ANALOG DEVICES AD 5421 BREZ Datasheet

Page 1
Loop-Powered, 4 mA to 20 mA DAC
AD5421
Rev. E
nished by Analog Devices is believed to be accurate and reliable. However, no
Trademarks and registered trademarks are the property of their respective owners.
Fax: 781.461.3113 ©2011–2012 Analog Devices, Inc. All rights reserved.
R
SET
24kΩ
SYNC SCLK
SDIN
SDO
LDAC
RANGE0 RANGE1
ALARM_CURRENT_DIRECTION
FAULT
R
INT
/R
EXT
INPUT
REGISTER CONTROL
LOGIC
GAIN/OFFSET ADJUSTMENT
REGISTERS
TEMPERATURE
SENSOR
REFOUT2 REFIN CINR
EXT1
R
EXT2
REFOUT1
VREF
16
16-BIT
DAC
LOOP VOLTAGE MONITOR
V
LOOP
DV
DD
IODV
DD
AD5421
VOLTAGE
REGULATOR
REG_SEL0 REG_SEL1 REG_SEL2 REG
OUT
REG
IN
DRIVE
LOOP–
11.5kΩ 52Ω
09128-001
COM
Data Sheet

FEATURES

16-bit resolution and monotonicity Pin selectable NAMUR-compliant ranges
4 mA to 20 mA
3.8 mA to 21 mA
3.2 mA to 24 mA
NAMUR-compliant alarm currents
Downscale alarm current = 3.2 mA
Upscale alarm current = 22.8 mA/24 mA Total unadjusted error (TUE): 0.05% maximum INL error: 0.0035% FSR maximum Output TC: 3 ppm/°C typical Quiescent current: 300 µA maximum Flexible SPI-compatible serial digital interface
with Schmitt triggered inputs On-chip fault alerts via FAULT pin or alarm current Automatic readback of fault register on each write cycle Slew rate control function Gain and offset adjust registers On-chip reference TC: 4 ppm/°C maximum Selectable regulated voltage output Loop voltage range: 5.5 V to 52 V Temperature range: −40°C to +105°C TSSOP and LFCSP packages

APPLICATIONS

Industrial process control 4 mA to 20 mA loop-powered transmitters Smart transmitters HART network connectivity
16-Bit, Serial Input,

GENERAL DESCRIPTION

The AD5421 is a complete, loop-powered, 4 mA to 20 mA digital-to-analog converter (DAC) designed to meet the needs of smart transmitter manufacturers in the industrial control indu s t r y. The DAC provides a high precision, fully integrated, low cost solution in compact TSSOP and LFCSP packages.
The AD5421 includes a regulated voltage output that is used to power itself and other devices in the transmitter. This regulator provides a regulated 1.8 V to 12 V output voltage. The AD5421 also contains 1.22 V and 2.5 V references, thus eliminating the need for a discrete regulator and voltage reference.
The AD5421 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 30 MHz and allows for simple connection to commonly used microprocessors and microcontrollers via a SPI-compatible, 3-wire interface.
The AD5421 is guaranteed monotonic to 16 bits. It provides
0.0015% integral nonlinearity, 0.0012% offset error, and
0.0006% gain error under typical conditions.
The AD5421 is available in a 28-lead TSSOP and a 32-lead LFCSP specified over the extended industrial temperature range of −40°C to +105°C.

COMPANION LOW POWER PRODUCTS

HART Modem: AD5700, AD5700-1 Microcontroller: ADuCM360
Information fur responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.

FUNCTIONAL BLOCK DIAGRAM

Figure 1.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700
www.analog.com
Page 2
AD5421 Data Sheet
TABLE OF CONTENTS
Features .............................................................................................. 1
Applications ....................................................................................... 1
General Description ......................................................................... 1
Companion Low Power Products .................................................. 1
Functional Block Diagram .............................................................. 1
Revision History ............................................................................... 2
Specifications ..................................................................................... 3
AC Performance Characteristics ................................................ 8
Timing Characteristics ................................................................ 8
Absolute Maximum Ratings .......................................................... 10
Thermal Resistance .................................................................... 10
ESD Caution ................................................................................ 10
Pin Configuration and Function Descriptions ........................... 11
Typ i cal Performance Characteristics ........................................... 13
Terminology .................................................................................... 19
Theory of Operation ...................................................................... 20
Fault Alerts .................................................................................. 20
External Current Setting Resistor ............................................ 21
Loop Current Range Selection .................................................. 21
Connection to Loop Power Supply .......................................... 21
On-Chip ADC ............................................................................ 22
Voltage Regulator ....................................................................... 22
Loop Current Slew Rate Control .............................................. 22
Power-On Default ...................................................................... 23
HART Communications ........................................................... 23
Serial Interface ................................................................................ 25
Input Shift Register .................................................................... 25
Register Readback ...................................................................... 25
DAC Register .............................................................................. 26
Control Register ......................................................................... 27
Fault Register .............................................................................. 28
Offset Adjust Register ................................................................ 29
Gain Adjust Register .................................................................. 29
Applications Information .............................................................. 31
Determining the Expected Total Error.................................... 31
Thermal and Supply Considerations ....................................... 33
Outline Dimensions ....................................................................... 34
Ordering Guide .......................................................................... 35

REVISION HISTORY

7/12—Rev. D to Rev. E
Changes to Figure 1 and Companion Products Section ............. 1
Changes to Pin LOOP− Description ........................................... 12
Changes to Applications Information Section and Figure 49... 31
Added Figure 50 .............................................................................. 32
5/12—Rev. C to Rev. D
Changes to Features Section and Applications Section; Added
Companion Products Section ......................................................... 1
Changes to Line Regulation Parameter, Table 1 ........................... 5
Updated Outline Dimensions ....................................................... 33
12/11—Rev. B to Rev. C
Change to REFOUT1 Pin, Capacitive Load Parameter, Test
Conditions, Table 1 ........................................................................... 4
Change to REG
Conditions, Table 1 ........................................................................... 5
Changes to ESD Parameter, Table 6 ............................................. 10
12/11—Rev. A to Rev. B
Added 32-Lead LFCSP ....................................................... Universal
Changes to the Specifications Section, Table 1 ............................. 3
Output, Capacitive Load Parameter, Test
OUT
Changes to Table 7 .......................................................................... 10
Added Figure 5, Renumbered Sequentially ................................ 11
Changes to Table 8 .......................................................................... 11
Changes to Figure 33 ...................................................................... 17
Changes to the On-Chip ADC Section ....................................... 22
Changes to Figure 46 ...................................................................... 23
Changes to Figure 48 ...................................................................... 24
Changes to the Register Readback Section ................................. 25
Updated Outline Dimensions ....................................................... 33
Changes to Ordering Guide .......................................................... 34
5/11—Rev. 0 to Rev. A
Changes to REG
in Table 8 .......................................................................................... 10
Change to Figure 45 ....................................................................... 22
Changes to Input Shift Register Section, Table 11, and Register
Readback Section ............................................................................ 24
Changes to Figure 48 ...................................................................... 30
2/11—Revision 0: Initial Version
, REFOUT1, and REFOUT2 Pin Descriptions
IN
Rev. E | Page 2 of 36
Page 3
Data Sheet AD5421
ACCURACY, INTERNAL R
−0.06
±0.011
+0.06
% FSR
B grade, TA = 25°C
−0.041
±0.0065
+0.041
% FSR
B grade and C grade, TA = 25°C
23.97
24.03
mA
3.2 mA to 24 mA range
TUE Long-Term Stability
40 ppm FSR
Drift after 1000 hours at TA = 125°C

