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FEATURES 4 mA to 20 mA Current Output HART(R) Compatible 16-Bit Resolution and Monotonicity 0.01% Integral Nonlinearity 5 V or 3 V Regulator Output 2.5 V and 1.25 V Precision Reference 750 A Quiescent Current max Programmable Alarm Current Capability Flexible High Speed Serial Interface 16-Lead SOIC and PDIP Packages
DATA CLOCK LATCH REF IN (+2.5V)
Loop-Powered 4 mA to 20 mA DAC AD421
FUNCTIONAL BLOCK DIAGRAM
REF OUT1 REF OUT2 (+2.5V) (+1.25V) LV VCC 75k
112.5k
AD421
BANDGAP REFERENCE
134k DRIVE 121k COMP BOOST
LOCAL OSCILLATOR INPUT SHIFT REGISTER DAC LATCH 16-BIT SIGMADELTA DAC SWITCHED CURRENT SOURCES AND FILTERING
GENERAL DESCRIPTION
40 80k LOOP RTN
The AD421 is a complete, loop-powered, digital to 4 mA to 20 mA converter, designed to meet the needs of smart transmitter manufacturers in the Industrial Control industry. It provides a high precision, fully integrated, low cost solution in a compact 16-lead package. The AD421 is ideal for extending the resolution of smart 4 mA to 20 mA transmitters at very low cost. The AD421 includes a selectable regulator that is used to power itself and other devices in the transmitter. This regulator provides either a +5 V, +3.3 V or +3 V regulated output voltage. The part also contains +1.25 V and +2.5 V precision references. The AD421 thus eliminates the need for a discrete regulator and voltage reference. The only external components required are a number of passive components and a pass transistor to span large loop voltages. The AD421 can be used with standard HART FSK protocol communication circuitry without any degradation in specified performance. The high speed serial interface is capable of operating at 10 Mbps and allows for simple connection to commonly-used microprocessors and microcontrollers via a standard three-wire serial interface. The sigma-delta architecture of the DAC guarantees 16-bit monotonicity while the integral nonlinearity for the AD421 is 0.01%. The part provides a zero scale 4 mA output current with 0.1% offset error and a 20 mA full-scale output current with 0.2% gain error. The AD421 is available in a 16-lead, 0.3 inch-wide, plastic DIP and in a 16-lead, 0.3 inch-wide, SOIC package. The part is specified over the industrial temperature range of -40C to +85C.
POWER-ON RESET
COM
C1 C2 C3
PRODUCT HIGHLIGHTS
1. The AD421 is a single chip, high performance, low cost solution for generating 4 mA to 20 mA signals for smart industrial control transmitters. 2. The AD421's regulated supply voltage can be used to power any additional circuits in the transmitter. The regulated output value is pin selectable as either +3 V, +3.3 V or +5 V. 3. The AD421's on-chip references can provide a precision reference voltage to other devices in the system. This reference voltage can be either +1.25 V or +2.5 V. 4. The AD421 is fully compatible with standard HART circuitry or other similar FSK protocols. 5. With the addition of a single discrete transistor, the AD421 can be operated from VCC + 2 V min to a maximum of the breakdown voltage of the pass transistor. 6. The AD421 converts the digital data to current with 16-bit resolution and monotonicity. Full-scale settling time to 0.1% typically occurs within 8 ms. 7. The AD421 features a programmable alarm current capability that allows the transmitter to send out of range currents to indicate a transducer fault.
HART is a registered trademark of the HART Communication Foundation.
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Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 (c) Analog Devices, Inc., 2000
AD421-LOOP-POWERED SPECIFICATIONS
Parameter OUTPUT CHARACTERISTICS Current Loop Voltage Compliance3 Full-Scale Settling Time Output Impedance AC Loop Voltage Sensitivity VOLTAGE REGULATOR Output Voltage (VCC) 3 V Mode 3.3 V Mode 5 V Mode Externally Available Current Line Regulation Load Regulation B Versions2 VCC + 2 350 8 25 2 Units V min V max ms typ M typ A/V typ
(Using DN25D1 as pass transistor as per Figure 3; REF IN = REF OUT2; TA = TMIN to TMAX unless otherwise noted)
Conditions/Comments
DN25D Breakdown Voltage Settling Time to 0.1%, C1 = C2 = 10 nF, C3 = 3.3 nF 1200 Hz to 2200 Hz
2.95/3.05 3.25/3.35 4.95/5.05 3.25 1 15
V min/V max V min/V max V min/V max mA min V/V typ V/mA typ
