage Compliance V + 2 V min CC 350 V max DN25D Breakdown Voltage Full-Scale Settling Time 8 ms typ Settling Time to ±0.1%, C1 = C2 = 10 nF, C3 = 3.3 nF Output Impedance 25 MΩ typ AC Loop Voltage Sensitivity 2 µA/V typ 1200 Hz to 2200 Hz VOLTAGE REGULATOR Output Voltage (V ) CC 3 V Mode 2.95/3.05 V min/V max 3 V Nominal. LV Pin Connected to V CC 3.3 V Mode 3.25/3.35 V min/V max 3.3 V Nominal. LV Pin Connected Through 0.01 µF to V CC 5 V Mode 4.95/5.05 V min/V max 5 V Nominal. LV Pin Connected to COM Externally Available Current 3.25 mA min Assuming 4 mA Flowing in the Loop Line Regulation 1 µV/V typ Load Regulation 15 µV/mA typ DAC SPECIFICATIONS (V = +3 V to +5 V; REF IN = REF OUT2; T = T to T unless otherwise noted) CC A MIN MAX 2 Parameter B Versions Units Conditions/Comments ACCURACY Resolution 16 Bits Monotonicity 16 Bits min Integral Nonlinearity ±0.01 % of FS max FS = Full-Scale Output Current 4 Offset (4 mA) @ +25°C ±0.1 % of FS max V = 5 V CC Offset Drift ±25 ppm of FS/°C max Includes On-Chip Reference Drift 4 Total Output Error (20 mA) @ +25°C ±0.2 % of FS max V = 5 V CC Total Output Drift ±50 ppm of FS/°C max Includes On-Chip Reference Drift V Supply Sensitivity 50 nA/mV max 25 nA/mV Typical CC VOLTAGE REFERENCE REF OUT2 Output Voltage 2.49/2.51 V min/V max 2.5 V Nominal Drift ±40 ppm/°C max 20 ppm/°C Typical from –40°C to +25°C and –2.5 ppm/°C Typical from +25°C to +85°C Externally Available Current 0.5 mA min V Supply Sensitivity 150 µV/V max 15 µV/V Typical CC Output Impedance 3 Ω typ Noise (0.1 Hz–10 Hz) 6 µV (p-p) typ REF OUT1 5 Output Voltage 1.24/1.26 V min/V max 1.25 V Nominal, 100 kΩ Load to COM Drift ±50 ppm/°C max 20 ppm/°C Typical from –40°C to +25°C and 2 ppm/°C Typical from +25°C to +85°C Externally Available Current 0.5 mA min V Supply Sensitivity 150 µV/V max 15 µV/V Typical CC Output Impedance 3 Ω typ Noise (0.1 Hz–10 Hz) 4 µV (p-p) typ REF IN Input Resistance 40 kΩ typ DIGITAL INPUTS V (Logic 1) 0.75 × V V min IH CC V (Logic 0) 0.25 × V V max IL CC I ±10 µA max V = V IH IN CC I ±10 µA max V = 0 V IL IN Data Coding Binary Data Rate 10 Mbps max POWER SUPPLIES Operating Range +2.95 to +5.05 V min to V max Functional to 7 V Quiescent Current @ V = 3 V 650 µA max 475 µA Typical CC @ V = 5 V 750 µA max 575 µA Typical CC NOTES 1 The DN25D is available from Supertex, Inc., 1350 Bordeaux Drive, Sunnyvale, CA 94089. 2 Temperature range is –40°C to +85°C. 3 The max current loop voltage compliance is determined by the pass transistor breakdown voltage and is 350 V for the DN25D. 4 With V = 3 V, the transfer function shifts negative by typically 0.25%; a 16 kΩ resistor connected between COM and LOOPRTN will approximately compensate for the V CC CC supply sensitivity in moving from 5 V to 3 V by skewing the gain of the AD421. 5 100 kΩ resistor only required if this reference is being used in application circuits. Specifications subject to change without notice. REV. C –2– AD421 1, 2, 3 TIMING CHARACTERISTICS (V = +3 V to +5 V, T = T to T unless otherwise noted) CC A MIN MAX Parameter (B Versions) Units Conditions/Comments t 100 ns min Data Clock Period CK t 50 ns min Data Clock Low Time CL t 50 ns min Data Clock High Time CH t 30 ns min Data Stable Width DW t 30 ns min Data Setup Time DS t 0 ns min Data Hold Time DH t 50 ns min Latch Delay Time LD t 50 ns min Latch Low Time LL t 50 ns min Latch High Time LH NOTES 1 Guaranteed by characterization at initial product release, not production tested. 2 See Figures 1 and 2. 3 All input signals are specified with tr = tf = 5 ns (10% to 90% of V ) and timed from a voltage level of (V + V )/2; tr and tf should not exceed 1 µs on any digital CC IN IL input. Specifications subject to change without notice. CLOCK WORD "N" WORD "N +1" 1 DATA 10 1 1 0011 0 0 1 0 0 1 1 1 0 0 1 LATCH Figure 1. Serial Interface Waveforms (Normal Data Load) t CK t CL CLOCK t CH t t DS DH DATA t DW t LD t LL LATCH t LH Figure 2. Serial Interface Timing Diagram –3– REV. C (MSB) B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 (LSB) B15 B14 B13 B12 AD421 ABSOLUTE MAXIMUM RATINGS* PIN CONFIGURATION (T = +25°C unless otherwise noted) A DIP and SOIC DRIVE, BOOST, COMP to COM . . . –0.5 V to V + 0.5 V CC LOOP RTN to COM . . . . . . . . . . . . . . . . . . . –2 V to + 0.5 V Digital Input Voltage to COM . . . . . . . –0.5 V to V + 0.5 V CC 1 16 REF OUT1 V CC Operating Temperature Range REF OUT2 2 15 BOOST Commercial (B Version) . . . . . . . . . . . . . . –40°C to +85°C REF IN 3 14 COMP Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C LV 4 AD421 13 DRIVE Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150°C LATCH 5 12 C1 TOP VIEW Plastic DIP Package, Power Dissipation . . . . . . . . . . 670 mW (NOT TO SCALE) CLOCK 6 11 C2 θ Thermal Impedance . . . . . . . . . . . . . . . . . . . . 