SPECIFICATIONS

Loop voltage = 24 V; REFIN = 2.5 V external; RL = 250 Ω; external NMOS connected; all loop current ranges; all specifications T
to T
MIN
Table 1.
Parameter1 Min Typ Max Unit Test Conditions/Comments
Resolution 16 Bits
Tot al Unadjusted Error (TUE)2 −0.126 +0.126 % FSR C grade
−0.041 ±0.0064 +0.041 % FSR C grade, TA = 25°C
−0.18 +0.18 % FSR B grade
−0.27 +0.27 % FSR A grade
−0.08 ±0.011 +0.08 A grade, TA = 25°C
TUE Long-Term Stability 210 ppm FSR Drift after 1000 hours at TA = 125°C
Relative Accuracy (INL) −0.0035 ±0.0015 +0.0035 % FSR C grade
−0.012 ±0.006 +0.012 % FSR B grade
−0.024 ±0.01 +0.024 % FSR A grade
Differential Nonlinearity (DNL) −1 +1 LSB Guaranteed monotonic
Offset Error −0.056 +0.056 % FSR B grade and C grade
−0.008 ±0.0008 +0.008 % FSR B grade and C grade, TA = 25°C
−0.11 ±0.0008 +0.11 % FSR A grade
Offset Error TC3 1 ppm FSR/°C
Gain Error −0.107 +0.107 % FSR B grade and C grade
−0.035 ±0.0058 +0.035 % FSR B grade and C grade, TA = 25°C
−0.2 ±0.0058 +0.2 % FSR A grade
Gain Error TC3 4 ppm FSR/°C
Full-Scale Error −0.126 +0.126 % FSR B grade and C grade
, unless otherwise noted.
MAX
SET
−0.25 ±0.0065 +0.25 % FSR A grade Full-Scale Error TC3 5 ppm FSR/°C Downscale Alarm Current 3.19 3.21 mA Upscale Alarm Current 22.77 22.83 mA 4 mA to 20 mA and 3.8 mA to 21 mA
ranges
ACCURACY, EXTERNAL R
Resolution 16 Bits Total Unadjusted Error (TUE)2 −0.048 +0.048 % FSR C grade
−0.027 ±0.002 +0.027 % FSR C grade, TA = 25°C
−0.08 +0.08 % FSR B grade
−0.04 ±0.003 +0.04 % FSR B grade, TA = 25°C
Relative Accuracy (INL) −0.0035 ±0.0015 +0.0035 % FSR C grade
−0.012 ±0.006 +0.012 % FSR B grade Differential Nonlinearity (DNL) −1 +1 LSB Guaranteed monotonic Offset Error −0.021 +0.021 % FSR
−0.007 ±0.0012 +0.007 % FSR TA = 25°C Offset Error TC3 0.5 ppm FSR/°C Gain Error −0.03 +0.03 % FSR
−0.023 ±0.0006 +0.023 % FSR TA = 25°C
(24 kΩ) Assumes ideal resistor, B grade and
SET
C grade only; not specified for A grade
Rev. E | Page 3 of 36
Page 4
AD5421 Data Sheet
Downscale Alarm Current
3.08 3.21
mA
0.1 Hz to 10 Hz
50 nA p-p
Temperature Coefficient
1.5 4 ppm/°C
C grade
Thermal Hysteresis3
285 ppm
First temperature cycle
Parameter1 Min Typ Max Unit Test Conditions/Comments
Gain Error TC3 1 ppm FSR/°C Full-Scale Error −0.047 +0.047 % FSR
−0.028 ±0.0017 +0.028 % FSR TA = 25°C Full-Scale Error TC3 1 ppm FSR/°C
Upscale Alarm Current 22.78 23 mA 4 mA to 20 mA and 3.8 mA to 21 mA
ranges
23.99 24.01 mA 3.2 mA to 24 mA range OUTPUT CHARACTERISTICS3
Loop Compliance Voltage4 LOOP− + 5.5 V REG LOOP− + 12.5 V REG Loop Current Long-Term Stability 100 ppm FSR Drift after 1000 hours at TA = 125°C,
15 ppm FSR Drift after 1000 hours at TA = 125°C,
Loop Current Error vs. REG
OUT
Load
1.2 µA/mA Loop current = 12 mA, load current
Current Resistive Load 0 2 kΩ See Figure 20 for a load line graph Inductive Load 50 mH Stable operation Power Supply Sensitivity 0.1 µA/V Loop current = 12 mA Output Impedance 12 400 MΩ Output TC 3 ppm FSR/°C Loop current = 12 mA, internal R 1 ppm FSR/°C Loop current = 12 mA, external R Output Noise
< 5.5 V, loop current = 24 mA
OUT
= 12 V, loop current = 24 mA
OUT
loop current = 12 mA, internal R
loop current = 12 mA, external R
from REG
= 5 mA
OUT
SET
SET
SET
SET
500 Hz to 10 kHz 0.2 mV rms HART bandwidth; measured across
500 Ω load
Noise Spectral Density 195 nA/√Hz At 1 kHz
256 nA/√Hz At 10 kHz REFERENCE INPUT (REFIN PIN)3
Reference Input Voltage5 2.5 V For specified performance DC Input Impedance 75 800 MΩ
REFERENCE OUTPUTS
REFOUT1 Pin
Output Voltage 2.498 2.5 2.503 V TA = 25°C
2 8 ppm/°C B grade 4 10 ppm/°C A grade Output Noise (0.1 Hz to 10 Hz)3 7.5 µV p-p Noise Spectral Density3 245 nV/√Hz At 1 kHz 70 nV/√Hz At 10 kHz Output Voltage Drift vs. Time3 200 ppm Drift after 1000 hours at TA = 125°C Capacitive Load3 10 nF Recommended operation Load Current
3, 6
4 mA Short-Circuit Current3 6.5 mA Short circuit to COM Power Supply Sensitivity3 2 12 µV/V
5 ppm Second temperature cycle Load Regulation3 0.1 0.2 mV/mA Measured at 0 mA and 1 mA loads Output Impedance 0.1
REFOUT2 Pin
Output Voltage 1.18 1.227 1.28 V TA = 25°C Output Impedance 72 kΩ
Rev. E | Page 4 of 36
Page 5
Data Sheet AD5421
Externally Available Current
3.15
mA
Assuming 4 mA flowing in the loop
Inductive Load
50 mH
Stable operation
Load Regulation
45 mV/mA
Measured at 0 mA and 3 mA loads
Hysteresis
0.21 V
IODVDD = 1.8 V
I
0.01% FSR
I
+ 0.01% FSR
Parameter1 Min Typ Max Unit Test Conditions/Comments
REG
OUTPUT Voltage regulator output
OUT
Output Voltage 1.8 12 V See Table 10 Output Voltage TC3 110 ppm/°C Output Voltage Accuracy −4 ±2 +4 %
Short-Circuit Current 23 mA Line Regulation3 500 μV/V Internal NMOS 10 μV/V External NMOS Load Regulation3 8 mV/mA
Capacitive Load 2 10 µF Recommended operation
ADC ACCURACY
Die Temperature ±5 °C V
Input ±1 %
LOOP
DVDD OUTPUT Can be overdriven up to 5.5 V
Output Voltage 3.17 3.3 3.48 V Externally Available Current
Short-Circuit Current 7.7 mA
3, 6
and during HART communications
3, 6
3.15 mA Assuming 4 mA flowing in the loop and during HART communications
DIGITAL INPUTS3 SCLK,
SYNC
, SDIN,
LDAC Input High Voltage, VIH 0.7 × IODVDD V Input Low Voltage, VIL 0.25 × IODVDD V
0.63 V IODVDD = 3.3 V
1.46 V IODVDD = 5.5 V Input Current −0.015 +0.015 µA Per pin Pin Capacitance 5 pF Per pin
DIGITAL OUTPUTS3
SDO Pin
Output Low Voltage, VOL 0.4 V Output High Voltage, VOH IODVDD − 0.5 V High Impedance Leakage
−0.01 +0.01 µA
Current
High Impedance Output
5 pF
Capacitance
FAULT Pin
Output Low Voltage, VOL 0.4 V Output High Voltage, VOH IODVDD − 0.5 V
FA ULT THRESHOLDS
I
Under
LOOP
I
Over
LOOP
LOOP
LOOP
mA mA
Temp 140°C 133 °C Fault removed when temperature
≤ 125°C
Temp 100°C 90 °C Fault removed when temperature
≤ 85°C
V
6V 0.3 V Fault removed when V
LOOP
V
12V 0.6 V Fault removed when V
LOOP
LOOP
LOOP
≥ 0.4 V ≥ 0.7 V
Rev. E | Page 5 of 36
Page 6
AD5421 Data Sheet
Parameter1 Min Typ Max Unit Test Conditions/Comments
POWER REQUIREMENTS
REGIN 5.5 52 V With respect to LOOP− IODVDD 1.71 5.5 V With respect to COM Quiescent Current 260 300 µA
1
Temperature range: −40°C to +105°C; typical at +25°C.
2
Total unadjusted error is the total measured error (offset error + gain error + linearity error + output drift over temperature) after factory calibration of the AD5421.
System level total error can be reduced using the offset and gain registers.
3
Guaranteed by design and characterization; not production tested.
4
The voltage between LOOP− and REGIN must be 5.5 V or greater.
5
The AD5421 is factory calibrated with an external 2.5 V reference connected to REFIN.
6
This is the current that the output is capable of sourcing. The load current originates from the loop and, therefore, contributes to the total current consumption figure.
Rev. E | Page 6 of 36
Page 7
Data Sheet AD5421
ACCURACY, INTERNAL R
ACCURACY, EXTERNAL R
(24 kΩ)
Assumes ideal resistor
Loop voltage = 24 V; REFIN = REFOUT1 (2.5 V internal reference); RL = 250 Ω; external NMOS connected; all loop current ranges; all specifications T
Table 2.
Parameter
1, 2
Total Unadjusted Error (TUE)3 −0.157 +0.157 % FSR
−0.117 ±0.0172 +0.117 % FSR TA = 25°C Relative Accuracy (INL) −0.004 +0.004 % FSR
−0.004 ±0.0015 +0.004 % FSR TA = 25°C Offset Error −0.04 +0.04 % FSR
−0.025 ±0.0025 +0.025 % FSR TA = 25°C Offset Error TC 1 ppm FSR/°C Gain Error −0.128 +0.128 % FSR
−0.093 ±0.0137 +0.093 % FSR TA = 25°C Gain Error TC 5 ppm FSR/°C Full-Scale Error −0.157 +0.157 % FSR
−0.117 ±0.0172 +0.117 % FSR TA = 25°C Full-Scale Error TC 6 ppm FSR/°C
Total Unadjusted Error (TUE)3 −0.133 +0.133 % FSR
−0.133 ±0.0252 +0.133 % FSR TA = 25°C Relative Accuracy (INL) −0.004 +0.004 % FSR
−0.004 ±0.0015 +0.004 % FSR TA = 25°C Offset Error −0.029 +0.029 % FSR
−0.029 ±0.0038 +0.029 % FSR TA = 25°C Offset Error TC 0.5 ppm FSR/°C Gain Error −0.11 +0.11 % FSR
−0.106 ±0.0197 +0.106 % FSR TA = 25°C Gain Error TC 2 ppm FSR/°C Full-Scale Error −0.133 +0.133 % FSR
−0.133 ±0.0252 +0.133 % FSR TA = 25°C Full-Scale Error TC 2 ppm FSR/°C
1
Temperature range: −40°C to +105°C; typical at +25°C.
2
Specifications guaranteed by design and characterization; not production tested.
3
Total unadjusted error is the total measured error (offset error + gain error + linearity error + output drift over temperature) after factory calibration of the AD5421.
System level total error can be reduced using the offset and gain registers.
to T
MIN
, unless otherwise noted.
MAX
C Grade
SET
SET
Unit Test Conditions/Comments Min Typ Max
Rev. E | Page 7 of 36
Page 8
AD5421 Data Sheet
t3
17
ns min
SCLK low time
1 0 1
2619
3000
3489
ms

AC PERFORMANCE CHARACTERISTICS

Loop voltage = 24 V; REFIN = 2.5 V external; RL = 250 Ω; all specifications T
Table 3.
Parameter1 Min Typ Max Unit Test Conditions/Comments
DYNAMIC PERFORMANCE
Loop Current Settling Time 50 µs To 0.1% FSR, CIN = open circuit Loop Current Slew Rate 400 µA/µs CIN = open circuit AC Loop Voltage Sensitivity 1.3 µA/V 1200 Hz to 2200 Hz, 5 V p-p, RL = 3 kΩ
1
Temperature range: −40°C to +105°C; typical at +25°C.

TIMING CHARACTERISTICS

Loop voltage = 24 V; REFIN = 2.5 V external; RL = 250 Ω; all specifications T
MIN
MIN
to T
to T
, unless otherwise noted.
MAX
.
MAX
Table 4.
Parameter
1, 2, 3
Limit at T
MIN
, T
Unit Description
MAX
t1 33 ns min SCLK cycle time t2 17 ns min SCLK high time
t4 17 ns min t5 10 ns min SCLK falling edge to t6 25 µs min Minimum
falling edge to SCLK falling edge setup time
SYNC
rising edge
SYNC
high time
SYNC t7 5 ns min Data setup time t8 5 ns min Data hold time t9 25 µs min t10 10 ns min t11 70 ns max SCLK rising edge to SDO valid (C t12 0 ns min t13 70 ns max
1
Guaranteed by design and characterization; not production tested.
2
All input signals are specified with tR = tF = 5 ns (10% to 90% of DVDD) and timed from a voltage level of 1.2 V.
3
See Figure 2 and Figure 3.
rising edge to
SYNC
pulse width low
LDAC
falling edge to SCLK rising edge setup time
SYNC
rising edge to SDO tristate (C
SYNC
LDAC
falling edge
L SDO
Table 5. SPI Watchdog Timeout Periods
Parameter1
T0 T1 T2
Min Typ Max Unit
0 0 0 43 50 59 ms 0 0 1 87 100 117 ms 0 1 0 436 500 582 ms 0 1 1 873 1000 1163 ms 1 0 0 1746 2000 2326 ms
= 30 pF)
= 30 pF)
L SDO
1 1 0 3493 4000 4652 ms 1 1 1 4366 5000 5814 ms
1
Specifications guaranteed by design and characterization; not production tested.
Rev. E | Page 8 of 36
Page 9
Data Sheet AD5421

Timing Diagrams

t
1
10 11 12 22
t
2
D14 D13 D2 D1D15
t
11
24
23
D0D16D23
t
13
t
5
t9t
10
SCLK
SDIN
SDO
SYNC
t
12
2
t
7
89
t
3
t
8
D15 D14 D13 D2 D1 D0
1
t
4
t
6
LDAC
09128-002
Figure 2. Serial Interface Timing Diagram
SCLK
SDIN
SDO
SYNC
18924
D23 D15 D0
INPUT WORD SPECIFIES REGISTER TO BE READ
D16
UNDEFINED DATA
Figure 3. Readback Timing Diagram
1
D23 D0D15
SPECIFIE D RE GISTER DATA CLOCKED OUT
89
D16
NOP OR REGISTER ADDRESS
D15 D0
24
09128-003
Rev. E | Page 9 of 36
Page 10
AD5421 Data Sheet
REFIN to COM
−0.3 V to +7 V
Junction Temperature (T
)
125°C