3 V Nominal. LV Pin Connected to VCC 3.3 V Nominal. LV Pin Connected Through 0.01 F to VCC 5 V Nominal. LV Pin Connected to COM Assuming 4 mA Flowing in the Loop
DAC SPECIFICATIONS (V
Parameter ACCURACY Resolution Monotonicity Integral Nonlinearity Offset (4 mA) @ +25C4 Offset Drift Total Output Error (20 mA) @ +25C4 Total Output Drift VCC Supply Sensitivity VOLTAGE REFERENCE REF OUT2 Output Voltage Drift Externally Available Current VCC Supply Sensitivity Output Impedance Noise (0.1 Hz-10 Hz) REF OUT1 Output Voltage Drift Externally Available Current VCC Supply Sensitivity Output Impedance Noise (0.1 Hz-10 Hz) REF IN Input Resistance DIGITAL INPUTS VIH (Logic 1) VIL (Logic 0) IIH IIL Data Coding Data Rate POWER SUPPLIES Operating Range Quiescent Current @ VCC = 3 V @ VCC = 5 V
NOTES
1 2
CC = +3 V to +5 V; REF IN = REF OUT2; TA = TMIN to TMAX
unless otherwise noted)
B Versions2 16 16 0.01 0.1 25 0.2 50 50
Units Bits Bits min % of FS max % of FS max ppm of FS/C max % of FS max ppm of FS/C max nA/mV max
Conditions/Comments
FS = Full-Scale Output Current VCC = 5 V Includes On-Chip Reference Drift VCC = 5 V Includes On-Chip Reference Drift 25 nA/mV Typical
2.49/2.51 40 0.5 150 3 6 1.24/1.26 50 0.5 150 3 4 40 0.75 x VCC 0.25 x VCC 10 10 Binary 10 +2.95 to +5.05 650 750
V min/V max ppm/C max mA min V/V max typ V (p-p) typ V min/V max ppm/C max mA min V/V max typ V (p-p) typ k typ V min V max A max A max Mbps max V min to V max A max A max
2.5 V Nominal 20 ppm/C Typical from -40C to +25C and -2.5 ppm/C Typical from +25C to +85C 15 V/V Typical
1.25 V Nominal, 100 k Load to COM5 20 ppm/C Typical from -40C to +25C and 2 ppm/C Typical from +25C to +85C 15 V/V Typical
VIN = VCC VIN = 0 V
Functional to 7 V 475 A Typical 575 A Typical
The DN25D is available from Supertex, Inc., 1350 Bordeaux Drive, Sunnyvale, CA 94089. 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 VCC = 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 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.
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AD421 TIMING CHARACTERISTICS1, 2, 3 (V
Parameter tCK tCL tCH tDW tDS tDH tLD tLL tLH 100 50 50 30 30 0 50 50 50
CC
= +3 V to +5 V, TA = TMIN to TMAX unless otherwise noted)
Units ns min ns min ns min ns min ns min ns min ns min ns min ns min Conditions/Comments Data Clock Period Data Clock Low Time Data Clock High Time Data Stable Width Data Setup Time Data Hold Time Latch Delay Time Latch Low Time Latch High Time
(B Versions)
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 CC ) and timed from a voltage level of (V IN + VIL )/2; tr and tf should not exceed 1 s on any digital input. Specifications subject to change without notice.
CLOCK
WORD "N" DATA 1
(MSB) B15
WORD "N +1" 1
B6
0
B14
11
B13 B12
0
B11
0
B10
1
B9
0
B8
0
B7
1
B5
1
B4
0
B3
0
B2
11
B1 B0 (LSB)
1
B15
0
B14
0
B13
1
B12
LATCH
Figure 1. Serial Interface Waveforms (Normal Data Load)
tC K tC L
CLOCK
tC H tD S
DATA
tD H
tD W tL D tLL
LATCH
tL H
Figure 2. Serial Interface Timing Diagram
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AD421
ABSOLUTE MAXIMUM RATINGS*
(TA = +25C unless otherwise noted)
PIN CONFIGURATION DIP and SOIC
DRIVE, BOOST, COMP to COM . . . -0.5 V to VCC + 0.5 V LOOP RTN to COM . . . . . . . . . . . . . . . . . . . -2 V to + 0.5 V Digital Input Voltage to COM . . . . . . . -0.5 V to VCC + 0.5 V Operating Temperature Range Commercial (B Version) . . . . . . . . . . . . . . - 40C to +85C Storage Temperature Range . . . . . . . . . . . . -65C to +150C Junction Temperature . . . . . . . . . . . . . . . . . . . . . . . . +150C Plastic DIP Package, Power Dissipation . . . . . . . . . . 670 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 116C/W Lead Temperature (Soldering, 10 sec) . . . . . . . . . . . . 260C SOIC Package, Power Dissipation . . . . . . . . . . . . . . . 450 mW JA Thermal Impedance . . . . . . . . . . . . . . . . . . . . 110C/W Lead Temperature, Soldering Vapor Phase (60 sec) . . . . . . . . . . . . . . . . . . . . . +215C Infrared (15 sec) . . . . . . . . . . . . . . . . . . . . . . . . . +220C
*
REF OUT1 1 REF OUT2 2 REF IN 3 LV 4 LATCH 5 CLOCK 6 DATA 7 LOOP RTN 8
16 VCC 15 BOOST 14 COMP
AD421
13 DRIVE C1
12 TOP VIEW (NOT TO SCALE)
11 C2 10 C3 9 COM
ORDERING GUIDE
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the 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 conditions for extended periods may affect device reliability.