116°C/W JA DATA 7 10 C3 Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . 260°C LOOP RTN 8 9 COM SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW θ Thermal Impedance . . . . . . . . . . . . . . . . . . . . 110°C/W JA Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215°C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220°C ORDERING GUIDE * Stresses above those listed under Absolute Maximum Ratings may cause perma- Temperature Package nent damage to the device. This is a stress rating only; functional operation of the * Model Range Option device at these or any other conditions above those listed in the operational sections of this specification is not implied. Exposure to absolute maximum rating AD421BN –40°C to +85°C N-16 conditions for extended periods may affect device reliability. AD421BR –40°C to +85°C R-16 AD421BRRL –40°C to +85°C R-16; Reeled SOIC EVAL-AD421EB Evaluation Board *N = Plastic DIP, R = SOIC. CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily WARNING! accumulate on the human body and test equipment and can discharge without detection. Although these devices feature proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD ESD SENSITIVE DEVICE precautions are recommended to avoid performance degradation or loss of functionality. –4– REV. C AD421 PIN FUNCTION DESCRIPTIONS Pin No. Mnemonic Function 1 REF OUT1 Reference Output 1. A precision +1.25 V reference is provided at this pin. It is intended as a precision ref- erence source for other devices in the transmitter. REF OUT1 is a buffered output capable of providing up to 0.5 mA to external circuitry. If REF OUT 1 is required to sink current, a resistive load of 100 kΩ to COM should be added. (See Reference section.) 2 REF OUT2 Reference Output 2. A precision +2.5 V reference is provided at this pin. To operate the AD421 with its own reference, REF OUT2 should be connected to REF IN. It can also be used as a precision reference source for other devices in the transmitter. REF OUT2 is a buffered output capable of providing up to 0.5 mA to external circuitry. 3 REF IN Voltage Reference Input. The reference voltage for the AD421 is applied to this pin and it sets the span for the AD421. The nominal reference voltage for the AD421 is +2.5 V for correct operation. This can be sup- plied using an external reference source or by using the part’s own REF OUT2 voltage. 4 LV Regulated Voltage Control Input. The LV input controls the loop gain of the servo amplifier to set V . CC With LV connected to COM, the regulator voltage is set to 5 V nominal. If the LV input is connected through 0.01 µF to V , the regulated voltage is nominally 3.3 V. With LV connected to V the regulated voltage, CC CC V , is 3 V nominal. CC 5 LATCH DAC Latch Input. Logic Input. A rising edge of the LATCH signal loads the data from the serial input shift register to the DAC latch and hence updates the output of the DAC. The number of clock cycles provided between latch pulses determines whether the DAC is in alarm or normal current mode. (See Digital Inter- face section.) 6 CLOCK Data Clock Input. Data on the DATA input is clocked into the shift register on the rising edge of this CLOCK input. The period of this clock equals the input serial data bit rate. This serial clock rate can be up to 10 MHz. If 16 clock cycles are provided between LATCH pulses then the data on the DATA input is accepted as normal 4–20 mA data. If more than 16 clock cycles are provided between LATCH pulses, the data is assumed to be alarm current data (see Digital Interface section). 7 DATA Data Input. The data to be loaded to the AD421 input shift register is applied to this input. Data should be valid on the rising edge of the CLOCK input. 8 LOOP RTN Loop Return Output. LOOP RTN is the return path for current flowing in the current loop. 9 COM Common. This is the reference potential for the AD421 analog and digital inputs and outputs and for the voltage regulator output. 10 C3 Filtering Capacitor. A low dielectric absorption capacitor ceramic capacitor should be connected between this pin and COM for internal filtering of the switched current sources. 11 C2 Filtering Capacitor. See C3 description. 12 C1 Filtering Capacitor. See C3 description. 13 DRIVE Output from the Voltage Regulator Loop. The DRIVE signal controls the external pass transistor to establish and maintain the correct V level programmed by the LV inputs while providing the necessary bias as the loop cur- CC rent is programmed from 4 mA to 20 mA. 14 COMP Compensation Capacitor Input. A capacitor connected between COMP and DRIVE is required to stabilize the feedback loop formed with the regulator op amp and the external pass transistor. 15 BOOST This open collector pin sinks the necessary current from the loop so that the current flowing into BOOST plus the current flowing into COM is equal to the programmed loop current. 