ABSOLUTE MAXIMUM RATINGS

TA = 25°C, unless otherwise noted. Transient currents of up to 100 mA do not cause SCR latch-up.
Table 6.
Parameter Rating
REGIN to COM −0.3 V to +60 V REG
to COM −0.3 V to +14 V
OUT
Digital Inputs to COM,
RANGE0, RANGE1, R ALARM_CURRENT_DIRECTION, REG_SEL0, REG_SEL1, REG_SEL2
Digital Inputs to COM
SCLK, SDIN,
Digital Outputs to COM,
SDO, FAULT
REFOUT1, REFOUT2 −0.3 V to +4.7 V V
to COM −0.3 V to +60 V
LOOP
LOOP− to COM −5 V to +0.3 V DVDD to COM −0.3 V to +7 V IODVDD to COM −0.3 V to +7 V R
, CIN to COM −0.3 V to +4.3 V
EXT1
R
to COM −0.3 V to +0.3 V
EXT2
DRIVE to COM −0.3 V to +11 V Operating Temperature Range (TA)
Industrial −40°C to +105°C
Storage Temperature Range −65°C to +150°C
Power Dissipation (T Lead Temperature,
Soldering (10 sec)
ESD
Human Body Model 3 kV Field Induced Charged Device
Model
Machine Model 200 V
SYNC
,
INT/REXT
LDAC
,
J MAX
−0.3 V to DVDD + 0.3 V or +7 V (whichever is less)
−0.3 V to IODVDD + 0.3 V or +7 V (whichever is less)
−0.3 V to IODVDD + 0.3 V or +7 V (whichever is less)
− TA)/θJA
J MAX
JEDEC Industry Standard J-STD-020
2 kV
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 indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.

THERMAL RESISTANCE

θJA is specified for the worst-case conditions, that is, a device soldered in a circuit board for surface-mount packages.
Table 7. Thermal Resistance
Package Type θJA θJC Unit
28-Lead TSSOP_EP (RE-28-2) 32 9 °C/W 24-Lead LFCSP_WQ (CP-32-11) 40 7 °C/W

ESD CAUTION

Rev. E | Page 10 of 36
Page 11
Data Sheet AD5421
1 2 3 4 5 6 7 8
9 10 11 12 13 14
28 27 26 25 24 23 22 21 20 19 18 17 16 15
SDO SCLK SYNC
FAULT
LDAC
SDIN
IODV
DD
REG
IN
DRIVE V
LOOP
R
EXT1
R
EXT2
LOOP–
DV
DD
ALARM_CURRENT_DIRECTION
R
INT
/R
EXT
COM COM
RANGE1
RANGE0
C
IN
REFOUT1 REFOUT2
REG_SEL1 REG_SEL2
REG_SEL0
REFIN
REG
OUT
TOP VIEW
(Not to S cale)
AD5421
NOTES
1. THE EXPO S E D P ADDLE SHOULD BE CONNECTED TO THE SAME POTENT IAL AS THE CO M P IN AND TO A COPP E R P LANE FOR OPTIM UM THERMAL PERF ORMANCE.
09128-004
09128-100
PIN 1 INDICATOR
1SDIN 2LDAC 3FAULT 4COM 5DV
DD
6ALARM CURRENT DI RE CTION 7R
INT/REXT
8RANGE 0
24
V
LOOP
23 LOOP– 22 R
EXT2
21 R
EXT1
20 C
IN
19 REFOUT1 18 REFOUT2 17 REFIN
9
RANGE 1
10
COM
11
COM
12
NC
13
REG_SEL2
14
REG_SEL1
15
REG_SEL0
16NC
32
SYNC
31
SCLK
30
SDO
29
IODV
DD
28
REG
OUT
27
REG
IN
26
DRIVE
25
NC
AD5421
TOP VIEW
(Not to S cale)
NOTES
1. NO CONNECT. DO NOT CONNECT TO THIS PIN.
2. THE E X P OSED PADDLE SHOULD BE CONNECT E D TO THE SAME POT E NTIAL AS THE COM PIN AND TO A COPPER PLANE FO R OPTIMUM THERMAL PERF ORMANCE.
Digital Interface Supply Pin. Digital thresholds are referenced to the voltage applied to this pin. A

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

Figure 4. TSSOP Pin Configuration
Table 8. Pin Function Descriptions
Pin No.
TSSOP LFCSP
Mnemonic Description
1 29 IODVDD
voltage from 1.71 V to 5.5 V can be applied to this pin.
2 30 SDO Serial Data Output. Used to clock data from the input shift register. Data is clocked out on the
rising edge of SCLK and is valid on the falling edge of SCLK.
3 31 SCLK Serial Clock Input. Data is clocked into the input shift register on the falling edge of SCLK. This
input operates at clock speeds up to 30 MHz.
4 32
Frame Synchronization Input, Active Low. This is the frame synchronization signal for the serial
SYNC
interface. When
is low, data is transferred on the falling edge of SCLK. The input shift
SYNC
register data is latched on the rising edge of 5 1 SDIN Serial Data Input. Data must be valid on the falling edge of SCLK. 6 2
Load DAC Input, Active Low. This pin is used to update the DAC register and, consequently, the
LDAC
output current. If
of
. If
SYNC
LDAC
is tied permanently low, the DAC register is updated on the rising edge
LDAC
is held high during the write cycle, the input register is updated, but the output
update is delayed until the falling edge of 7 3 FAU LT Fault Alert Output Pin, Active High. This pin is asserted high when a fault is detected. Detectable
faults are loss of SPI interface control, communication error (PEC), loop current out of range,
insufficient loop voltage, and overtemperature. For more information, see the Fault Alerts
section. 8 5 DVDD 3.3 V Digital Power Supply Output. This pin should be decoupled to COM with 100 nF and 4.7 µF
9 6 ALARM_
10 7 R
11, 12 8, 10 RANGE0,
CURRENT_ DIRECTION
RANGE1
Current Setting Resistor Select. When this pin is connected to DVDD, the internal current setting
INT/REXT
capacitors.
Alarm Current Direction Select. This pin is used to select whether the alarm current is upscale
(22.8 mA/24 mA) or downscale (3.2 mA). Connecting this pin to DVDD selects an upscale alarm
current (22.8 mA/24 mA); connecting this pin to COM selects a downscale alarm current (3.2 mA).
For more information, see the Power-On Default section.
resistor is selected. When this pin is connected to COM, the external current setting resistor is
selected. An external resistor can be connected between the R
Digital Input Pins. These two pins select the loop current range (see the Loop Current Range
Selection section).
Rev. E | Page 11 of 36
Figure 5. LFCSP Pin Configuration
.
SYNC
LDAC
. The
pin should not be left unconnected.
LDAC
EXT1
and R
EXT2
pins.
Page 12
AD5421 Data Sheet
21
20
CIN
External Capacitor Connection and HART FSK Input. An external capacitor connected from CIN to
Voltage Input Pin. Voltage input range is 0 V to 2.5 V. The voltage applied to this pin is digitized to
N/A1
9, 16, 25
NC
No Connect. Do not connect to this pin.
Pin No.
TSSOP LFCSP
13, 14 4, 11, 12 COM Ground Reference Pin for the AD5421. It is recommended that a 4.7 V Zener diode be placed
15, 16,
13, 14, 15 REG_SEL2,
17
18 17 REFIN Reference Voltage Input. V 19 18 REFOUT2 Internal Reference Voltage Output (1.22 V). It is recommended to connect a 100 nF capacitor
20 19 REFOUT1 Internal Reference Voltage Output (2.5 V). It is recommended to connect a 100 nF capacitor from
22, 23 21, 22 R
24 23 LOOP− Loop Current Return Pin. As shown in Figure 1, the COM and LOOP− pins can be used to sense
25 23 V
26 26 DRIVE Gate Connection for External Depletion Mode MOSFET. For more information, see the
27 27 REGIN Voltage Regulator Input. The loop voltage can be connected directly to this pin. Or, to reduce on-
28 28 REG
Mnemonic Description
between the LOOP− and COM pins. See the Applications Information section for more information.
REG_SEL1,
These three pins together select the regulator output (REG section).
) voltage (see the Voltage Regulator
OUT
REG_SEL0
= 2.5 V for specified performance.
REFIN
from this pin to COM.
this pin to COM.
COM implements an output slew rate control function (see the Loop Current Slew Rate Control section). HART FSK signaling can also be coupled through a capacitor to this pin (see the HART Communications section).
, R
EXT1
Connection for External Current Setting Resistor. A precision 24 kΩ resistor can be connected
EXT2
between these pins for improved performance.
the loop current across the internal 52 Ω resistor. Note that the voltage measured at LOOP− will be negative with respect to COM.
LOOP
eight bits, which are available in the fault register. This pin can be used for general-purpose voltage monitoring, but it is intended for monitoring of the loop supply voltage. Connecting the loop voltage to this pin via a 20:1 resistor divider allows the AD5421 to monitor and feedback the loop voltage. The AD5421 also generates an alert if the loop voltage is close to the minimum operating value (see the
Loop Voltage Fault section).
Connection to Loop Power Supply section.
chip power dissipation, an external pass transistor can be connected at this pin to stand off the loop voltage. For more information, see the Connection to Loop Power Supply section.
Voltage Regulator Output. Pin selectable values are from 1.8 V to 12 V via the REG_SEL0,
OUT
REG_SEL1, and REG_SEL2 pins (see the Voltage Regulator section).
EPAD EPAD Exposed Pad The exposed paddle should be connected to the same potential as the COM pin and to a copper
plane for optimum thermal performance.
1
N/A means not applicable.
Rev. E | Page 12 of 36
Page 13
Data Sheet AD5421
1.0
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
0 60k50k40k30k20k10k
INL ERROR ( LSB)
DAC CODE
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
T
A
= 25°C 4mA TO 20mA RANGE EXT V
REF
EXT R
SET
09128-005
1.0
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.4
0.6
0.8
0 60k50k40k30k20k10k
DNL ERROR (L S B)
DAC CODE
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
T
A
= 25°C 4mA TO 20mA RANGE EXT V
REF
EXT R
SET
09128-006
0.01
–0.06
–0.05
–0.04
–0.03
–0.02
–0.01
0
0 60k50k40k30k20k10k
TOTAL UNADJUS TED ERROR (% FS R)
DAC CODE
09128-007
V
REF
EXT, R
SET
EXT, NMOS EXT, 24V
V
REF
EXT, R
SET
EXT, NMOS INT, 24V
V
REF
EXT, R
SET
EXT, NMOS INT, 52V
V
REF
INT, R
SET
INT, NMOS EXT, 24V
V
REF
INT, R
SET
INT, NMOS INT, 24V
V
REF
INT, R
SET
INT, NMOS INT, 52V
R
LOAD
= 250Ω
T
A
= 25°C
4mA TO 20mA RANGE
0.015
–0.010
–0.005
0
0.005
0.010
–40 85603510–15
OFFSET ERROR (% FSR)
TEMPERATURE (°C)
EXT V
REF
, INT R
SET
EXT V
REF
, EXT R
SET
INT V
REF
, INT R
SET
INT V
REF
, EXT R
SET
V
LOOP
= 24V 4mA TO 20mA RANGE R
LOAD
= 250Ω
EXT NMOS
09128-008
0.03
–0.06
–0.05
–0.04
–0.03
–0.02
–0.01
0
0.01
0.02
–40 85603510–15
GAIN ERROR ( % FSR)
TEMPERATURE (°C)
EXT V
REF
, INT R
SET
EXT V
REF
, EXT R
SET
INT V
REF
, INT R
SET
INT V
REF
, EXT R
SET
V
LOOP
= 24V 4mA TO 20mA RANGE R
LOAD
= 250Ω
EXT NMOS
09128-009
0.0012
–0.0008
–0.0006
–0.0004
–0.0002
0
0.0002
0.0004
0.0006
0.0008
0.0010
–40 8560
35
MAX INL
MIN INL
10–15
INL ERROR ( % FSR)
TEMPERATURE (°C)
EXT V
REF
, INT R
SET
EXT V
REF
, EXT R
SET
INT V
REF
, INT R
SET
INT V
REF
, EXT R
SET
V
LOOP
= 24V 4mA TO 20mA RANGE R
LOAD
= 250Ω
09128-010