Model AD421BN AD421BR AD421BRRL EVAL-AD421EB
Temperature Range -40C to +85C -40C to +85C -40C to +85C Evaluation Board
Package Option* N-16 R-16 R-16; Reeled SOIC
*N = Plastic DIP, R = SOIC.
CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily 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 precautions are recommended to avoid performance degradation or loss of functionality.
WARNING!
ESD SENSITIVE DEVICE
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AD421
PIN FUNCTION DESCRIPTIONS
Pin No. Mnemonic 1 REF OUT1
Function Reference Output 1. A precision +1.25 V reference is provided at this pin. It is intended as a precision reference 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 k to COM should be added. (See Reference section.) 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. 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 supplied using an external reference source or by using the part's own REF OUT2 voltage. Regulated Voltage Control Input. The LV input controls the loop gain of the servo amplifier to set VCC. With LV connected to COM, the regulator voltage is set to 5 V nominal. If the LV input is connected through 0.01 F to VCC, the regulated voltage is nominally 3.3 V. With LV connected to VCC the regulated voltage, VCC, is 3 V nominal. 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 Interface section.) 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). 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. Loop Return Output. LOOP RTN is the return path for current flowing in the current loop. Common. This is the reference potential for the AD421 analog and digital inputs and outputs and for the voltage regulator output. 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. Filtering Capacitor. See C3 description. Filtering Capacitor. See C3 description. Output from the Voltage Regulator Loop. The DRIVE signal controls the external pass transistor to establish and maintain the correct VCC level programmed by the LV inputs while providing the necessary bias as the loop current is programmed from 4 mA to 20 mA. 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. 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. Power Supply. VCC is the power supply input of the AD421 and it also provides the voltage regulator output, 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. VCC should be decoupled to COM with a 2.2 F capacitor.
2
REF OUT2
3
REF IN
4
LV
5
LATCH
6
CLOCK
7 8 9 10 11 12 13
DATA LOOP RTN COM C3 C2 C1 DRIVE
14 15 16
COMP BOOST VCC
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AD421
CIRCUIT DESCRIPTION Table I. FET Characteristics
The AD421 is designed for use in loop-powered 4-20 mA smart transmitter applications. A smart transmitter, as a remote instrument, controls its current output signal on the same pair of wires from which it receives its power. The AD421 essentially provides three primary functions in the smart transmitter. These functions are a DAC function for converting the microprocessor/ microcontroller's digital data to analog format, a current amplifier which sets the current flowing in the loop and a voltage regulator to provide a stable operating voltage from the loop supply. The part also contains a high speed serial interface, two buffered output references and a clock oscillator circuit. The different sections of the AD421 are discussed in more detail below.
Voltage Regulator
FET Type IDSS BVDS VPINCHOFF Power Dissipation
N-Channel Depletion Mode 24 mA min (VLOOP - VCC) min VCC max 24 mA x (VLOOP - VCC) min
where VCC is the operating voltage of the AD421 and VLOOP is the loop voltage. The DN25D FET transistor from Supertex1 meets all the above requirements for the FET. Other suitable transistors include ND2020L and ND2410L, both from Siliconix. There are a number of external components required to compensate the regulator loop and ensure stable operation. The capacitor from the VCC pin to the COM pin is required to stabilize the regulator loop. To provide additional compensation for the regulator loop, a compensation capacitor of 0.01 F should be connected between the COMP and DRIVE pins and an external circuit of a 1 k resistor and a 1000 pF capacitor in series should be connected between DRIVE and COM to stabilize this feedback loop formed with the regulator op amp and the external pass transistor.
DAC Section
The voltage regulator consists of an op amp, bandgap reference and an external depletion mode FET pass transistor. This circuit is required to regulate the loop voltage that powers the AD421 itself and the rest of the transmitter circuitry. Figure 3 shows the voltage regulator section of the AD421 plus the associated external circuitry for a VCC of 3.3 V.
LOOP(+) VCC TO EXTERNAL CIRCUITRY 2.2 F COM DN25D
0.01 F LV 112.5k VCC 75k 134k
AD421
BANDGAP REFERENCE
1.21V 121k
DRIVE COMP 0.01 F 1k 1000pF
The AD421 contains a 16-bit sigma-delta DAC to convert the digital information loaded to the input latch into a current. The sigma-delta architecture is particularly useful for the relatively low bandwidth requirements of the industrial control environment because of its inherent monotonicity at high resolution. The AD421 guarantees monotonicity to the 16-bit level. The sigma-delta DAC consists of a second order modulator followed by a continuous time filter. The single bit stream from the modulator controls a switched current source. This current source is then filtered by three resistor-capacitor filter sections. The resistors for each of the filter sections are on-chip while the capacitors are external on the C1-C3 pins. To meet the specified full-scale settling on the part, low dielectric absorption capacitors (NPO) are required. Suitable values for these capacitors are C1 = 0.01 F, C2 = 0.01 F, and C3 = 0.0033 F.