16 V Power Supply. V is the power supply input of the AD421 and it also provides the voltage regulator output, CC CC driven by the external pass transistor. It is used both to bias the AD421 itself and to provide power for the rest of the smart transmitter circuitry. The LV input determines the regulated voltage output to be either 3 V, 3.3 V or 5 V nominal. Alternatively, a separate power supply can be connected to this pin to power the AD421. V should be decoupled to COM with a 2.2 µF capacitor. CC –5– REV. C AD421 CIRCUIT DESCRIPTION Table I. FET Characteristics The AD421 is designed for use in loop-powered 4–20 mA smart FET Type N-Channel Depletion Mode transmitter applications. A smart transmitter, as a remote in- strument, controls its current output signal on the same pair of I 24 mA min DSS wires from which it receives its power. The AD421 essentially BV (V – V ) min DS LOOP CC provides three primary functions in the smart transmitter. These functions are a DAC function for converting the microprocessor/ V V max PINCHOFF CC microcontroller’s digital data to analog format, a current amp- Power Dissipation 24 mA × (V – V ) min LOOP CC lifier which sets the current flowing in the loop and a voltage regulator to provide a stable operating voltage from the loop where V is the operating voltage of the AD421 and V is CC LOOP supply. The part also contains a high speed serial interface, two the loop voltage. buffered output references and a clock oscillator circuit. The 1 different sections of the AD421 are discussed in more detail The DN25D FET transistor from Supertex meets all the above below. requirements for the FET. Other suitable transistors include ND2020L and ND2410L, both from Siliconix. Voltage Regulator The voltage regulator consists of an op amp, bandgap reference There are a number of external components required to com- and an external depletion mode FET pass transistor. This cir- pensate the regulator loop and ensure stable operation. The capacitor from the V pin to the COM pin is required to cuit is required to regulate the loop voltage that powers the CC AD421 itself and the rest of the transmitter circuitry. Figure 3 stabilize the regulator loop. shows the voltage regulator section of the AD421 plus the associ- To provide additional compensation for the regulator loop, a ated external circuitry for a V of 3.3 V. CC compensation capacitor of 0.01 µF should be connected between the COMP and DRIVE pins and an external circuit LOOP(+) of a 1 kΩ resistor and a 1000 pF capacitor in series should be V TO EXTERNAL CC DN25D CIRCUITRY connected between DRIVE and COM to stabilize this feed- back loop formed with the regulator op amp and the external 2.2�F 0.01�F pass transistor. COM V LV CC DAC Section 75k� 112.5k� The AD421 contains a 16-bit sigma-delta DAC to convert the AD421 digital information loaded to the input latch into a current. The 134k� sigma-delta architecture is particularly useful for the relatively DRIVE 1.21V low bandwidth requirements of the industrial control environ- BANDGAP 0.01�F COMP REFERENCE ment because of its inherent monotonicity at high resolution. 121k� 1k� The AD421 guarantees monotonicity to the 16-bit level. 1000pF The sigma-delta DAC consists of a second order modulator followed by a continuous time filter. The single bit stream from Figure 3. AD421 Voltage Regulator Circuit to Provide the modulator controls a switched current source. This current V = 3.3 V CC source is then filtered by three resistor-capacitor filter sections. The signal on the LV pin selects the voltage to which V CC The resistors for each of the filter sections are on-chip while regulates by changing the gain of the resistor divider between the capacitors are external on the C1–C3 pins. To meet the the op amp inverting input and the V pin. As the LV pin CC specified full-scale settling on the part, low dielectric absorption varies between COM and V , the voltage from the regulator CC capacitors (NPO) are required. Suitable values for these capacitors loop varies between 3 V and 5 V nominal. With LV connected are C1 = 0.01 µF, C2 = 0.01 µF, and C3 = 0.0033 µF. to COM, the regulated voltage is 5 V; with LV connected Current Amplifier through a 0.01 µF capacitor to V , the regulated voltage is CC The DAC output current drives the second section, an opera- 3.3 V while if LV is connected to V , the regulated voltage CC tional amplifier and NPN transistor which acts as a current is 3 V. amplifier to set the current flowing through the LOOP RTN The range of loop voltages that can be used by the configuration pin. Figure 4 shows the current amplifier section of the AD421. shown in Figure 3 is determined by the FET breakdown and An 80 kΩ resistor connected between the DAC output