TYPICAL PERFORMANCE CHARACTERISTICS

Figure 6. Integral Nonlinearity Error vs. Code
Figure 7. Differential Nonlinearity Error vs. Code
Figure 9. Offset Error vs. Temperature
Figure 10. Gain Error vs. Temperature
Figure 8. Total Unadjusted Error vs. Code
Figure 11. Integral Nonlinearity Error vs. Temperature
Rev. E | Page 13 of 36
Page 14
AD5421 Data Sheet
0.5
0.4
0.3
0.2
0.1
0
MAX DNL
V
= 24V
LOOP
4mA TO 20mA RANGE R
= 250
LOAD
–0.1
DNL ERRO R (LSB)
–0.2
MIN DNL
–0.3
–0.4
–0.5
–40 856035
10–15
TEMPERATURE (° C)
Figure 12. Differential Nonlinearity Error vs. Temperature
09128-011
0.0006
MAX INL
0.0004
0.0002
0
MIN INL
–0.0002
INL ERROR (% FSR)
R
= 250
LOAD
T
= 25°C
–0.0004
–0.0006
A
3.8mA TO 21mA RANGE EXT V
REF
EXT R
SET
0605040
302010
LOOP SUPPLY VOLTAGE (V)
Figure 15. Integral Nonlinearity Error vs. Loop Supply Voltage
09128-014
0.04
0.03
0.02
0.01
0
–0.01
V
= 24V
LOOP
–0.02
4mA TO 20mA RANGE R
= 250
LOAD
–0.03
EXT NMOS
EXT V
, INT R
REF
TOTAL UNADJUSTED ERROR (% FSR)
–0.04
–0.05
–0.06
EXT V INT V INT V
–40 85603510–15
REF
REF
REF
, EXT R , INT R , EXT R
SET
SET
SET
SET
TEMPERATURE (° C)
Figure 13. Total Unadjusted Error vs. Temperature
0.04
0.03
0.02
0.01
0
–0.01
V
= 24V
LOOP
–0.02
4mA TO 20mA RANGE R
= 250
LOAD
–0.03
EXT NMOS
EXT V
, INT R
REF
FULL-SCAL E ERROR (% FSR)
–0.04
–0.05
–0.06
EXT V INT V INT V
–40 85603510–15
REF
REF
REF
, EXT R , INT R , EXT R
SET
SET
SET
SET
TEMPERATURE (° C)
Figure 14. Full-Scale Error vs. Temperature
0.0029
0.0027
0.0025
0.0023
0.0021
0.0019 R
= 250
LOAD
T
= 25°C
A
3.8mA TO 21mA RANGE
0.0017
TOTAL UNADJUSTED ERROR (% FSR)
09128-012
0.0015
EXT V
REF
EXT R
SET
0605040302010
LOOP SUPPLY VOLTAGE (V)
09128-015
Figure 16. Total Unadjusted Error vs. Loop Supply Voltage
0.0024 R
= 250
LOAD
T
= 25°C
A
3.8mA TO 21mA RANGE
0.0022 EXT V
REF
EXT R
0.0020
SET
0.0018
0.0016
0.0014
OFFS ET ERRO R (% FSR)
0.0012
0.0010
0605040302010
09128-013
LOOP SUPPLY VOLTAGE (V)
09128-016
Figure 17. Offset Error vs. Loop Supply Voltage
Rev. E | Page 14 of 36
Page 15
Data Sheet AD5421
0.0015
–0.0025
–0.0020
–0.0015
–0.0010
–0.0005
0
0.0005
0.0010
0 605040302010
GAIN ERROR ( % FSR)
LOOP SUPPLY VOLTAGE (V)
R
LOAD
= 250Ω
T
A
= 25°C
3.8mA TO 21mA RANG E EXT V
REF
EXT R
SET
09128-017
0.0030
0.0025
0.0020
0.0015
0.0010
0.0005
0
–0.0005
0 605040302010
FULL-S CALE ERROR (% FS R)
LOOP SUPPLY VOLTAGE (V)
R
LOAD
= 250Ω
T
A
= 25°C
3.8mA TO 21mA RANG E EXT V
REF
EXT R
SET
09128-018
2000
0
250
500
750
1000
1250
1500
1750
0 5040302010
LOAD RESI STANCE (Ω)
LOOP SUPPLY VOLTAGE (V)
T
A
= 25°C
EXT V
REF
I
LOOP
= 24mA
EXT R
SET
OPERATI NG AREA
09128-019
4.70
4.35
4.40
4.45
4.50
4.55
4.60
4.65
–40 100806040200–20
COMPLI ANCE V OLTAGE HE ADROOM (V)
TEMPERATURE (°C)
R
LOAD
= 250Ω
3.2mA TO 24mA RANG E EXT V
REF
I
LOOP
= 24mA
09128-020
7
6
5
4
3
2
1
0
0 5.04.54.03.53.02.52.01.51.00.5
LOOP CURRE NT ERROR (µA)
REG
OUT
LOAD CURRENT ( mA)
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
T
A
= 25°C
I
LOOP
= 20mA
09128-021
8
–8
–6
–4
–2
0
2
4
6
0 10987654321
VOLTAGE ACROSS 250Ω LOAD RESISTOR (µV)
TIME (Seconds)
V
LOOP
= 24V EXT NMOS EXT V
REF
I
LOOP
= 4mA
R
LOAD
= 250Ω
T
A
= 25°C
09128-022
Figure 18. Gain Error vs. Loop Supply Voltage
Figure 19. Full-Scale Error vs. Loop Supply Voltage
Figure 21. Compliance Voltage Headroom vs. Temperature
Figure 22. Loop Current Error vs. REG
Load Current
OUT
Figure 20. Load Resistance Load Line vs. Loop Supply Voltage
(Voltage Between LOOP− and REG
)
IN
Figure 23. Loop Current Noise, 0.1 Hz to 10 Hz Bandwidth
Rev. E | Page 15 of 36
Page 16
AD5421 Data Sheet
1.0
–1.0
–0.8
–0.6
–0.4
–0.2
0
0.2
0.6
0.8
0.4
0 1.00.90.8
0.70.60.50.40.30.20.1
VOLTAGE ACROSS 500Ω LOAD RESISTOR (mV)
TIME (Seconds)
V
LOOP
= 24V EXT NMOS INT V
REF
I
LOOP
= 4mA
R
LOAD
= 500Ω
T
A
= 25°C
1.33mV p-p
0.2mV rms
09128-023
6
5
4
3
2
1
0
–40 –30 –20 –10 0 10 20 30 40
VOLTAGE ACROSS 250Ω LOAD RESISTOR (V)
TIME (µs)
FALLING
RISING
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
T
A
= 25°C
C
IN
= OPEN CIRCUI T
09128-025
6
5
4
3
2
1
0
–1.0 –0.5 0 0.5 1.0 1.5 2.0 2.5 3.0
VOLTAGE ACROSS 250Ω LOAD RESISTOR (V)
TIME (ms)
FALLING
RISING
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
T
A
= 25°C
C
IN
= 22nF
09128-026
0.244
0.226
0.228
0.230
0.232
0.234
0.236
0.238
0.240
0.242
0 0.5 1.0 1.5 2.0
IODV
DD
CURRENT (µA)
DIGITAL LOGIC VOLTAGE (V)
DECREASING
INCREASING
IODVDD = 1.8V T
A
= 25°C
09128-027
0.60
0.55
0.50
0.45
0.40 0 3.53.02.52.01.51.00.5
IODV
DD
CURRENT (µA)
DIGITAL LOGIC VOLTAGE (V)
IODVDD = 3.3V T
A
= 25°C
DECREASING
INCREASING
09128-028
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6 0 654321
IODV
DD
CURRENT (µA)
DIGITAL LOGIC VOLTAGE (V)
IODV
DD
= 5V
T
A
= 25°C
DECREASING
INCREASING
09128-029
Figure 24. Loop Current Noise, 500 Hz to 10 kHz Bandwidth
(HART Bandwidth)
Figure 25. Full-Scale Loop Current Step
Figure 27. IODVDD Current vs. Digital Logic Voltage, Increasing and
Decreasing, IODV
= 1.8 V
DD
Figure 28. IODVDD Current vs. Digital Logic Voltage, Increasing and
Decreasing, IODV
= 3.3 V
DD
Figure 26. Full-Scale Loop Current Step, CIN = 22 nF
Figure 29. IODVDD Current vs. Digital Logic Voltage, Increasing and
Decreasing, IODV
DD
= 5 V
Rev. E | Page 16 of 36
Page 17
Data Sheet AD5421
1.85
1.84
1.83
1.82
1.81
1.80
1.79
1.78
1.77
1.76
0
–50
–45
–40
–35
–30
–25
–20
–15
–10
–5
0 12
0 0.250.200.150.100.05
108642
REG
OUT
VOLTAGE (V)
REG
OUT
VOLTAGE CHANGE (mV)
REG
OUT
LOAD CURRENT ( mA)
REG
OUT
LOAD CURRENT ( mA)
V
LOOP
= 24V EXT NMOS T
A
= 25°C
09128-030
263.5
258.5
259.0
259.5
260.0
260.5
261.0
261.5
262.0
262.5
263.0
0 605040302010
QUIESCENT CURRENT (µA)
LOOP SUPPLY VOLTAGE (V)
TA = 25°C
09128-031
266
257
258
259
260
261
262
263
264
265
–40 100806040200–20
QUIESCENT CURRENT (µA)
TEMPERATURE (°C)
V
LOOP
= 24V EXT NMOS V
IH
= IODV
DD
V
IL
= COM
T
A
= 25°C
09128-032
3.5
0
0
–50
–100
–150
–200
–250
0.5
1.0
1.5
2.0
2.5
3.0
0 1 2 3 4 5
DV
DD
OUTPUT VOLTAGE (V)
DV
DD
OUTPUT VOLTAGE CHANGE (mV)
DVDD LOAD CURRENT (mA)
09128-101
V
LOOP
= 24V EXT NMOS T
A
= 25°C
4
–4
–3
–2
–1
0
1
2
3
0 10854 976321
REFOUT1 VOLTAGE NOISE (µV)
TIME (Seconds)
V
LOOP
= 24V EXT NMOS T
A
= 25°C
09128-034
3.0
2.5
2.0
1.5
1.0
0.5
0
1
–5
–4
–3
–2
–1
0
0 7654321
REFOUT1 VOLTAGE (V)
REFOUT1 V OLTAGE CHANGE (mV)
REFOUT1 LOAD CURRENT (mA)
V
LOOP
= 24V EXT NMOS TA = 25°C
09128-035
Figure 30. REG
Voltage vs. Load Current
OUT
Figure 31. Quiescent Current vs. Loop Supply Voltage
Figure 33. DV
Output Voltage vs. Load Current
DD
Figure 34. REFOUT1 Voltage Noise, 0.1 Hz to 10 Hz Bandwidth
Figure 32. Quiescent Current vs. Temperature
Figure 35. REFOUT1 Voltage vs. Load Current
Rev. E | Page 17 of 36
Page 18
AD5421 Data Sheet
2.5012
2.4994
2.4996
2.4998
2.5000
2.5002
2.5004
2.5006
2.5008
2.5010
–40 100806040200–20
REFOUT1 VOLTAGE (V)
TEMPERATURE (°C)
60 DEVICES SHOWN
09128-036
30
0
5
10
15
20
25
0
0.25
0.50
0.75
1.00
1.25
1.50
1.75
2.00
2.25
2.50
2.75
3.00
3.25
3.50
3.75
4.00
4.25
4.50
4.75
5.00
POPULATION (%)
TEMPERATURE COEFFICIENT (ppm/°C)
MEAN TC = 1.5p pm/°C
09128-037
250
0
50
100
150
200
–40 100806040200–20
ADC CODE (Decimal )
DIE TEMPERATURE (°C)
V
LOOP
= 24V EXT NMOS R
LOAD
= 250Ω
I
LOOP
= 3.2mA
09128-038
250
200
150
100
50
0
0 2.52.01.51.00.5
ADC CODE (Decimal )
V
LOOP
PIN INPUT VOLTAGE (V)
V
LOOP
= 24V EXT NMOS T
A
= 25°C
09128-039
Figure 36. REFOUT1 Voltage vs. Temperature, 60 Devices Shown
(C Grade Device)
Figure 37. REFOUT1 Temperature Coefficient Histogram
(C Grade Device)
Figure 38. On-Chip ADC Code vs. Die Temperature
Figure 39. On-Chip ADC Code vs. V
Pin Input Voltage
LOOP
Rev. E | Page 18 of 36
Page 19
Data Sheet AD5421