Current Amplifier
Figure 3. AD421 Voltage Regulator Circuit to Provide VCC = 3.3 V
The signal on the LV pin selects the voltage to which VCC regulates by changing the gain of the resistor divider between the op amp inverting input and the VCC pin. As the LV pin varies between COM and VCC, the voltage from the regulator loop varies between 3 V and 5 V nominal. With LV connected to COM, the regulated voltage is 5 V; with LV connected through a 0.01 F capacitor to VCC, the regulated voltage is 3.3 V while if LV is connected to VCC, the regulated voltage is 3 V. The range of loop voltages that can be used by the configuration shown in Figure 3 is determined by the FET breakdown and saturation voltages. The external FET parameters such as Vgs (off), IDSS and transconductance must be chosen so that the op amp output on the DRIVE pin can control the FET operating point while swinging in the range from VCC to COM. The main characteristics for selecting the FET pass transistor are as follows:
The DAC output current drives the second section, an operational amplifier and NPN transistor which acts as a current amplifier to set the current flowing through the LOOP RTN pin. Figure 4 shows the current amplifier section of the AD421. An 80 k resistor connected between the DAC output and loop return is used as a sampling resistor to determine current. The base drive to the NPN transistor servos the voltage across the 40 resistor to equal the voltage across the 80 k resistor.
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AD421
Reference Section
SWITCHED CURRENT SOURCES
AD421
BOOST
80k 40 LOOP RTN
Figure 4. Current Amplifier
The BOOST pin is normally tied to the VCC pin. As the DAC input code varies from all zeros to full scale, the output current from the NPN transistor and thus the total loop current varies from 4 mA to 20 mA. With BOOST and VCC tied together, the external FET (DN25D) has to supply the full range of loop current (4 mA to 20 mA).
Digital Interface
The AD421 contains an on-chip 1.21 V bandgap reference which is used as part of the voltage regulator loop. A bandgap 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 provides a buffered +1.25 V reference voltage which can supply up to 0.5 mA of external current. The REF OUT2 pin provides 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 reference, simply connect the REF OUT2 pin to the REF IN pin of the device. Alternatively, the part can be used with an external reference by connecting the external reference between REF IN and COM. When REF OUT1 and REF OUT2 are used in application circuits, external 4.7 F capacitors are required on the reference pins to provide compensation and ensure stable operation of the references. These capacitors can be omitted if the internal references are not required.
The digital interface on the AD421 consists of just three wires: 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 into an input shift register on the rising edge of the CLOCK signal and is transferred to the DAC latch on the rising edge of the LATCH signal. The timing diagrams for the serial interface are shown in Figure 1 and Figure 2. 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. The first form is where the AD421 operates over its normal 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 transmitter than there is a problem with the transducer. The AD421 counts the number of clock pulses which it receives between LATCH signals as a means of determining whether the data clocked in is 4 mA to 20 mA data or alarm current data. If there are 16 rising clock edges between successive LATCH pulses, then the data being loaded to the input shift register is assumed to be normal 4 mA to 20 mA data. On the rising edge of the LATCH signal, the input shift register data is transferred 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 between 4 mA for all 0s and 20 mA for all 1s (see Table II). Data transferred to the AD421 should be MSB first. If there are more than 16 clock pulses between successive LATCH pulses, then the data being loaded to the input shift register is assumed to be alarm current data. In this case, the AD421 accepts 17 bits of data into its shift register. For situations where there are more than 17 clocks in the serial write operation (for example, 24 clocks in a 3 x 8-bit transfer from the serial port of a microcontroller) the AD421 simply accepts the last 17 bits of the serial write operation. Data transferred in this serial write operation is LSB last (i.e., the MSB is loaded on the 17th rising clock edge prior to the LATCH pulse). On the rising edge of the LATCH signal, the input shift register data is transferred to the DAC latch in a 17-bit parallel transfer. In this case, the 17 bits of data in the DAC latch program the output current between 0 mA for all 0s and 32 mA for all 1s (see Table III). However, in practice the AD421 cannot reliably produce a current less than 3.5 mA or more than 24 mA.
4.7 F REF OUT1 (1.25V) REF OUT2 (2.5V)
4.7 F LV VCC 75k 134k 2.5V BANDGAP REFERENCE 1.21V 121k DRIVE
AD421
50k
112.5k
50k
Figure 5. Reference Section
REF OUT2 is sensed internally, and if more than 0.5 mA is drawn externally from this reference, the chip goes into a power on reset state. In this state the sigma-delta DAC is disabled, the internal oscillator is stopped and the input data latch is cleared. REF OUT1 has limited current sinking capability. If REF OUT1 is required to sink current, a resistive load of 100 k to COM should be added in addition to the 4.7 F capacitor.
