Datasheet

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XTR108

SBOS187C – OCTOBER 2001 – REVISED JULY 2005

www.ti.com

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of Texas Instruments

standard warranty. Production processing does not necessarily include

testing of all parameters.

SPI is a trademark of Motorola, Inc. All other trademarks are the property of their respective owners.

4-20mA, TWO-WIRE TRANSMITTER

“Smart” Programmable with Signal Conditioning

FEATURES

● COMPLETE TRANSMITTER + RTD

LINEARIZATION

● TWO-WIRE, 4-20mA OUTPUT

● VOLTAGE OUTPUT (0.5V to 4.5V)

● ELIMINATES POTENTIOMETERS AND

TRIMMING

● DIGITALLY CALIBRATED

● 5V SUB-REGULATOR OUTPUT

● SERIAL SPI



BUS INTERFACE

● SSOP-24 PACKAGE

APPLICATIONS

● REMOTE RTD TRANSMITTERS

● PRESSURE BRIDGE TRANSMITTERS

● STRAIN GAGE TRANSMITTERS

● SCADA REMOTE DATA ACQUISITION

● WEIGHING SYSTEMS

● INDUSTRIAL PROCESS CONTROL

DESCRIPTION

The XTR108 is a “smart,” programmable, 4-20mA, two-wire

transmitter designed for temperature and bridge sensors.

Zero, span, and linearization errors in the analog signal

path can be calibrated via a standard digital serial interface,

eliminating manual trimming. Non-volatile external EEPROM

stores calibration settings.

The all-analog signal path contains an input multiplexer,

autozeroed programmable-gain instrumentation amplifier, dual

programmable current sources, linearization circuit, voltage

reference, sub-regulator, internal oscillator, control logic, and

an output current amplifier. Programmable level shifting

compensates for sensor DC offsets. Selectable

up- and down-scale output indicates out-of-range and burn-

out per NAMUR NE43. Automatic reset is initiated when

supply is lost.

Current sources, steered through the multiplexer, can be

used to directly excite RTD temperature sensors, pressure

bridges, or other transducers. An uncommitted op amp can

be used to convert current into a voltage. The XTR108 is

specified for –40°C to +85°C.

SPI and

Control Circuits

4-20mA

EEPROM

Linearization

Excitation

Ret

Multiplexer

Gain and Offset

SDIO

SCLK

CS2

CS1

V/I-0

V/I-1

V/I-2

V/I-3

V/I-4

V/I-5

RTD

PGA V/I

LOAD

XTR108

Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of

Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

Summary of content (32 pages)