TERMINOLOGY

Total Unadjusted Error
Tota l u nadjusted error (TUE) is a measure of the total output error. TUE consists of INL error, offset error, gain error, and output drift over temperature, in the case of maximum TUE. TUE is expressed in % FSR.
Relative Accuracy or Integral Nonlinearity (INL) Error
Relative accuracy, or integral nonlinearity (INL) error, is a measure of the maximum deviation in the output current from a straight line passing through the endpoints of the transfer function. INL error is expressed in % FSR.
Differential Nonlinearity (DNL) Error
Differential nonlinearity (DNL) error is the difference between the measured change and the ideal 1 LSB change between any two adjacent codes. A specified differential nonlinearity of ±1 LSB maximum ensures monotonicity.
Offset Error
Offset error is a measure of the output error when zero code is loaded to the DAC register and is expressed in % FSR.
Offset Error Temperature Coefficient (TC)
Offset error TC is a measure of the change in offset error with changes in temperature and is expressed in ppm FSR/°C.
Gain Error
Gain error is a measure of the span error of the DAC. It is the deviation in slope of the DAC transfer function from the ideal and is expressed in % FSR.
Gain Error Temperature Coefficient (TC)
Gain error TC is a measure of the change in gain error with changes in temperature and is expressed in ppm FSR/°C.
Full-Scale Error
Full-scale error is a measure of the output error when full-scale code is loaded to the DAC register and is expressed in % FSR.
Full-Scale Error Temperature Coefficient (TC)
Full-scale error TC is a measure of the change in full-scale error with changes in temperature and is expressed in ppm FSR/°C.
Loop Compliance Voltage Headroom
Loop compliance voltage headroom is the minimum voltage between the LOOP− and REG
pins for which the output
IN
current is equal to the programmed value.
Output Temperature Coefficient (TC)
Output TC is a measure of the change in the output current at 12 mA with changes in temperature and is expressed in ppm FSR/°C.
Voltage Reference Thermal Hysteresis
Voltage reference thermal hysteresis is the difference in output voltage measured at +25°C compared to the output voltage measured at +25°C after cycling the temperature from +25°C to
−40°C to +105°C and back to +25°C. The hysteresis is specified for the first and second temperature cycles and is expressed in mV.
Voltage Reference Temperature Coefficient (TC)
Voltage reference TC is a measure of the change in the reference output voltage with a change in temperature. The voltage refer­ence TC is calculated using the box method, which defines the TC as the maximum change in the reference output voltage over a given temperature range. Voltage reference TC is expressed in ppm/°C as follows:
REF_MAX
=
TC
REF_NOM
VV
REF_MIN
×
Temp_RangeV
6
10×
 
where:
V
is the maximum reference output voltage measured
REF_MAX
over the total temperature range.
V
is the minimum reference output voltage measured
REF_MIN
over the total temperature range.
V
is the nominal reference output voltage, 2.5 V.
REF_NOM
Temp_Range is the specified temperature range
(−40°C to +105°C).
Rev. E | Page 19 of 36
Page 20
SDIN
SYNC
SCLK
UPDATE ON SY NC HIGH
MSB
D23
LSB
D0
24-BIT DATA
24-BIT DATA TRANSFER—NO ERROR CHECKING
SDIN
FAULT
SYNC
SCLK
UPDATE AFT E R S Y NC HIGH
ONLY IF ERROR CHECK PASSE D
FAULT PIN GOES HIGH
IF ERROR CHE CK FAILS
MSB
D31
LSB
D8 D7 D0
24-BIT DATA 8-BIT FCS
32-BIT DATA TRANSFER W ITH ERROR CHECKI NG
09128-049
AD5421 Data Sheet

THEORY OF OPERATION

The AD5421 is an integrated device designed for use in loop­powered, 4 mA to 20 mA smart transmitter applications. In a single chip, the AD5421 provides a 16-bit DAC and current amplifier for digital control of the loop current, a voltage regulator to power the entire transmitter, a voltage reference, fault alert functions, a flexible SPI-compatible serial interface, gain and offset adjust registers, as well as other features and functions. The features of the AD5421 are described in the following sections.

FAULT ALERTS

The AD5421 provides a number of fault alert features. All faults are signaled to the controller via the FAULT pin and the fault register. In the case of a loss of communication between the AD5421 and the microcontroller (SPI fault), the AD5421 programs the loop current to an alarm value. If the controller detects that the FAULT pin is set high, it should then read the fault register to determine the cause of the fault.

SPI Fault

The SPI fault is asserted if there is no valid communication to any register of the AD5421 for more than a user-defined period. The user can program the time period using the SPI watchdog timeout bits of the control register. The SPI fault bit of the fault register indicates the fault on the SPI bus. Because this fault is caused by a loss of communication between the controller and the AD5421, the loop current is also forced to the alarm value.
The direction of the alarm current (downscale or upscale) is selected via the ALARM_CURRENT_DIRECTION pin. Connecting this pin to DV (22.8 mA/24 mA); connecting this pin to COM selects a downscale alarm current (3.2 mA).

Packet Error Checking

To verify that data has been received correctly in noisy environ­ments, the AD5421 offers the option of error checking based on an 8-bit cyclic redundancy check (CRC). Packet error checking (PEC) is enabled by writing to the AD5421 with a 32-bit serial frame, where the least significant eight bits are the frame check sequence (FCS). The device controlling the AD5421 should generate the 8-bit FCS using the following polynomial:
The 8-bit FCS is appended to the end of the data-word, and 32 data bits are sent to the AD5421 before If the check is valid, the data is accepted. If the check fails, the FAULT pin is asserted and the PEC bit of the fault register is set.
C(x) = x
8
+ x 2 + x + 1
After the fault register is read, the PEC bit is reset low and the FAULT pin returns low.
selects an upscale alarm current
DD
SYNC
is taken high.
Rev. E | Page 20 of 36
In the case of data readback, if the AD5421 is addressed with a 32-bit frame, it generates the 8-bit frame check sequence and appends it to the end of the 24-bit data stream to create a 32-bit data stream.
Figure 40. PEC Timing

Current Loop Fault

The current loop (I
) fault is asserted when the actual loop
LOOP
current is not within ±0.01% FSR of the programmed loop current. If the measured loop current is less than the programmed loop current, the I
Under bit of the fault register is set. If the
LOOP
measured loop current is greater than the programmed loop current, the I
Over bit of the fault register is set. The FAULT
LOOP
pin is set to logic high in either case.
An I
Over condition occurs when the value of the load current
LOOP
sourced from the AD5421 (via REG or DV to flow in the loop. An I
) is greater than the loop current that is programmed
DD
under condition occurs when there
LOOP
, REFOUT1, REFOUT2,
OUT
is insufficient compliance voltage to support the programmed loop current, caused by excessive load resistance or low loop supply voltage.

Overtemperature Fault

There are two overtemperature alert bits in the fault register: Temp 100°C and Temp 140°C. If the die temperature of the AD5421 exceeds either 100°C or 140°C, the appropriate bit is set. If the Temp 140°C bit is set in the fault register, the FAULT pin is set to logic high.
Page 21
Data Sheet AD5421
19MΩ
1MΩ
R
L
LOOP–
V
LOOP
COM
REG
IN
V
LOOP
AD5421
09128-048
R
L
LOOP–
DRIVE
COM
REG
IN
V
LOOP
AD5421
09128-050
R
L
200kΩ
LOOP–
DRIVE
COM
REG
IN
V
LOOP
AD5421
T1
DN2540
BSP129
09128-051

Loop Voltage Fault

There are two loop voltage alert bits in the fault register: V
12V and V
LOOP
6V. If the voltage between the V
LOOP
LOOP
and COM pins falls below 0.6 V (corresponding to a 12 V loop supply value), the V
12V bit is set; this bit is cleared when
LOOP
the voltage returns above 0.7 V. Similarly, if the voltage between the V 6 V loop supply value), the V when the voltage returns above 0.4 V. If the V
and COM pins falls below 0.3 V (corresponding to a
LOOP
6V bit is set; this bit is cleared
LOOP
6V bit is set in
LOOP
the fault register, the FAULT pin is set to logic high.
Figure 41 illustrates how a resistor divider enables the monitor­ing of the loop supply with the V
input. The recommended
LOOP
resistor divider consists of a 1 MΩ and a 19 MΩ resistor that provide a 20:1 ratio, allowing the 2.5 V input range of the V
LOOP
pin to monitor loop supplies up to 50 V. With a 20:1 divider ratio, the preset V
6V and V
LOOP
12V alert bits of the fault register
LOOP
generate loop supply faults according to their stated values. If another divider ratio is used, the fault bits generate faults at values that are not equal to 6 V and 12 V.