USING THE AD421
The AD421 can be programmed for normal 4 mA to 20 mA operation or for alarm current operation. For normal operation, the coding is 16-bit straight (natural) binary over an output current range of 4 mA to 20 mA. For alarm current operation, the coding is also straight binary but with 17 bits of resolution over twice the span, 0 mA to 32 mA, although the part should not be programmed outside the range of 3.5 mA to 24 mA. To determine whether data written to the part is normal 4 mA to 20 mA data or alarm current data, the number of clock pulses between two successive LATCH pulses are counted. If the number of pulses is 0-16 (modulo 32), it chooses normal mode; if it is 17-31 (modulo 32), it chooses alarm current range.
4 mA to 20 mA Coding
Table II shows the ideal input-code-to-output-current relationship for normal operation of the AD421. The output current values shown assume a REF IN voltage of +2.5 V. With a REF IN of +2.5 V, 1 LSB = 16 mA/65,536 = 244 nA. Figure 6 shows a timing diagram for programming the AD421 for normal 4 mA to 20 mA operation, the AD421 outputting a current -7-
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AD421
of 11.147 mA. With 16 clock pulses between consecutive latch signals data written is for normal 4 mA to 20 mA operation.
Table II. Ideal Input/Output Code Table for 4 mA to 20 mA Operation
CLOCK WORD "N" DATA X X XX XX X 0
X X X X X X X (MSB)B16
00 111 1 00
B15 B14 B13 B12 B11 B10 B9 B8
00000000
B7 B6 B5 B4 B3 B2 B1 (LSB)B0
Code 0000 0000 0000 0000 0000 0000 0000 0001 0000 0000 0000 0010 0100 0000 0000 0000 1000 0000 0000 0000 1100 0000 0000 0000 1111 1111 1111 1101 1111 1111 1111 1110 1111 1111 1111 1111
Output Current 4 mA 4.000244 mA 4.000488 mA 8 mA 12 mA 16 mA 19.999268 mA 19.999512 mA 19.999756 mA
LATCH
Figure 7. Write Cycle for Programming Alarm Current Data
MICROPROCESSOR INTERFACING AD421 - MC68HC11 (SPI BUS) INTERFACE
Figure 8 shows a typical interface between the AD421 and the Motorola MC68HC11 SPI (Serial Peripheral Interface) bus. 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" DATA 1
(MSB) B15
WORD "N +1" 1 001
B3 B2
SCK
CLOCK
0110
B13 B12 B14
010
0
11
1
1
B15
00 1
B13 B12 B14
68HC11
MOSI SS DATA IN LATCH
AD421*
B11 B10
B1 B0 (LSB)
B9 B8 B7
B6 B5 B4
LATCH
* ADDITIONAL PINS OMITTED FOR CLARITY
Figure 8. AD421 to 68HC11 Interface Figure 6. Write Cycle for 4 mA to 20 mA Operation
Alarm Current Coding
A typical routine such as the one shown below begins by initializing the state of the various SPI data and control registers.
INIT LDAA STAA LDAA STAA LDAA STAA NEXTPT LDAA BSR JMP SENDAT LDY BCLR STAA WAIT1 LDAA BPL LDAA STAA WAIT2 LDAA BPL BSET RTS #$2F PORTD #$38 DDRD #$50 SPCR MSBY SENDAT NEXTPT #$1000 $08,Y,$20 SPDR SPSR WAIT1 LSBY SPDR SPSR WAIT2; $08,Y,$20 ;SS = 1; SCK = 0; MOSI = 1 ;SEND TO SPI OUTPUTS ;SS, SCK, MOSI = OUTPUTS ;SEND DATA DIRECTION INFO ;DABL INTRPTS, SPI IS MASTER & ON ;CPOL = 0, CPHA = 0, 1MHZ BAUDRATE ;LOAD ACCUM W/UPPER 8 BITS ;JUMP TO DAC OUTPUT ROUTINE ;INFINITE LOOP ;POINT AT ON-CHIP REGISTERS ;DRIVE SS (LATCH) LOW ;SEND MS-BYTE TO SPI DATA REG ;CHECK STATUS OF SPIE ;POLL FOR END OF X-MISSION ;GET LOW 8 BITS FROM MEMORY ;SEND LS-BYTE TO SPI DATA REG ;CHECK STATUS OF SPIE ;POLL FOR END OF X-MISSION ;DRIVE SS HIGH TO LATCH DATA
Table III shows the ideal input-code-to-output-current relationship for alarm current programming of the AD421. In this case, the equivalent span is 0 mA to 32 mA but a reliable operating span is 3.5 mA to 24 mA. The part may give an indeterminate output for code values outside the range given in the table. As a result, the user is advised to restrict the code programmed to the part in alarm current mode to within the range shown in Table III. Figure 7 shows a timing diagram for loading an alarm current of 3.75 mA to the AD421 with an 8-bit microcontroller using three 8-bit writes. The output current values shown assume a REF IN voltage of +2.5 V. With a REF IN of +2.5 V, an ideal 1 LSB = 32 mA/ 131,072 = 244 nA.