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XT R XTR108 108 SBOS187C – OCTOBER 2001 – REVISED JULY 2005 4-20mA, TWO-WIRE TRANSMITTER “Smart” Programmable with Signal Conditioning FEATURES DESCRIPTION ● COMPLETE TRANSMITTER + RTD LINEARIZATION ● TWO-WIRE, 4-20mA OUTPUT ● VOLTAGE OUTPUT (0.5V to 4.5V) ● ELIMINATES POTENTIOMETERS AND TRIMMING ● DIGITALLY CALIBRATED ● 5V SUB-REGULATOR OUTPUT ● SERIAL SPI BUS INTERFACE ● SSOP-24 PACKAGE The XTR108 is a “smart,” programmable, 4-20mA, two-wire transmitter designed for temperature and bridge sensors.
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ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Loop Supply Voltage, VPS ............................ Dependent on External FET XTR Supply Voltage, External VS (Referenced to IRET Pin) ............ +5.5V This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with appropriate precautions. Failure to observe proper handling and installation procedures can cause damage. Input Voltage to Multiplexer (Referenced to IRET Pin) ........
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ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = –40°C to +85°C. At TA = +25°C, VPS = 24V, and Supertex DN2540 external depletion-mode FET transistor, unless otherwise noted, all voltages measured with respect to IRET pin. XTR108EA PARAMETER PGA (Cont.) Input Impedance: Differential Input Impedance: Common-Mode Voltage Noise, 0.
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ELECTRICAL CHARACTERISTICS (Cont.) Boldface limits apply over the specified temperature range, TA = –40°C to +85°C. At TA = +25°C, VPS = 24V, and Supertex DN2540 external depletion-mode FET transistor, unless otherwise noted, all voltages measured with respect to IRET pin.
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PIN CONFIGURATION Top View SSOP V/I-0 1 24 OPA +IN V/I-1 2 23 OPA –IN V/I-2 3 22 OPA OUT V/I-3 4 21 REFOUT V/I-4 5 20 REFIN V/I-5 6 19 RSET XTR108 CFILTER 7 18 CS1 RLIN 8 17 SCLK VO 9 16 SDIO IIN 10 15 CS2 IO 11 14 VGATE IRET 12 13 VS PIN ASSIGNMENTS PIN V/I-0 V/I-1 V/I-2 V/I-3 V/I-4 V/I-5 CFILTER RLIN VO IIN IO IRET VS VGATE CS2 SDIO SCLK CS1 RSET REFIN REFOUT OPA OUT OPA –IN OPA +IN NAME FUNCTION Channel 0 and/or IREF Out Channel 1 and/or IREF Out Chann
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TYPICAL CHARACTERISTICS At TA = +25°C, V+ = 24V, unless otherwise noted. RVI = 6.34kΩ. COMMON-MODE REJECTION vs FREQUENCY TRANSCONDUCTANCE vs FREQUENCY 90 G = 400 60 50 80 G = 200 40 Rejection (20log mA ) V Transconductance (20log mA ) V 70 G = 50 30 20 10 0 –10 G = 6.25 70 60 G = 100 50 40 30 G = 6.
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TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, V+ = 24V, unless otherwise noted. RVI = 6.34kΩ. LARGE INPUT STEP RESPONSE VIN VIN LARGE INPUT STEP RESPONSE 20mA IOUT IOUT 20mA 4mA 4mA PGA Gain = 6.25 CFILT = 0 PGA Gain = 6.25 CFILT = 0.
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TYPICAL CHARACTERISTICS (Cont.) At TA = +25°C, V+ = 24V, unless otherwise noted. RVI = 6.34kΩ. INPUT NOISE POWER DENSITY PGA PEAK-TO-PEAK NOISE (RTI) 1000 1.25µV/div Noise Density (nV√Hz) BW: 0.1Hz to 10Hz 100 CFILT = 0.01 10 1 10 100 1k 5s/div 10k Frequency (Hz) OVERVIEW 8 EE PROM SDIO CS2 The XTR108 is a 4-20mA current-loop transmitter that allows the user to digitally adjust the gain, offset, and linearity correction of the analog output to calibrate the sensor.
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THEORY OF OPERATION REFERENCE The XTR108 has an on-board precision bandgap voltage reference with output at pin 21 (REFOUT). The value of the reference is factory-trimmed to 1.193V, with a typical temperature drift of 5ppm/°C. Pins 21 (REFOUT) and 20 (REFIN) must be connected together to use the internal reference. External circuitry, such as a voltage excited sensor or an Analog-to-Digital Converter (ADC), can be connected to the REFOUT pin.