and loop saturation voltages. The external FET parameters such as Vgs return is used as a sampling resistor to determine current. The (off), I and transconductance must be chosen so that the op DSS base drive to the NPN transistor servos the voltage across the amp output on the DRIVE pin can control the FET operating 40 Ω resistor to equal the voltage across the 80 kΩ resistor. point while swinging in the range from V to COM. CC The main characteristics for selecting the FET pass transistor are as follows: –6– REV. C AD421 Reference Section SWITCHED The AD421 contains an on-chip 1.21 V bandgap reference AD421 CURRENT which is used as part of the voltage regulator loop. A bandgap SOURCES BOOST reference is also used to generate two references voltages which are available for use external to the AD421. Figure 5 shows the reference section of the AD421. The REF OUT1 pin 80k� provides a buffered +1.25 V reference voltage which can supply 40� up to 0.5 mA of external current. The REF OUT2 pin provides LOOP RTN a +2.5 V reference voltage which is also capable of providing 0.5 mA of external current. To use the AD421 with its own Figure 4. Current Amplifier reference, simply connect the REF OUT2 pin to the REF IN The BOOST pin is normally tied to the V pin. As the DAC CC pin of the device. Alternatively, the part can be used with an input code varies from all zeros to full scale, the output current external reference by connecting the external reference between from the NPN transistor and thus the total loop current varies REF IN and COM. from 4 mA to 20 mA. With BOOST and V tied together, the CC When REF OUT1 and REF OUT2 are used in application external FET (DN25D) has to supply the full range of loop circuits, external 4.7 µF capacitors are required on the reference current (4 mA to 20 mA). pins to provide compensation and ensure stable operation of the Digital Interface references. These capacitors can be omitted if the internal refer- The digital interface on the AD421 consists of just three wires: ences are not required. DATA, CLOCK and LATCH. The interface connects directly to the serial ports of commonly-used microcontrollers without the need for any external glue logic. Data is loaded MSB first 4.7�F 4.7�F into an input shift register on the rising edge of the CLOCK REF OUT1 REF OUT2 (1.25V) (2.5V) V LV CC signal and is transferred to the DAC latch on the rising edge of the LATCH signal. The timing diagrams for the serial interface 75k� AD421 112.5k� are shown in Figure 1 and Figure 2. 50k� 134k� The data to be loaded to the AD421’s input shift register takes two forms; normal 4 mA to 20 mA data or alarm current data. 2.5V 1.21V DRIVE BANDGAP 50k� The first form is where the AD421 operates over its normal REFERENCE 121k� 4 mA to 20 mA output range with 16 bits of resolution between these endpoints. The second form allows the user to program a current value outside this range as an indication from the trans- mitter than there is a problem with the transducer. The AD421 Figure 5. Reference Section counts the number of clock pulses which it receives between REF OUT2 is sensed internally, and if more than 0.5 mA is LATCH signals as a means of determining whether the data drawn externally from this reference, the chip goes into a power clocked in is 4 mA to 20 mA data or alarm current data. on reset state. In this state the sigma-delta DAC is disabled, the If there are 16 rising clock edges between successive LATCH internal oscillator is stopped and the input data latch is cleared. pulses, then the data being loaded to the input shift register is REF OUT1 has limited current sinking capability. If REF assumed to be normal 4 mA to 20 mA data. On the rising edge OUT1 is required to sink current, a resistive load of 100 kΩ of the LATCH signal, the input shift register data is transferred to COM should be added in addition to the 4.7 µF capacitor. to the DAC latch in a 16-bit parallel transfer. In this case, the 16 bits of data in the DAC latch program the output current USING THE AD421 between 4 mA for all 0s and 20 mA for all 1s (see Table II). The AD421 can be programmed for normal 4 mA to 20 mA Data transferred to the AD421 should be MSB first. operation or for alarm current operation. For normal operation, If there are more than 16 clock pulses between successive the coding is 16-bit straight (natural) binary over an output LATCH pulses, then the data being loaded to the input shift current range of 4 mA to 20 mA. For alarm current operation, register is assumed to be alarm current data. In this case, the the coding is also straight binary but with 17 bits of resolution AD421 accepts 17 bits of data into its shift register. For situa- over twice the span, 0 mA to 32 mA, although the part should tions where there are more than 17 clocks in