Table III. Ideal Input/Output Code Table for Alarm Current Operation
Code 0 0011 1000 0000 0000 0 0011 1100 0000 0000 0 0100 0000 0000 0000 0 1000 0000 0000 0000 1 0000 0000 0000 0000 1 0100 0000 0000 0000 1 0110 0000 0000 0000 1 1000 0000 0000 0000
Output Current 3.5 mA 3.75 mA 4 mA 8 mA 16 mA 20 mA 22 mA 24 mA -8-
The SPI data port is configured to process data in 8-bit bytes. The most significant data byte (MSBY) is retrieved from memory and processed by the SENDAT routine. The SS pin is driven low by indexing into the PORTD data register and clear Bit 5. The MSBY is then sent to the SPI data register where it is automatically transferred to the AD421 internal shift resistor.
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AD421
The HC11 generates the requisite eight clock pulses with data valid on the rising edges. After the MSBY is transmitted, the least significant byte (LSBY) is loaded from memory and transmitted in a similar fashion. To complete the transfer, the LATCH pin is driven high when loading the complete 16-bit word into the AD421.
CLOCK VCC 2.2 F VCC 10k VCC CLOCK VCC 0.1 F
AD421 TO MICROWIRE INTERFACE
The flexible serial interface of the AD421 is also compatible with the National Semiconductor MICROWIRE interface. The MICROWIRE interface is used in microcontrollers such as the COP400 and COP800 series of processors. A generic interface to use the MICROWIRE interface is shown in Figure 9. The G1, SK, and SO pins of the MICROWIRE interface respectively connect to the LATCH, CLOCK, and DATA IN pins of the AD421.
AD421*
10k LATCH VCC 10k DATA IN DATA IN COM LATCH
SK
CLOCK
* ADDITIONAL PINS OMITTED FOR CLARITY
MICROWIRE
SO G1 DATA IN LATCH
AD421*
Figure 10. Opto-Isolated Interface
APPLICATIONS SECTION Basic Operating Configuration
* ADDITIONAL PINS OMITTED FOR CLARITY
Figure 9. AD421 to MICROWIRE Interface
Opto-Isolated Interface
The AD421 has a versatile serial 3-wire serial interface making it ideal for minimizing the number of control lines required for isolation of the digital system from the control loop. In intrinsically safe applications or due to noise, safety requirements, or distance, it may be necessary to isolate the AD421 from the controller. This can easily be achieved by using opto-isolators. Figure 10 shows an opto-isolated interface to the AD421 where CLOCK, DATAIN and LATCH are driven from opto-couplers. Be aware of signal inversion across the opto-couplers. If optocouplers with relatively slow rise and fall times are used, Schmitt triggers may be required on the digital inputs to prevent erroneous data being presented to the DAC.
Figure 11 shows the basic connection diagram for the AD421 operating at 5 V. This circuit shows the minimum of external components to operate the AD421. In the diagram, the AD421's regulator loop in conjunction with the DN25D pass transistor provides the VCC voltage for the AD421 itself and for other devices in the transmitter. The VCC pin should be well decoupled with a 2.2 F capacitor to ensure regulator stability and to absorb power glitches on the VCC line of the AD421 and other devices in the system. If the AD421 is operated with VCC = 3 V, the transfer function shifts negative. To correct for this a 16 k resistor connected between COM and LOOPRTN will approximately compensate for the VCC supply sensitivity in moving from 5 V to 3 V by adjusting the gain of the AD421.
VCC TO EXTERNAL CIRCUITRY 4.7 F COM REF IN REF OUT2 REF OUT1 2.2 F COM LV
DN25D
VCC DRIVE
DATA CLOCK
COMP 0.01 F 1k
VLOOP
AD421
LATCH BOOST LOOP RTN
1000pF
COM COM TO EXTERNAL CIRCUITRY
C1 0.01 F
C2 0.01 F
C3 0.0033 F
Figure 11. Basic Connection Diagram
REV. C
-9-
AD421
A capacitor of 0.01 F connected between COMP and DRIVE is required to stabilize the feedback loop formed with the regulator op amp and the external pass transistor. An external snubber circuit of 1 k and 1000 pF is required between the DRIVE pin and COM and a 0.1 F cap between COMP and DRIVE to stabilize the feedback loop formed by the regulator op amp and the external pass transistor. The internal 2.5 V reference on the AD421 is used as the reference for the AD421 and this has to be decoupled with a 4.7 F capacitor for compensation and stability purposes. The sigmadelta DAC on the part consists of a second order modulator followed by a continuous time filter. The resistors for each of the filter sections are on-chip while the capacitors are external on the C1 to C3 pins. To meet the specified full-scale settling on the part, low dielectric absorption capacitors (NPO) are required. Suitable values for these capacitors are C1 = C2 = 0.01 F, and C3 = 0.0033 F. The digital interface on the AD421 consists of just three wires: 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 into an input shift register on the rising edge of the CLOCK signal and is transferred to the DAC latch on the rising edge of the LATCH signal.