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FIGURE 2. XTR108 Internal Block Diagram. www.ti.com RTD RCM V/I-5 V/I-4 V/I-3 V/I-2 V/I-1 V/I-0 RZ1 RZ2 RZ3 RZ4 RZ5 OPA OUT 1 Multiplexer 10 XTR108 SBOS187C 2 Σ OPA –IN 0.01µF CFILT PGA ILIN DAC IREF DAC OPA +IN 15.8kΩ RLIN CS2 12.1kΩ RSET IIN OSC RVI 6.34kΩ VOUT SDIO SCLK SPI and Control Circuits Linearization Circuit Zero DAC CS1 2.
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OVERALL PROGRAM COARSE DAC FINE DAC VOLTAGE REFERRED TO VO PIN WITH RESPECT TO IRET CURRENT REFERRED TO IOUT PIN VZERO = VZ PROGRAM + VZ COARSE + VZ FINE IZERO = IZ PROGRAM + IZ COARSE + IZ FINE V Z PROGRAM = V Z COARSE = V Z FINE = 3.5V REF 8 I Z PROGRAM = V REF N13 • 80 4 I Z COARSE = V REF N12 • 80 64 I Z FINE = 175V REF 8R VI 5V REF N13 • 8R VI 4 5V REF N12 • 8R VI 64 NOTE: N13 and N12 are assigned decimal values of registers 13 and 12, respectively. TABLE II.
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means that one LSB of the coarse DAC is equal to 16 fine LSBs, and the full-scale range of the fine DAC is equal to 16 coarse LSBs. This effectively produces 12-bit adjustment resolution. This allows the user to set pre-calculated values before the calibration, using the coarse DAC only and adjust the reference current output level with the fine DAC during the calibration process. LINEARIZATION CIRCUIT AND RLIN RESISTOR The XTR108 incorporates circuitry for correcting a secondorder sensor nonlinearity.
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The uncommitted amplifier can be used for a variety of purposes, such as voltage sensor excitation, buffering the REFOUT pin, four-wire RTD connection, or sensing the bridge voltage for temperature compensation. CONTROL REGISTERS POWER-GOOD/POWER-ON RESET DESCRIPTION OF CONTROL REGISTERS In case of a supply brownout condition or short interruption, the XTR108 power-good detection circuit will initiate a chip reset that will cause all registers to be reset to 0’s and a cycle of EEPROM read to begin.
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Address = 4: Control Register 2 If the RBD bit is set to ‘1’, the automatic read-back from the EEPROM will be disabled after a valid checksum byte is received in Register 15. This bit is read from the EEPROM during a read-back by the XTR108 and allows the user to program the XTR108 to read the EEPROM data once (instead of continuously), and then disables the automatic read-back function. The XTR108 will continuously read the EEPROM if RBD is set to ‘0’.
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Address = 9: IREF Output Configuration Register This register connects the reference currents to the various multiplexed input pins. IREF connection codes are given in Table XIII. Address = 10: Fine IREF Adjust Register This register sets the code to the 8-bit Fine DAC that adjusts the magnitude of both reference currents. The DAC output value has a bipolar range (for each reference current) and can be calculated using the equations in Table III.
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Hi-Z SCLK t8 t8 DIO 0 0 0 t9 0 0 0 1 1 0 0 0 Instruction/Address to EEPROM 0 0 1 0 0 Data from EEPROM CS2 FIGURE 4. Timing Diagram for the XTR108 Continuous Readback Cycle. (See Table XIV for timing key.) SCLK t10 t11 DIO t13 CS2 t12 CS1 FIGURE 5. Interrupting an XTR108 EEPROM Readback Cycle. (See Table XIV for timing key.) As long as CS1 is held LOW, the external system can write to the EEPROM. See Figure 7 for this timing.
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CS1 t7 t1 t5 SCLK t2 t2 t3 t4 t6 DIO FIGURE 6. Timing Diagram for Writing to and Reading From the XTR108 with EEPROM Readback Disabled. (See Table XIV for timing key.) SCLK DIO 0 1 1 1 1 1 1 1 Instruction to XTR108 Data to/from EEPROM CS1 t14 CS2 FIGURE 7. Writing to and Reading From the EEPROM Device From External Controller. (See Table XIV for timing key.