LOOP CURRENT RANGE SELECTION

To select the loop current range, connect the RANGE0 and RANGE1 pins to the COM and DV
pins, as shown
DD
in Table 9.
Table 9. Selecting the Loop Current Range
RANGE1 Pin RANGE0 Pin Loop Current Range
COM COM 4 mA to 20 mA COM DVDD 3.8 mA to 21 mA DVDD COM 3.2 mA to 24 mA DVDD DVDD 3.8 mA to 21 mA

CONNECTION TO LOOP POWER SUPPLY

The AD5421 is powered from the 4 mA to 20 mA current loop. Typically, the power supply is located far from the transmitter device and has a value of 24 V. The AD5421 can be connected directly to the loop power supply and can tolerate a voltage up to a maximum of 52 V (see Figure 42).
Figure 41. Resistor Divider Connection at V
LOOP

EXTERNAL CURRENT SETTING RESISTOR

The 24 kΩ resistor R output voltage to a current, which is then mirrored with a gain of 221 to the LOOP− pin. The stability of the loop current over temperature is dependent on the temperature coefficient of R
Tabl e 1 and Table 2 outline the performance specifications of the AD5421 with both the internal R 24 kΩ R
resistor. Using the internal R
SET
justed error of better than 0.126% FSR can be expected. Using an external resistor gives improved performance of 0.048% FSR. This specification assumes an ideal resistor; the actual performance depends on the absolute value and temperature coefficient of the resistor used. For more information, see the Determining the Expected Total Error section.
, shown in Figure 1, converts the DAC
SET
resistor and an external,
SET
resistor, a total unad-
SET
Pin
SET
Rev. E | Page 21 of 36
Figure 42. Direct Connection of the AD5421 to Loop Power Supply
Figure 42 shows how the AD5421 is connected directly to the loop power supply. An alternative power connection is shown in Figure 43, which shows a depletion mode N-channel MOSFET connected between the AD5421 and the loop power supply. The use of this device keeps the voltage drop across the AD5421 at approximately 12 V, limiting the worst-case on-chip power dissi­pation to 288 mW (12 V × 24 mA = 288 mW). If the AD5421 is
.
connected directly to the loop supply as shown in Figure 42, the potential worst-case on-chip power dissipation for a 24 V loop power supply is 576 mW (24 V × 24 mA = 576 mW). The power dissipation changes in proportion to the loop power supply voltage.
Figure 43. MOSFET Connecting the AD5421 to Loop Power Supply
Page 22
AD5421 Data Sheet
COM
DVDD
DVDD
3.3
LOOP–
R
DAC
V-TO-I
CIRCUITRY
C
IN
C
SLEW
09128-052
DAC
SLEW
R
t
C
×=5
nF68
220,155
ms5
×
=
SLEW
C
nF133
220,155
ms10
×
=
SLEW
C
6
5
4
3
2
1
0
–2 2218141062
VOLTAGE ACROSS 250Ω LOAD RESISTOR (V)
TIME (ms)
C
SLEW
= 267nF
C
SLEW
= 133nF
C
SLEW
= 68nF
09128-053

ON-CHIP ADC

The AD5421 contains an on-chip ADC used to measure and feed back to the fault register either the temperature of the die or the voltage between the V
and COM pins. The select ADC
LOOP
input bit (Bit D8) of the control register selects the parameter to be converted. A conversion is initiated with command byte 00001000 (necessary only if auto fault readback is disabled). This command byte powers on the ADC and performs the conversion. A read of the fault register returns the conversion result. If auto readback of the fault register is required, the ADC must first be powered up by setting the on-chip ADC bit (Bit D7) of the control register.
Because the FAULT pin can go high for as long as 30 μs, care is required when performing a die temperature measurement after a readback of the V
LOOP
voltage.

VOLTAGE REGULATOR

The on-chip voltage regulator provides a regulated voltage out­put to supply the AD5421 and the remainder of the transmitter circuitry. The output voltage range is from 1.8 V to 12 V and is selected by the states of three digital input pins (see Table 10). The regulator output is accessed at the REG
Table 10. Setting the Voltage Regulator Output
REG_SEL2 REG_SEL1 REG_SEL0
COM COM COM 1.8 COM COM DVDD 2.5 COM DVDD COM 3.0
pin.
OUT
Regulated Output Voltage (V)
The resistance of the DAC is typically 15.22 kΩ for the 4 mA to 20 mA and 3.8 mA to 21 mA loop current ranges. The DAC resistance changes to 16.11 kΩ when the 3.2 mA to 24 mA loop current range is selected.
The time constant of the circuit is expressed as
τ = R
DAC
× C
SLEW
Taking five time constants as the required time to reach the final value, C
can be determined for a desired response time, t,
SLEW
as follows:
where: t is the desired time for the output current to reach its final value.
R
is the resistance of the DAC core, either 15.22 kΩ or
DAC
16.11 kΩ, depending on the selected loop current range.
For a response time of 5 ms,
For a response time of 10 ms,
The responses for both of these configurations are shown in Figure 45.
DVDD COM COM 5.0 DVDD COM DVDD 9.0 DVDD DVDD COM 12.0

LOOP CURRENT SLEW RATE CONTROL

The rate of change of the loop current can be controlled by connecting an external capacitor between the C COM. This reduces the rate of change of the loop current. The output resistance of the DAC (R C
capacitor generate a time constant that determines the
SLEW
response of the loop current (see Figure 44).
Figure 44. Slew Capacitor Circuit
) together with the
DAC
pin and
IN
Figure 45. 4 mA to 20 mA Step with Slew Rate Control
The CIN pin can also be used as a coupling input for HART FSK signaling. The HART signal must be ac-coupled to the C
IN
input. The capacitor through which the HART signal is coupled must be considered in the preceding calculations, where the total capacitance is C
SLEW
+ C
. For more information, see
HART
the HART Communications section.
Rev. E | Page 22 of 36
Page 23
Data Sheet AD5421
09128-054
R
L
200kΩ
LOOP–
DRIVE
COMC
IN
REG
IN
V
LOOP
AD5421
HART_OUT
HART_IN
HART
MODEM
C
HART
C
SLEW
HART
SLEW
HART
C
CC +
=5.4
( )
SLEW
HART
DAC
dB
CCR
f
+××π×=2
1
3
12
10
8
6
4
2
0
150
–150
–100
–50
0
50
100
–50 –30 –10 10 30 50
VOLTAGE ACROSS 500Ω LOAD RESISTOR (V)
OUTPUT OF HART DIGITAL FILTER (mV)
HCF_TOOL-31
TIME (ms)
09128-060

POWER-ON DEFAULT

The AD5421 powers on with all registers loaded with their default values and with the loop current in the alarm state set to 3.2 mA or 22.8 mA/24 mA (depending on the state of the ALARM_ CURRENT_DIRECTION pin and the selected range). The AD5421 remains in this state until it is programmed with new values. The SPI watchdog timer is enabled by default with a timeout period of 1 sec. If there is no communication with the AD5421 within 1 sec of power-on, the FAULT pin is set.

HART COMMUNICATIONS

The AD5421 can be interfaced to a Highway Addressable Remote Transducer (HART) modem to enable HART digital communications over the 2-wire loop connection. Figure 46 shows how the modem frequency shift keying (FSK) output is connected to the AD5421.
Figure 46. Connecting a HART Modem to the AD5421
To achieve a 1 mA p-p FSK current signal on the loop, the voltage at the C
pin must be 111 mV p-p. Assuming a 500 mV p-p
IN
output from the HART modem, this means that the signal must be attenuated by a factor of 4.5. The following equation can be used to calculate the values of the C
HART
and C
capacitors.
SLEW
To achieve a 500 Hz high-pass 3 dB frequency cutoff, the com­bined values of C
HART
and C
should be 21 nF. To ensure the
SLEW
correct HART signal amplitude on the current loop, the final values for the capacitors are C
= 4.7 nF and C
HART
= 16.3 nF.
SLEW

Output Noise During Silence and Analog Rate of Change

The AD5421 has a direct influence on two important specifi­cations relating to the HART communications protocol: output noise during silence and analog rate of change. Figure 24 shows the measurement of the AD5421 output noise in the HART extended bandwidth; the noise measurement is 0.2 mV rms, within the required 2.2 mV rms value.
To meet the analog rate of change specification, the rate of change of the 4 mA to 20 mA current must be slow enough so that it does not interfere with the HART digital signaling. This is determined by forcing a full-scale loop current change through a 500 Ω load resistor and applying the resulting voltage signal to the HART digital filter (HCF_TOOL-31). The peak amplitude of the signal at the filter output must be less than 150 mV. To achieve this, the rate of change of the loop current must be restricted to less than approximately 1.3 mA/ms.
The output of the AD5421 naturally slews at approximately 880 mA/ms, a rate that is far too great to comply with the HART specifications. To reduce the slew rate, a capacitor can be connected from the C
pin to COM, as described in the Loop
IN
Current Slew Rate Control section. To reduce the slew rate enough so that the HART specification is met, a capacitor value in the region of 4.7 µF is required, resulting in a full-scale transition time of 500 ms. Many applications regard this time as too slow, in which case the slew rate needs to be digitally controlled by writing a sequence of codes to the DAC register so that the output response follows the desired curve.
Figure 47 shows a digitally controlled full-scale step and the resulting filter output. In Figure 47, it can be seen that the peak amplitude of the filter output signal is less than the required 150 mV, and the transition time is approximately 30 ms.
From this equation, the ratio of C ratio of the capacitor values sets the amplitude of the HART FSK signal on the loop. The absolute values of the capacitors set the response time of the loop current, as well as the bandwidth presented to the HART signal connected at the C bandwidth must pass frequencies from 500 Hz to 10 kHz. The two capacitors and the internal impedance, R pass filter. The 3 dB frequency of this high-pass filter should be less than 500 Hz and can be calculated as follows:
HART
to C
is 1 to 3.5. This
SLEW
IN
, form a high-
DAC
pin. The
Figure 47. Digitally Controlled Full-Scale Step and Resulting HART Digital
Filter Output Signal
Rev. E | Page 23 of 36
Page 24
AD5421 Data Sheet
09128-061
R
L
LOOP–
COM
C
IN
REG
IN
V
LOOP
AD5421
FROM HART MODEM
47nF
168nF
Figure 48 shows the circuit diagram for this measurement. The 47 nF and 168 nF capacitor values for C adequate filtering of the digital steps, ensuring that they do not cause interference.
HART
and C
SLEW
provide
Figure 48. Circuit Diagram for Figure 47
Rev. E | Page 24 of 36
Page 25
Data Sheet AD5421
00000101
Load DAC
Address/command byte
Data-word

SERIAL INTERFACE

The AD5421 is controlled by a versatile, 3-wire serial interface that operates at clock rates up to 30 MHz. It is compatible with the SPI, QSPI™, MICROWIRE®, and DSP standards. Figure 2 shows the timing diagram. The interface operates with either a continuous or noncontinuous gated burst clock.
The write sequence begins with a falling edge of the
SYNC signal; data is clocked in on the SDIN data line on the falling edge of SCLK. On the rising edge of
SYNC
, the 24 bits of data are latched; the data is transferred to the addressed register and the programmed function is executed (either a change in DAC output or mode of operation).
If packet error checking on the SPI interface is required using cyclic redundancy codes, an additional eight bits must be written to the AD5421, creating a 32-bit serial interface. In this case, 32 bits are written to the AD5421 before
SYNC
is brought high.