the serial write not be programmed outside the range of 3.5 mA to 24 mA. To operation (for example, 24 clocks in a 3 × 8-bit transfer from the determine whether data written to the part is normal 4 mA to serial port of a microcontroller) the AD421 simply accepts the 20 mA data or alarm current data, the number of clock pulses last 17 bits of the serial write operation. Data transferred in this between two successive LATCH pulses are counted. If the num- serial write operation is LSB last (i.e., the MSB is loaded on the ber of pulses is 0–16 (modulo 32), it chooses normal mode; if it 17th rising clock edge prior to the LATCH pulse). On the rising is 17–31 (modulo 32), it chooses alarm current range. edge of the LATCH signal, the input shift register data is trans- 4 mA to 20 mA Coding ferred to the DAC latch in a 17-bit parallel transfer. In this Table II shows the ideal input-code-to-output-current relation- case, the 17 bits of data in the DAC latch program the output ship for normal operation of the AD421. The output current current between 0 mA for all 0s and 32 mA for all 1s (see Table values shown assume a REF IN voltage of +2.5 V. With a III). However, in practice the AD421 cannot reliably produce a REF IN of +2.5 V, 1 LSB = 16 mA/65,536 = 244 nA. Figure 6 current less than 3.5 mA or more than 24 mA. shows a timing diagram for programming the AD421 for normal 4 mA to 20 mA operation, the AD421 outputting a current –7– REV. C AD421 of 11.147 mA. With 16 clock pulses between consecutive latch CLOCK signals data written is for normal 4 mA to 20 mA operation. WORD "N" Table II. Ideal Input/Output Code Table DATA XX XX XX X0100 010 11 000000 00 for 4 mA to 20 mA Operation Code Output Current 0000 0000 0000 0000 4 mA LATCH 0000 0000 0000 0001 4.000244 mA 0000 0000 0000 0010 4.000488 mA Figure 7. Write Cycle for Programming Alarm Current 0100 0000 0000 0000 8 mA Data 1000 0000 0000 0000 12 mA MICROPROCESSOR INTERFACING 1100 0000 0000 0000 16 mA AD421 – MC68HC11 (SPI BUS) INTERFACE 1111 1111 1111 1101 19.999268 mA Figure 8 shows a typical interface between the AD421 and the 1111 1111 1111 1110 19.999512 mA Motorola MC68HC11 SPI (Serial Peripheral Interface) bus. 1111 1111 1111 1111 19.999756 mA The SCK, MOSI and SS pins of the 68HC11 are respectively connected to the CLOCK, DATA IN and LATCH pins of the AD421. CLOCK WORD "N +1" WORD "N" CLOCK SCK AD421* 68HC11 DATA 10 1 1 0011 0 0 11 0 0 1 1 100 1 MOSI DATA IN LATCH SS * ADDITIONAL PINS OMITTED FOR CLARITY LATCH Figure 8. AD421 to 68HC11 Interface A typical routine such as the one shown below begins by initializ- Figure 6. Write Cycle for 4 mA to 20 mA Operation ing the state of the various SPI data and control registers. Alarm Current Coding INIT LDAA #$2F ;SS = 1; SCK = 0; MOSI = 1 Table III shows the ideal input-code-to-output-current relation- STAA PORTD ;SEND TO SPI OUTPUTS ship for alarm current programming of the AD421. In this case, LDAA #$38 ;SS, SCK, MOSI = OUTPUTS the equivalent span is 0 mA to 32 mA but a reliable operating STAA DDRD ;SEND DATA DIRECTION INFO span is 3.5 mA to 24 mA. The part may give an indeterminate LDAA #$50 ;DABL INTRPTS, SPI IS MASTER & ON output for code values outside the range given in the table. As a STAA SPCR ;CPOL = 0, CPHA = 0, 1MHZ BAUDRATE result, the user is advised to restrict the code programmed to the NEXTPT LDAA MSBY ;LOAD ACCUM W/UPPER 8 BITS BSR SENDAT ;JUMP TO DAC OUTPUT ROUTINE part in alarm current mode to within the range shown in Table JMP NEXTPT ;INFINITE LOOP III. Figure 7 shows a timing diagram for loading an alarm cur- SENDAT LDY #$1000 ;POINT AT ON-CHIP REGISTERS rent of 3.75 mA to the AD421 with an 8-bit microcontroller BCLR $08,Y,$20 ;DRIVE SS (LATCH) LOW using three 8-bit writes. STAA SPDR ;SEND MS-BYTE TO SPI DATA REG The output current values shown assume a REF IN voltage of WAIT1 LDAA SPSR ;CHECK STATUS OF SPIE +2.5 V. With a REF IN of +2.5 V, an ideal 1 LSB = 32 mA/ BPL WAIT1 ;POLL FOR END OF X-MISSION LDAA LSBY ;GET LOW 8 BITS FROM MEMORY 131,072 = 244 nA. STAA SPDR ;SEND LS-BYTE TO SPI DATA REG WAIT2 LDAA SPSR ;CHECK STATUS OF SPIE Table III. Ideal Input/Output Code Table BPL WAIT2; ;POLL FOR END OF X-MISSION for Alarm Current Operation BSET $08,Y,$20 ;DRIVE SS HIGH TO LATCH DATA RTS Code Output Current The SPI data port is configured to process data in 8-bit bytes. 0 0011 1000 0000 0000 3.5 mA The most significant data byte (MSBY) is retrieved from 0 0011 1100 0000 0000 3.75 mA memory and processed by the SENDAT routine. The SS pin is driven low by indexing into the PORTD data register and clear 0 0100 0000 0000 0000 4 mA Bit 5. The MSBY is then sent to the SPI data register where it is 0 1000 0000 0000 0000 8 mA automatically transferred to the AD421 internal shift resistor. 