Reduce Power Load on External FET Smart Transmitter
The AD421 is intended for use in 4 mA to 20 mA smart transmitters. A smart transmitter is a system that incorporates a microprocessor system which is used for linearization and communication. Figure 13 shows a block diagram of a typical smart transmitter. In this example, the transmitter does not have any digital communication capabilities.
MEMORY
SENSORS
A/D CONVERTER
MICROPROCESSOR
D/A CONVERTER
4mA TO 20mA MEASUREMENT CIRCUIT
Figure 13. Typical Smart Transmitter
Figure 14 shows a typical smart transmitter application circuit using the AD421. The sensor voltage to be measured at the transmitter is converted using a high resolution sigma-delta converter such as the AD7714 or AD7715. These devices have an on-board PGA which can provide gains on the analog front end from 1 to 128. This allows for an analog input range as low as 10 mV which allows the transducer to be connected directly to the ADC. The AD7714/AD7715 have digital calibration techniques which are used to eliminate gain and offset errors. In addition, background calibration techniques are provided whereby the part continually calibrates itself and the user does not have to worry about issuing periodic calibration commands to remove effects of time and temperature drift. In normal operation the microprocessor reads the data from the AD7714/AD7715. After the data is processed by the microcontroller, the data is transferred from the serial port of the processor to the AD421 for transmission over the 4 to 20 mA loop back to the control center. The AD421 regulates the loop voltage to create power for the rest of the transmitter circuitry. In Figure 14, the derived VCC voltage is 3.3 V which is achieved by connecting the LV pin to VCC through 0.01 F. REF OUT2 provides the reference voltage for the AD421 itself while REF OUT1 provides the reference voltage for the AD7714/AD7715.
Figure 12 shows a circuit where an external NPN transistor is added to reduce the power loading on the FET. The FET will supply the VCC and an external high voltage NPN bipolar transistor can carry the BOOST current. The BOOST 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. The external NPN transistor reduces the external power load that the FET has to carry to less than 750 A if no other components share the VCC line and to less than 4 mA in applications that share the same VCC line as the AD421.
LOOP(+) VCC TO EXTERNAL CIRCUITRY 2.2 F COM LV VCC 75k 134k BANDGAP 1.21V REFERENCE 121k BOOST DRIVE COMP 0.01 F 1k 1000pF DN25D BC639/BC337
112.5k
AD421
80k 40 LOOP RTN LOOP(-)
Figure 12. External NPN Transistor Reduces Power Load on FET
-10-
REV. C
AD421
3.3V 0.1 F 2.2 F DN25D 0.01 F
SENSORS RTD mV TC AMBIENT TEMP SENSOR
DVDD AVDD 1.25V REF IN ANALOG TO DIGITAL CONVERTER 100k 4.7 F VCC 4.7 F
BOOST REF OUT1 REF OUT2 REF IN
VCC
LV LOOP POWER
0.01 F COMP
AD7714/ AD7715
MCLK IN
MICROCONTROLLER
CLOCK LATCH DATA
DRIVE 1k 1000pF
AD421
GND LOOP RTN C2
MCLK OUT CS DATA OUT SCLK DATA IN DGND AGND COM C1 C3
Figure 14. AD421 in Smart Transmitter Application
HART Interfacing
The HART protocol uses a frequency shift (FSK) keying technique based on the Bell 202 Communication Standard which is one of several standards used to transmit digital signals over the telephone lines. This technique is used to superimpose digital communication on to the 4 mA to 20 mA current loop connecting the central system to the transmitter in the field. Two different frequencies, 1200 Hz and 2200 Hz, are used to represent binary 1 and 0 respectively, as shown in Figure 15. These sine wave tones are superimposed on the dc signal at a low level with the average value of the sine wave signal being zero. This allows simultaneous analog and digital communications. 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 continue to work in systems that implement HART as the low-pass filtering usually present effectively removes the digital signal. A single pole 10 Hz low-pass filter effectively reduces the communication signal to a ripple of about 0.01% of the full-scale signal. The HART protocol specifies that master devices like a host control system or a hand held terminal transmit a voltage signal whereas a slave or field device transmits a current signal. The current signal is converted into a corresponding voltage by the loop load resistor.
APPROX +0.5mA
Figure 16 shows a block diagram of a smart and intelligent transmitter. An intelligent transmitter is a transmitter in which the functions of the microprocessor are shared between deriving the primary measurement signal, storing information regarding the transmitter itself, its application data and its location and also managing a communication system which enables two way communication to be superimposed on the same circuit that carries the measurement signal. A smart transmitter incorporating the HART protocol is an example of a smart intelligent transmitter.