INPUT SHIFT REGISTER

The input shift register is 24 bits wide (32 bits wide if CRC error checking of the data is required). Data is loaded into the device MSB first as a 24-/32-bit word under the control of a serial clock input, SCLK. The input shift register consists of an 8-bit address/ command byte, a 16-bit data-word, and an optional 8-bit CRC, as shown in Table 12 and Table 13.
The address/command byte decoding is described in Table 11.
Table 11. Address/Command Byte Functions
Address/Command Byte Function
00000001 Write to DAC register 00000010 Write to control register
Address/Command Byte Function
00000011 Write to offset adjust register 00000100 Write to gain adjust register
00000110 Force alarm current 00000111 Reset (it is recommended to wait
50 µs after a device reset before writing the next command)
00001000 Initiate V
measurement 00001001 No operation 10000001 Read DAC register 10000010 Read control register 10000011 Read offset adjust register 10000100 Read gain adjust register 10000101 Read fault register
/temperature
LOOP
The 16 bits of the data-word written following a load DAC, force alarm current, reset, initiate V
/temperature measurement,
LOOP
or no operation command byte are don’t cares (see Table 12 and Tabl e 13).

REGISTER READBACK

To re a d back a register, Bit D11 of the control register must be set to Logic 1 to disable the automatic readback of the fault register. The 16 bits of the data-word written following a read command are don’t cares (see Table 12 and Table 13).
The register data addressed by the read command is clocked out of SDO on the subsequent write command (see Figure 3).
Table 12. Input Shift Register
MSB LSB
D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Table 13. Input Shift Register with CRC
MSB LSB
D31
D30 D29 D28 D27 D26 D25 D24 D23 D22 D21 D20 D19 D18 D17 D16 D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
Address/command byte Data-word CRC
Rev. E | Page 25 of 36
Page 26
AD5421 Data Sheet
mA4
2
mA16
16
D I
LOOP
+×
 
 
=
mA8.3
2
mA2.17
16
D I
LOOP
+×
 
 
=

DAC REGISTER

The DAC register is a read/write register and is addressed as described in Table 11. The data programmed to the DAC register determines the loop current, as shown in the Ideal Output Transfer Function section and in Tab l e 15.

Ideal Output Transfer Function

The transfer function describing the relationship between the data programmed to the DAC register and the loop current is expressed by the following three equations.
For the 4 mA to 20 mA output range, the loop current can be expressed as follows:
Table 14. DAC Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
16-bit data
For the 3.8 mA to 21 mA output range, the loop current can be expressed as follows:
For the 3.2 mA to 24 mA output range, the loop current can be expressed as follows:
mA8.20
LOOP
=
 
D I
+×
16
2
mA2.3
where D is the decimal value of the DAC register.
Table 15. Relationship of DAC Register Code to Ideal Loop Current (Gain = 65,536; Offset = 0)
Ideal Loop Current (mA)
DAC Register Code
0x0000 4 3.8 3.2 0x0001 4.00024 3.80026 3.2003 … … 0x7FFF 11.9997 12.39974 13.5997 0x8000 12 12.4 13.6 … … 0xFFFE 19.9995 20.99947 23.9994 0xFFFF 19.9997 20.99974 23.9997
4 mA to 20 mA Range 3.8 mA to 21 mA Range 3.2 mA to 24 mA Range
Rev. E | Page 26 of 36
Page 27
Data Sheet AD5421
0 1 1
1 sec (default)

CONTROL REGISTER

The control register is a read/write register and is addressed as described in Tab l e 11. The data programmed to the control register determines the mode of operation of the AD5421.
Table 16. Control Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
SPI watchdog timeout
T0 T1 T2
SPI watchdog timer
Auto fault readback
Alarm on SPI fault
Table 17. Control Register Bit Descriptions
Control Bits Description
SPI watchdog timeout
The T0, T1, and T2 bits allow the user to program the watchdog timeout period. The watchdog timer is reset when a valid write to any AD5421 register occurs or when a NOP command is written.
T0 T1 T2 Timeout Period
0 0 0 50 ms 0 0 1 100 ms 0 1 0 500 ms
1 0 0 2 sec 1 0 1 3 sec 1 1 0 4 sec 1 1 1 5 sec
SPI watchdog timer
Auto fault readback
0 = SPI watchdog timer is enabled (default). 1 = SPI watchdog timer is disabled.
This bit specifies whether the fault register contents are automatically clocked out on the SDO pin on each write operation. (The fault register can always be addressed for readback.) 0 = fault register contents are clocked out on the SDO pin (default). 1 = fault register contents are not clocked out on the SDO pin.
Alarm on SPI fault
This bit specifies whether the loop current is forced to the alarm value when an SPI fault is detected (that is, the watchdog timer times out). When an SPI fault is detected, the SPI fault bit of the fault register and the FAULT pin are always set. 0 = loop current is forced to the alarm value when an SPI fault is detected (default). 1 = loop current is not forced to the alarm value when an SPI fault is detected.
Set min loop current
0 = normal operation (default). 1 = loop current is set to its minimum value so that the total current flowing in the loop consists only of the operating current of the AD5421 and its associated circuitry.
Select ADC input
0 = on-chip ADC measures the voltage between the V 1 = on-chip ADC measures the temperature of the AD5421 die.
On-chip ADC 0 = on-chip ADC is disabled (default).
1 = on-chip ADC is enabled.
Power down internal
0 = internal voltage reference is powered up (default). 1 = internal voltage reference is powered down and an external voltage reference source is required.
reference V
LOOP
alert
fault
This bit specifies whether the FAULT pin is set when the voltage between the V (The V 0 = FAU LT pin is not set when the V 1 = FAULT pin is set when the V
6V bit of the fault register is always set.)
LOOP
LOOP
− COM voltage falls to approximately 0.3 V.
LOOP
Set min loop current
Select ADC input
On-chip ADC
and COM pins (default).
LOOP
Power down internal reference
LOOP
− COM voltage falls to approximately 0.3 V.
V
LOOP
fault alert
Reserved
and COM pins falls to approximately 0.3 V.
Rev. E | Page 27 of 36
Page 28
AD5421 Data Sheet
11111111
2.49
−55

FAULT REGISTER

The read-only fault register is addressed as described in Tabl e 11. The bits in the fault register indicate a range of possible fault conditions.
Table 18. Fault Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
I
I
SPI PEC
LOOP
Over
LOOP
Under
Temp 140°C
Table 19. Fault Register Bit Descriptions
FA U LT
Fault Alert
Pin Set Description
SPI Yes This bit is set high to indicate the loss of the SPI interface signaling. This fault occurs if there is no valid
communication to the AD5421 over the SPI interface for more than the user-defined timeout period. The occurrence of this fault also forces the loop current to the alarm value if Bit D10 of the control register is at Logic 0. The alarm current direction is determined by the state of the ALARM_CURRENT_DIRECTION pin.
PEC (packet error check)
I
Over Yes This bit is set high when the actual loop current is greater than the programmed loop current.
LOOP
I
Under Yes This bit is set high when the actual loop current is less than the programmed loop current.
LOOP
Yes This bit is set high when an error in the SPI communication is detected using cyclic redundancy check (CRC)
error detection. See the Packet Error Checking section for more information.
Temp 140°C Yes This bit is set high to indicate an overtemperature fault. This bit is set if the die temperature of the AD5421
exceeds approximately 140°C. This bit is cleared when the temperature returns below approximately 125°C.
Temp 100°C No This bit is set high to indicate an increasing temperature of the AD5421. This bit is set if the die temperature of the
AD5421 exceeds approximately 100°C. This bit is cleared when the temperature returns below approximately 85°C.
V
6V Yes This bit is set high when the voltage between the V
LOOP
a 6 V loop supply voltage with 20:1 resistor divider connected at V above approximately 0.4 V.
V
12V No This bit is set high when the voltage between the V
LOOP
a 12 V loop supply voltage with 20:1 resistor divider connected at V above approximately 0.7 V.
V
/temper-
LOOP
ature value
N/A These eight bits represent either the voltage between the V
depending on the setting of Bit D8 of the control register (see the On-Chip ADC Transfer Function Equations section).
8-Bit Value V
00000000 0 +300
Temp 100°C
V
V
LOOP
6V
− COM Voltage (V) Die Temperature (°C)
LOOP
LOOP
12V
and COM pins falls below approximately 0.3 V (representing
LOOP
LOOP
and COM pins falls below approximately 0.6 V (representing
LOOP
and COM pins or the AD5421 die temperature,
LOOP
/temperature value
V
LOOP
). This bit is cleared when the voltage returns
). This bit is cleared when the voltage returns
LOOP

On-Chip ADC Transfer Function Equations

The transfer function equation for the measurement of the voltage between the V
where D is the 8-bit digital code returned by the on-chip ADC.
V
COM = (2.5/256) × D
LOOP
and COM pins is as follows:
LOOP
The transfer function equation for the die temperature is as follows:
Die Temperature = 125 − (1.771 × (D − 128))
where D is the 8-bit digital code returned by the on-chip ADC.
Rev. E | Page 28 of 36
Page 29
Data Sheet AD5421
32769
+1
32768 (default)
0
65534
−1

OFFSET ADJUST REGISTER

The offset adjust register is a read/write register and is addressed as described in Table 11.
Table 20. Offset Adjust Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
16-bit offset adjust data
Table 21. Offset Adjust Register Adjustment Range
Offset Adjust Register Data Digital Offset Adjustment (LSBs)
65535 +32767 65534 +32766 …
32767 −1 … … 1 −32767 0 −32768

GAIN ADJUST REGISTER

The gain adjust register is a read/write register and is addressed as described in Table 11.
Table 22. Gain Adjust Register Bit Map
MSB LSB
D15 D14 D13 D12 D11 D10 D9 D8 D7 D6 D5 D4 D3 D2 D1 D0
16-bit gain adjust data
Table 23. Gain Adjust Register Adjustment Range
Gain Adjust Register Data Digital Gain Adjustment at Full-Scale Output (LSBs)
65535 (default) 0
… 32769 −32767 32768 −32768 32767 −32769
1 −65534 0 −65535
Rev. E | Page 29 of 36
Page 30
AD5421 Data Sheet
( )
 
 
 
  
×
 
 
++ 768,32
2
mA16
mA4
16
Offset
×
×
 
 
= D
Gain
I
LOOP
16
16
2
2
mA2.17
×
×
 
 
= D
Gain
I
LOOP
16
16
2
2
mA8.20
( )
 
 
 
  
×
 
 
++ 768,32
2
mA8.20
mA2.3
16
Offset

Transfer Function Equations with Offset and Gain Adjust Values

When the offset adjust and gain adjust register values are taken into account, the transfer equations can be expressed as follows.
For the 4 mA to 20 mA output range, the loop current can be expressed as follows:
mA16
 
16
2
= D
I
LOOP
 
2
×
Gain
 
16
  
×
 
For the 3.8 mA to 21 mA output range, the loop current can be expressed as follows:
For the 3.2 mA to 24 mA output range, the loop current can be expressed as follows:
where:
D is the decimal value of the DAC register. Gain is the decimal value of the gain adjust register. Offset is the decimal value of the offset adjust register.
Note that the offset adjust register cannot adjust the zero-scale output value downward.
 