1 0000 0000 0000 0000 16 mA 1 0100 0000 0000 0000 20 mA 1 0110 0000 0000 0000 22 mA 1 1000 0000 0000 0000 24 mA –8– REV. C (MSB) B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 B0 (LSB) B15 B14 B13 B12 X X X X X X X (MSB)B16 B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 (LSB)B0 AD421 The HC11 generates the requisite eight clock pulses with data V CC valid on the rising edges. After the MSBY is transmitted, the least significant byte (LSBY) is loaded from memory and 2.2�F 0.1�F V transmitted in a similar fashion. To complete the transfer, the CC LATCH pin is driven high when loading the complete 16-bit 10k� V CC word into the AD421. CLOCK CLOCK V AD421 TO MICROWIRE INTERFACE CC AD421* The flexible serial interface of the AD421 is also compatible 10k� with the National Semiconductor MICROWIRE interface. The LATCH LATCH MICROWIRE interface is used in microcontrollers such as the V COP400 and COP800 series of processors. A generic interface CC to use the MICROWIRE interface is shown in Figure 9. The 10k� G1, SK, and SO pins of the MICROWIRE interface respec- DATA IN DATA IN tively connect to the LATCH, CLOCK, and DATA IN pins of the AD421. COM * ADDITIONAL PINS OMITTED FOR CLARITY CLOCK SK MICROWIRE AD421* Figure 10. Opto-Isolated Interface SO DATA IN APPLICATIONS SECTION G1 LATCH Basic Operating Configuration Figure 11 shows the basic connection diagram for the AD421 * ADDITIONAL PINS OMITTED FOR CLARITY operating at 5 V. This circuit shows the minimum of external Figure 9. AD421 to MICROWIRE Interface components to operate the AD421. In the diagram, the AD421’s Opto-Isolated Interface regulator loop in conjunction with the DN25D pass transistor The AD421 has a versatile serial 3-wire serial interface making provides the V voltage for the AD421 itself and for other CC it ideal for minimizing the number of control lines required for devices in the transmitter. The V pin should be well decou- CC isolation of the digital system from the control loop. In intrinsi- pled with a 2.2 µF capacitor to ensure regulator stability and to cally safe applications or due to noise, safety requirements, or absorb power glitches on the V line of the AD421 and other CC distance, it may be necessary to isolate the AD421 from the devices in the system. If the AD421 is operated with V = 3 V, CC controller. This can easily be achieved by using opto-isolators. the transfer function shifts negative. To correct for this a 16 kΩ Figure 10 shows an opto-isolated interface to the AD421 where resistor connected between COM and LOOPRTN will approxi- CLOCK, DATAIN and LATCH are driven from opto-couplers. mately compensate for the V supply sensitivity in moving from CC Be aware of signal inversion across the opto-couplers. If opto- 5 V to 3 V by adjusting the gain of the AD421. couplers with relatively slow rise and fall times are used, Schmitt triggers may be required on the digital inputs to prevent errone- ous data being presented to the DAC. V TO EXTERNAL CC DN25D CIRCUITRY 2.2�F 4.7�F COM REF OUT2 REF OUT1 COM LV REF IN V CC DRIVE V LOOP 0.01�F COMP DATA CLOCK 1k� AD421 1000pF BOOST LATCH LOOP RTN COM C1 C2 C3 COM TO EXTERNAL 0.01�F0.01�F 0.0033�F CIRCUITRY Figure 11. Basic Connection Diagram –9– REV. C AD421 A capacitor of 0.01 µF connected between COMP and DRIVE Smart Transmitter is required to stabilize the feedback loop formed with the The AD421 is intended for use in 4 mA to 20 mA smart trans- regulator op amp and the external pass transistor. An external mitters. A smart transmitter is a system that incorporates a snubber circuit of 1 kΩ and 1000 pF is required between the microprocessor system which is used for linearization and DRIVE pin and COM and a 0.1 µF cap between COMP and communication. Figure 13 shows a block diagram of a typical DRIVE to stabilize the feedback loop formed by the regulator smart transmitter. In this example, the transmitter does not have op amp and the external pass transistor. any digital communication capabilities. The internal 2.5 V reference on the AD421 is used as the refer- ence for the AD421 and this has to be decoupled with a 4.7 µF MEMORY capacitor for compensation and stability purposes. The sigma- delta DAC on the part consists of a second order modulator 4mA TO 20mA followed by a continuous time filter. The resistors for each of A/D MICRO- D/A SENSORS CONVERTER PROCESSOR CONVERTER the filter sections are on-chip while the capacitors are external MEASUREMENT CIRCUIT on the C1 to C3 pins. To meet the specified full-scale settling on the part, low dielectric absorption capacitors (NPO) are Figure 13. Typical Smart Transmitter required. Suitable values for these capacitors are C1 = C2 = Figure 14 shows a typical smart transmitter application circuit 0.01 µF, and C3 = 0.0033 µF. using the AD421. The digital interface on the AD421 consists of just three wires: The sensor voltage to be measured at the transmitter is con- DATA, CLOCK and LATCH. The interface connects directly verted using a high resolution sigma-delta converter such as to the serial ports of commonly-used microcontrollers without the AD7714 or AD7715. These devices have an on-board PGA the need for any external glue logic. Data is loaded into an input which can provide gains on the analog front end from 1 to 128. shift register on the rising edge of the CLOCK signal and is This allows for an analog input range as low as 10 mV which transferred to the DAC latch on the rising edge of the LATCH allows the transducer to be connected directly to the ADC. The signal. AD7714/AD7715 have digital calibration techniques which are Reduce Power Load on External FET used to eliminate gain and offset errors. In addition, back- Figure 12 shows a circuit where an external NPN transistor is ground calibration techniques are provided whereby the part added to reduce the power loading on the FET. The FET will continually calibrates itself and the user does not have to supply the V and an external high voltage NPN bipolar tran- CC worry about issuing periodic calibration commands to remove sistor can carry the BOOST current. The BOOST pin sinks the effects of time and temperature drift. necessary current from the loop so that the current flowing into In normal operation the microprocessor reads the data from the BOOST plus the current flowing into COM is equal to the AD7714/AD7715. After the data is processed by the micro- programmed loop current. The external NPN transistor reduces controller, the data is transferred from the serial port of the the external power load that the FET has to carry to less than processor to the AD421 for transmission over the 4 to 20 mA 750 µA if no other components share the V line and to less CC loop back to the control center. than 4 mA in applications that share the same V line as the CC The AD421 regulates the loop voltage to create power for the AD421. rest of the transmitter circuitry. In Figure 14, the derived V CC voltage is 3.3 V which is achieved by connecting the LV pin to LOOP(+) V through 0.01 µF. REF OUT2 provides the reference volt- V TO EXTERNAL CC CC DN25D CIRCUITRY age for the AD421 itself while REF OUT1 provides the refer- BC639/BC337 2.2�F ence voltage for the AD7714/AD7715. COM LV V CC 75k� 112.5k� AD421 134k� DRIVE 1.21V BANDGAP 0.01�F COMP REFERENCE 1k� 121k� 1000pF BOOST 80k� 40� LOOP RTN LOOP(–) Figure 12. External NPN Transistor Reduces Power Load on FET –10– REV. C AD421 DN25D 3.3V 0.01�F 2.2�F 0.1�F DV AV DD DD BOOST V LV CC SENSORS 1.25V RTD REF OUT1 REF IN mV � TC LOOP REF OUT2 4.7�F ANALOG 100k� 4.7�F POWER 0.01�F V TO REF IN CC AMBIENT DIGITAL COMP TEMP CONVERTER SENSOR CLOCK DRIVE AD7714/ MICROCONTROLLER LATCH 1k� AD7715 DATA 1000pF MCLK IN AD421 GND MCLK OUT CS LOOP RTN DATA OUT COM SCLK DATA IN C1 C2 C3 AGND DGND Figure 14. AD421 in Smart Transmitter Application HART Interfacing Figure 16 shows a block diagram of a smart and intelligent The HART protocol uses a frequency shift (FSK) keying tech- transmitter. An intelligent transmitter is a transmitter in which nique based on the Bell 202 Communication Standard which is the functions of the microprocessor are shared between deriving one of several standards used to transmit digital signals over the primary measurement signal, storing information regarding the telephone lines. This technique is used to superimpose the transmitter itself, its application data and its location and digital communication on to the 4 mA to 20 mA current loop also managing a communication system which enables two way connecting the central system to the transmitter in the field. communication to be superimposed on the same circuit that Two different frequencies, 1200 Hz and 2200 Hz, are used to carries the measurement signal. A smart transmitter incorporat- represent binary 1 and 0 respectively, as shown in Figure 15. ing the HART protocol is an example of a smart intelligent These sine wave tones are superimposed on the dc signal at a transmitter. low level with the average value of the sine wave signal being zero. This allows simultaneous analog and digital communica- MEMORY tions. Additionally, no dc component is added to the existing 4 mA to 20 mA signal regardless of the digital data being sent over the line. Consequently, existing analog instruments con- 4mA TO 20mA A/D MICRO- D/A tinue to work in systems that implement HART as the low-pass SENSORS CONVERTER PROCESSOR CONVERTER MEASUREMENT filtering usually present effectively removes the digital signal. A CIRCUIT single pole 10 Hz low-pass filter effectively reduces the commu- COMMUNICATION nication signal to a ripple of about ±0.01% of the full-scale SYSTEM signal. The HART protocol specifies that