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 AD421s 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 AD421s 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 40C to +85C. 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: 1 (Using DN25D as pass transistor as per Figure 3 AD421LOOP-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 M typ AC Loop Voltage Sensitivity 2 A/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 F 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 V/V typ Load Regulation 15 V/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) +25C 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) +25C 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 40C to +25C and 2.5 ppm/C Typical from +25C to +85C Externally Available Current 0.5 mA min V Supply Sensitivity 150 V/V max 15 V/V Typical CC Output Impedance 3 typ Noise (0.1 Hz10 Hz) 6 V (p-p) typ REF OUT1 5 Output Voltage 1.24/1.26 V min/V max 1.25 V Nominal, 100 k Load to COM Drift 50 ppm/C max 20 ppm/C Typical from 40C to +25C and 2 ppm/C Typical from +25C to +85C Externally Available Current 0.5 mA min V Supply Sensitivity 150 V/V max 15 V/V Typical CC Output Impedance 3 typ Noise (0.1 Hz10 Hz) 4 V (p-p) typ REF IN Input Resistance 40 k 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 A max V = V IH IN CC I 10 A 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 A max 475 A Typical CC V = 5 V 750 A max 575 A Typical CC NOTES 1 The DN25D is available from Supertex, Inc., 1350 Bordeaux Drive, Sunnyvale, CA 94089. 2 Temperature range is 40C to +85C. 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 k 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 k resistor only required if this reference is being used in application circuits. Specifications subject to change without notice. REV. C 2