++ 768,32
mA8.3
mA2.17
 
 
( )
×
16
2
Offset
Rev. E | Page 30 of 36
Page 31
Data Sheet AD5421

APPLICATIONS INFORMATION

Figure 49 shows a typical connection diagram for the AD5421 configured in a HART capable smart transmitter. Such a HART enabled smart transmitter was developed by Analog Devices as a reference demo circuit. This circuit, whose block diagram is shown in Figure 50, was verified and registered as an approved HART solution by the HART Communication Foundation.
To reduce power dissipation on the chip, a depletion mode MOSFET (T1), such as a DN2540 or BSP129, can be connected between the loop voltage and the AD5421, as shown in Figure 49. If a low loop voltage is used, T1 does not need to be inserted, and the loop voltage can connect directly to REG
(see Figure 42).
IN
In Figure 49, all interface signal lines are connected to the micro­controller. To reduce the number of interface signal lines, the LDAC
signal can be connected to COM, and the SDO and FAULT lines can be left unconnected. However, this configuration disables the use of the fault alert features.
Under normal operating conditions, the voltage between COM and LOOP− does not exceed 1.5 V, and the voltage at LOOP− is negative with respect to COM. If it is possible that the voltage at LOOP− may be forced positive with respect to COM, or if the voltage difference between LOOP− and COM may be forced in
excess of 5 V, a 4.7 V low leakage Zener diode should be placed between COM and the LOOP− pin, as shown in Figure 49, to protect the AD5421 from potential damage.

DETERMINING THE EXPECTED TOTAL ERROR

The AD5421 can be set up in a number of different configu­rations, each of which achieves different levels of accuracy, as described in Table 1 and Table 2. With the internal voltage reference and internal R of 0.157% of full-scale range can be expected for the C grade device over the temperature range of −40°C to +105°C.
Other configurations specify an external voltage reference, an external R external R
resistor, or both an external voltage reference and
SET
resistor. In these configurations, the specifications
SET
assume that the external voltage reference and external R resistor are ideal. Therefore, the errors associated with these components must be added to the data sheet specifications to determine the overall performance. The performance depends on the specifications of these components.
enabled, a maximum total error
SET
SET
ADuCM360
OPTIONAL
EMC FILTER
R1 470
4.7µF
0.1µF
1µF
AD5700/AD5700-1
TXD RXD RTS CD
0.1µF
IODVDDDVDDREG
RANGE0
RANGE1
ALARM_CURRENT_DIRECT ION R
INT/REXT
SYNC SCLK SDIN SDO FAU LT LDAC
COM
REFOUT2
0.1µF
V
CC
HART_OUT
ADC_IP
DGNDAGND
10µF
REF
OPTIONAL
MOSFET
DN2540 BSP129
OUT
AD5421
REG_SEL0
REG_SEL1
REG_SEL2
SETS REGULATOR
VOLTAGE
47nF 168nF
1.2M
1µF
1.2M 150pF
REG
C
IN
300pF
T1
200k
IN
DRIVE
V
LOOP
LOOP–
R
EXT1
R
EXT2
COMREFOUT1 REFIN
R1
OPTIONAL RESISTOR
150k
19M
1M
VZ = 4.7V
V
LOOP
R
L
Figure 49. AD5421 Application Diagram for HART Capable Smart Transmitter
Rev. E | Page 31 of 36
09128-055
Page 32
AD5421 Data Sheet
09128-200
ADC 0
PRESSURE SENSOR SIMULATION
TEMPERATURE SENSOR PT100
3.3V
ADC 1
ADC
DAC
ADuCM360
SRAM
FLASH
CLOCK
RESET
WATCHDOG
T1: CD T2: RTS T3: COM T4: TEST
SPI
UART
AD5700
AD5421
V
CC
HART_OUT
REF
ADC_IP
3.3V
3.3V
V
DD
C
IN
C_HART
C_SLEW
HART INPUT FILTER
COM
V-REGULATOR
TEMPERATURE
SENSOR
COM
REG
IN
V
LOOP
LOOP–
TEST CONNECTOR
+
50Ω
HART MODEM
MICRO-
CONTROLLER
WATCHDOG
TIMER
LEXC
DGNDAGND
Figure 50. Block Diagram—Analog Devices HART-Enabled Smart Transmitter Reference Demo Cir cuit
Rev. E | Page 32 of 36
Page 33
Data Sheet AD5421
FSR%102.0%)029.0(%)0435.0(%0015.0%048.0
22
=+++
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 20 40 60 80 100
POWER DISSIPATION (W)
AMBIENT TE M P E RATURE (°C)
09128-103
TSSOP
LFCSP
60
50
40
30
20
10
0
40 50 60 70 80 90 100
SUPPLY VOLTAGE (V)
AMBIENT TE M P E RATURE (°C)
09128-102
LFCSP
TSSOP
TT
mW625
32
105125=−
=
JA
AJ MAX
θ
TT
=×
JADMAXJ
PT
θ
( )( )
C77400228.052125 °=××
C87)32)0228.052((125
)(
°=××
=×
JAD MAXJ
PT
θ
Maximum
Maximum permitted supply voltage
V21
400228.0
105125
=
×
=
×
JALOOP
AMAXJ
I
TT
θ
V27
320228.0
105125
=
×
=
×
JALOOP
A MAXJ
I
TT
θ
To determine the absolute worst-case overall error, the reference and R
errors can be directly summed with the specified AD5421
SET
maximum error. For example, when using an external reference and external R
resistor, the maximum AD5421 error is 0.048%
SET
of full-scale range. Assuming that the absolute errors for the voltage reference and R
resistor are, respectively, 0.04% and
SET
0.05% with temperature coefficients of 3 ppm/°C and 2 ppm/°C, respectively, the overall worst-case error is as follows:
Worst-Case Error = AD5421 Error + V R
Absolute Error + R
SET
Absolute Error + V
REF
TC
SET
REF
TC +
Worst-Case Error =
0.048% + 0.04% + [(3/10
0.05% + [(2/10
6
) × 100 × 145]% = 0.21% FSR
6
) × 100 × 145]% +
This is the absolute worst-case value when the AD5421 operates over the temperature range of −40°C to +105°C. An error of this value is very unlikely to occur because the temperature coeffi­cients of the individual components do not exhibit the same drift polarity, and, therefore, an element of cancelation occurs. For this reason, the TC values should be added in a root of squares fashion.
A further improvement can be gained by performing a two-point calibration at zero scale and full scale, thus reducing the absolute errors of the voltage reference and R
resistor to a combined
SET
error of 1 LSB or 0.0015% FSR. After performing this calibration, the total maximum error becomes
Total Error =
Excessive junction temperature can occur if the AD5421 experiences elevated voltages across its terminals while regulating the loop current at a high value. The resulting junction temperature depends on the ambient temperature.
Tabl e 24 provides the bounds of operation at maximum ambient temperature and maximum supply voltage. This information is displayed graphically in Figure 51 and Figure 52. These figures assume that the exposed paddle is connected to a copper plane of approximately 6 cm
Figure 51. Maximum Power Dissipation vs. Ambient Temperature
2
.
To reduce this error value further, a voltage reference and R resistor with lower TC specifications must be chosen.

THERMAL AND SUPPLY CONSIDERATIONS

The AD5421 is designed to operate at a maximum junction temp­erature of 125°C. To ensure reliable and specified operation over the lifetime of the product, it is important that the device not be operated under conditions that cause the junction temperature to exceed this value.
Table 24. Thermal and Supply Considerations (External MOSFET Not Connected)
Parameter Description 32-Lead LFCSP 28-Lead TSSOP
Maximum
Power Dissipation
Maximum
Ambient Temperature
Supply Voltage
Maximum permitted power dissipation when operating at an ambient temperature of 105°C
Maximum permitted ambient temperature when operating from a supply of 52 V while regulating a loop current of 22.8 mA
when operating at an ambient temperature of 105°C while regulating a loop current of 22.8 mA
SET
Figure 52. Maximum Supply Voltage vs. Ambient Temperature
40
105125=−
mW500
AMAXJ
θ
=
JA
Rev. E | Page 33 of 36
Page 34
AD5421 Data Sheet
COMPLI ANT TO JEDEC STANDARDS MO-220-W HHD.
1
0.50
BSC
3.50 REF
BOTTOM VIEWTOP VIEW
PIN 1
INDICATOR
32
9
16
17
24
25
8
EXPOSED
PAD
PIN 1 INDICATOR
3.65
3.50 SQ
3.45
SEATING
PLANE
0.05 MAX
0.02 NOM
0.20 REF
COPLANARITY
0.08
0.30
0.25
0.18
5.10
5.00 SQ
4.90
0.80
0.75
0.70
FOR PROP E R CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CO NFIGURATI ON AND FUNCTIO N DE S CRIPTIONS SECTION OF THIS DATA SHEET.
0.50
0.40
0.30
0.25 MIN
04-02-2012-A
COMPLIANT TO JEDEC STANDARD
S MO-153-AET
05-08-2006-A
28
15
14
1
EXPOSED
PAD
(Pins Up)
9.80
9.70
9.
60
4.50
4.40
4.30
6.
40
BSC
3.05
3.00 2
.95
5.55
5.
50
5.45
FOR PROPER CONNECTION OF THE EXPOSED PAD, REFER TO THE PIN CONFIGURATION AND FUNCTION DESCRIPTIONS SECTION OF THIS DATA SHEET.
PIN 1
INDICATOR
BOTTOMVIEW
TOP VIEW
1.20 MAX
SEATING
PLANE
0.65 BSC
0.30
0.19
0.15 MAX
0.05 MIN COPLANARITY
0.10
1.05
1.00
0.80
0.20
0.09
0.25
8° 0°
0.75
0.60
0.45

OUTLINE DIMENSIONS

Figure 53. 32-Lead Lead Frame Chip Scale Package [LFCSP_WQ]
5 mm × 5 mm Body, Very Very Thin Quad
(CP-32-11)
Dimensions shown in millimeters
Figure 54. 28-Lead Thin Shrink Small Outline Package, Exposed Pad [TSSOP_EP]
(RE-28-2)
Dimensions shown in millimeters
Rev. E | Page 34 of 36
Page 35
Data Sheet AD5421
Model1
Temperature Range
Package Description
Package Option

ORDERING GUIDE

AD5421ACPZ-REEL7 −40°C to +105°C 32-Lead LFCSP_WQ CP-32-11 AD5421BCPZ-REEL7 −40°C to +105°C 32-Lead LFCSP_WQ CP-32-11 AD5421BREZ −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 AD5421BREZ-REEL −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 AD5421BREZ-REEL7 −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 AD5421CREZ −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 AD5421CREZ-RL −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 AD5421CREZ-RL7 −40°C to +105°C 28-Lead TSSOP_EP RE-28-2 EVAL-AD5421SDZ Evaluation Board
1
Z = RoHS Compliant Part.
Rev. E | Page 35 of 36
Page 36
AD5421 Data Sheet
©2011–2012 Analog Devices, Inc. All rights reserved. Trademarks and
NOTES
registered trademarks are the property of their respective owners. D09128-0-7/12(E)
Rev. E | Page 36 of 36
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