master devices like a host control system or a hand held terminal transmit a voltage Figure 16. Smart and Intelligent Transmitter signal whereas a slave or field device transmits a current signal. Figure 17 shows an example of the AD421 in a HART transmit- The current signal is converted into a corresponding voltage by ter application. Most of the circuit is as outlined in the smart the loop load resistor. transmitter as shown in Figure 14. The HART data transmitted on the loop is received by the transmitter using a bandpass filter APPROX and modem and the HART data is transferred to the micro- +0.5mA controller’s UART or asynchronous serial port. HART data to be transmitted on the loop is sent from the microcontroller’s UART or asynchronous serial port to the modem. It is then waveshaped before being coupled onto the AD421’s output at the C3 pin. The value of the coupling capacitor C is determined C by the waveshaper output and the C3 capacitor of the AD421. The blocks containing the Bell 202 Modem, waveshaper and bandpass filter come in a complete solution with the 20C15 from Symbios APPROX –0.5mA 1200Hz 2200Hz Logic, Inc., or HT2012 from SMAR Research Corp. “1” “0” For a more complete AD421-20C15 interface, please refer to Figure 15. HART Transmission of Digital Signals Application Note AN-534 on the Analog Devices’ website www.analog.com or contact your local sales office. –11– REV. C AD421 DN25D 3.3V 0.01�F 0.1�F 2.2�F DV AV DD DD BOOST V LV CC SENSORS 1.25V RTD REF IN REF OUT1 mV � TC LOOP 100k� 4.7�F 4.7�F REF OUT2 ANALOG POWER 0.01�F V TO CC REF IN COMP AMBIENT DIGITAL TEMP CONVERTER SENSOR DRIVE CLOCK AD7714/ MICROCONTROLLER LATCH AD7715 1k� DATA MCLK IN 1000pF AD421 GND MCLK OUT CS LOOP RTN DATA OUT COM SCLK DATA IN C1 C2 C3 AGND DGND C * C WAVEFORM BANDPASS HART SHAPER FILTER MODEM BELL 202 HT20C12/20C15 *FOR SELECTION OF C , REFER TO AN-534 C Figure 17. AD421 in HART Transmitter Application Current Source across R2. The ratio of R1 to R2 determines the current that Figure 18 shows an application circuit for the AD421 being flows in the load resistor R . I = [1 + R1/R2] × I , where I L L PROG L used as a current source. The current programmed to the is the current that flows in the load resistor R and I is the L PROG AD421 (4 mA to 20 mA) will develop a voltage across R1. current programmed to the AD421. R1 and R2 are external to This same voltage due to negative feedback will be generated the AD421 and will need to be matched resistors to obtain a highly accurate current source. 5 VOLT REGULATOR +5V OUT IN V S 2.2�F 10k� 4.7�F COM 10k� COM COM REF IN LV V REF OUT2 REF OUT1 CC 10k� DATA DRIVE DATA COMP BOOST AD421 CLOCK CLOCK LOOP RTN LATCH LATCH C1 C2 C3 COM R1 R2 0.01�F 0.01�F 0.0033�F R L V RETURN S Figure 18. AD421 in Programmable Current Source/Sink –12– REV. C AD421 Battery Backup Figure 19 shows an application circuit for the AD421 where the DN25D IN3611 micro and memory sections of the circuitry are protected against losing data if the loop is broken. The backup circuit switches IRFF9113 0.1�F 4.7�F IN3611 from V to battery voltage without a glitch when V power is CC CC V LOOP V CC lost. The IRFF9113 acts as a current source during normal 100k� 2.2�F MICRO/ operation and provides a constant charging current to the supercap MEMORY or Nicad. The loss of V drops the IRFF9113’s gate voltage to CC V SUPERCAP CC GND zero volts, which allows the battery or supercaps current to flow DRIVE through the MOSFETs channel and integral body diode to AD421* LOOP provide power for the micro and memory sections. To calibrate RTN this circuit, connect an ammeter in series with the battery or COM supercap. Then with V and the load present adjust the 100 kΩ CC *ADDITIONAL CIRCUITRY OMITTED FOR CLARITY potentiometer for the battery charging current recommended by Figure 19. Battery Backup Circuit the battery or supercap manufacturer. Nonrechargeable batteries should not be used in this application due to danger of explosion. –13– REV. C AD421 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 16-Lead Plastic DIP (N-16) 0.840 (21.34) 0.745 (18.92) 16 9 0.280 (7.11) 0.240 (6.10) 18 0.325 (8.26) 0.195 (4.95) 0.300 (7.62) PIN 1 0.060 (1.52) 0.115 (2.93) 0.015 (0.38) 0.210 (5.33) MAX 0.130 (3.30) 0.160 (4.06) MIN 0.115 (2.93) 0.015 (0.381) 0.022 (0.558) 0.100 0.070 (1.77) SEATING 0.008 (0.204) PLANE (2.54) 0.014 (0.356) 0.045 (1.15) BSC 16-Lead (Wide Body) Small Outline Package (R-16) 0.4133 (10.50) 0.3977 (10.00) 16 9 1 8 0.1043 (2.65) PIN 1 0.0291 (0.74) x 45� 0.0118 (0.30) 0.0926 (2.35) 0.0098 (0.25) 0.0040 (0.10) 0.0500 (1.27) 8� 0.0500 0.0192 (0.49) 0.0157 (0.40) 0� SEATING 0.0125 (0.32) (1.27) 0.0138 (0.35) PLANE BSC 0.0091 (0.23) –14– REV. C 0.2992 (7.60) 0.2914 (7.40) 0.4193 (10.65) 0.3937 (10.00) PRINTED IN U.S.A. C2105b–0–3/00 (rev. C)