MEMORY
SENSORS
A/D CONVERTER
MICROPROCESSOR
D/A CONVERTER
4mA TO 20mA MEASUREMENT CIRCUIT
COMMUNICATION SYSTEM
Figure 16. Smart and Intelligent Transmitter
APPROX -0.5mA
1200Hz "1"
2200Hz "0"
Figure 17 shows an example of the AD421 in a HART transmitter application. Most of the circuit is as outlined in the smart transmitter as shown in Figure 14. The HART data transmitted on the loop is received by the transmitter using a bandpass filter and modem and the HART data is transferred to the microcontroller'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 CC is determined 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 Logic, Inc., or HT2012 from SMAR Research Corp. For a more complete AD421-20C15 interface, please refer to Application Note AN-534 on the Analog Devices' website www.analog.com or contact your local sales office. -11-
Figure 15. HART Transmission of Digital Signals
REV. C
AD421
3.3V 0.1 F 2.2 F DN25D 0.01 F
SENSORS RTD mV TC AMBIENT TEMP SENSOR
DVDD AVDD REF IN ANALOG TO DIGITAL CONVERTER 100k 4.7 F VCC 1.25V
BOOST REF OUT1 4.7 F REF OUT2 REF IN CLOCK LATCH DATA
VCC
LV
0.01 F COMP DRIVE 1k
LOOP POWER
AD7714/ AD7715
MCLK IN
MICROCONTROLLER
AD421
GND LOOP RTN COM C1 C2 C3
1000pF
MCLK OUT CS DATA OUT SCLK DATA IN DGND AGND CC*
HART MODEM BELL 202
WAVEFORM SHAPER
BANDPASS FILTER
HT20C12/20C15
*FOR SELECTION OF CC, REFER TO AN-534
Figure 17. AD421 in HART Transmitter Application
Current Source
Figure 18 shows an application circuit for the AD421 being used as a current source. The current programmed to the AD421 (4 mA to 20 mA) will develop a voltage across R1. This same voltage due to negative feedback will be generated
across R2. The ratio of R1 to R2 determines the current that flows in the load resistor RL. IL = [1 + R1/R2] x IPROG, where IL is the current that flows in the load resistor RL and IPROG is the current programmed to the AD421. R1 and R2 are external to the AD421 and will need to be matched resistors to obtain a highly accurate current source.
5 VOLT REGULATOR OUT IN
+5V 10k 10k 10k DATA 2.2 F 4.7 F COM REF IN DATA REF OUT2 REF OUT1 LV VCC DRIVE COMP CLOCK CLOCK COM
VS
COM
AD421
BOOST LOOP RTN
LATCH
LATCH COM
C1
C2
C3
R1 R2 0.0033 F RL VS RETURN
0.01 F
0.01 F
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 micro and memory sections of the circuitry are protected against losing data if the loop is broken. The backup circuit switches from VCC to battery voltage without a glitch when VCC power is lost. The IRFF9113 acts as a current source during normal operation and provides a constant charging current to the supercap or Nicad. The loss of VCC drops the IRFF9113's gate voltage to zero volts, which allows the battery or supercaps current to flow through the MOSFETs channel and integral body diode to provide power for the micro and memory sections. To calibrate this circuit, connect an ammeter in series with the battery or supercap. Then with VCC and the load present adjust the 100 k potentiometer for the battery charging current recommended by the battery or supercap manufacturer. Nonrechargeable batteries should not be used in this application due to danger of explosion.
IN3611 0.1 F VCC IRFF9113
DN25D
4.7 F 100k
IN3611 VLOOP 2.2 F
MICRO/ MEMORY
GND SUPERCAP VCC
DRIVE
AD421*
LOOP RTN COM *ADDITIONAL CIRCUITRY OMITTED FOR CLARITY
Figure 19. Battery Backup Circuit
REV. C
-13-
AD421
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
16-Lead Plastic DIP (N-16)
C2105b-0-3/00 (rev. C)
0.840 (21.34) 0.745 (18.92)
16 1 9 8
0.280 (7.11) 0.240 (6.10) 0.060 (1.52) 0.015 (0.38) 0.130 (3.30) MIN
PIN 1 0.210 (5.33) MAX 0.160 (4.06) 0.115 (2.93) 0.022 (0.558) 0.014 (0.356) 0.100 (2.54) BSC
0.325 (8.26) 0.300 (7.62) 0.195 (4.95) 0.115 (2.93)
0.070 (1.77) SEATING 0.045 (1.15) PLANE
0.015 (0.381) 0.008 (0.204)
16-Lead (Wide Body) Small Outline Package (R-16)
0.4133 (10.50) 0.3977 (10.00)
16 9
1
8
PIN 1 0.0118 (0.30) 0.0040 (0.10)
0.1043 (2.65) 0.0926 (2.35)
0.2992 (7.60) 0.2914 (7.40) 0.4193 (10.65) 0.3937 (10.00)
0.0291 (0.74) x 45 0.0098 (0.25)
0.0500 (1.27) BSC
0.0192 (0.49) SEATING 0.0138 (0.35) PLANE
8 0.0125 (0.32) 0 0.0091 (0.23)
0.0500 (1.27) 0.0157 (0.40)
-14-
REV. C
PRINTED IN U.S.A.

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