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EEPROM DATA STORAGE The XTR108 automatically reads data from an SPI-compatible EEPROM device. The models 25C040 from MicroChip and the AT25010 from Atmel have been tested and are known to work. Equivalent devices with an SPI interface can be expected to work. The XTR108 will read data from addresses 4 through 15 of the EEPROM. The address in the EEPROM is the same as the address for the corresponding data in the XTR108. The XTR108 will not write data to the EEPROM.
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NOTE: (1) Zener Diode 36V: 1N4753A or General Semiconductor TransorbTM 1N6286A. Use lower voltage zener diodes with loop power supply voltages less than 30V for increased protection. See “Over-Voltage Surge Protection.” 10 V+ 0.01µF XTR108 14 D1(1) 1N4148 Diodes 13 RL Maximum VPS must be less than minimum voltage rating of zener diode. VPS IO 11 IRET 12 The diode bridge causes a 1.4V loss in loop supply voltage. FIGURE 8. Reverse Voltage and Over-Voltage Protection.
PAGE 20
6) Choose PGA gain from the available list and calculate the initial excitation current using: I REF1,2 = (I OUTMAX ) ( Step 3. • Calculate corrections using the following equations: ) – I OUTMIN • 1 – G LIN ( R MAX – R Z ) • R VI 50 • A PGA • ( R MAX – R MIN ) I REFA = R Z A = R MIN + Important: the PGA gain value should be chosen such that the IREF value is within ±35% of 5VREF/RSET to allow room for calibration adjustments without having to go to another span step.
PAGE 21
SAMPLE ERROR ANALYSIS Table XVII shows a detailed computation of the error accumulation. The sample error budget is based on a typical RTD circuit (Pt100, 200°C measurement span). Note that these calculations are based on typical characteristics where no maximum or minimum characteristic is available. The assumption is made that all errors are positive and additive.
PAGE 22
APPLICATIONS RTD CONNECTION METHODS Two-Wire Connection The simplest circuit that can be used to connect an RTD to the XTR108 is the two-wire connection shown in Figure 9. If the RTD is separated from the XTR108 by any distance the resistance of the lead wires can cause significant error in the reading. This wire resistance is noted as RLINE1 and RLINE2. If the RF filter is not required, then the PGA inputs could be taken from the same pins as are used for the current sources.
PAGE 23
R21 Equal line resistances here create a small common-mode voltage which is rejected by the XTR108. R22 R23 1 RLINE2 RLINE1 Multiplexer 2 R24 R25 RTD IRET RLINE3 3 Resistance in this line causes a small common-mode voltage which is rejected by the XTR108. RCM 0.01µF FIGURE 10. Three-Wire RTD Connection with Multiple Minimum Temperature Capabilities. VS RLINE1 RLINE2 1 2 RTD RLINE3 RLINE4 3 4 Multiplexer RZ OPA277 IRET RCM 0.01µF FIGURE 11. Four-Wire RTD Connection.
PAGE 24
VREF R1 Multiplexer R2 IRET FIGURE 12. Voltage Excited Bridge with Excitation Derived from VREF. VREF Multiplexer R1 IRET FIGURE 13. Voltage Excited Bridge with Uni-Directional Linearity by Control. 24 XTR108 www.ti.
PAGE 25
VOLTAGE OUTPUT MODE USING SIMPLE CURRENT PUMP In order for the voltage output mode of the XTR108 to operate properly, a negative voltage needs to be applied to the IRET pin (–200mV < IRET < –25mV). For systems without a negative supply a charge pump is an easy way to generate this voltage. Figure 14 shows a simple and inexpensive way to build this charge pump using two resistors, two capacitors, and two diodes (in an SOT package).
PAGE 26
COMMUNICATIONS WITH THE XTR108 USING A MICROCONTROLLER When communicating with the XTR108, special care must be taken to avoid getting a false clock. When CS1 is driven low, the false clock is generated because the microcontroller clock pin is in high-impedance state, which forces the clock pin to a logic high. Immediately after CS1 is driven low, the microcontroller drives the clock pin low. This sequence creates a glitch that the XTR interprets as a clock; see Figure 16.
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE MATERIALS INFORMATION www.ti.com 16-Aug-2012 TAPE AND REEL INFORMATION *All dimensions are nominal Device XTR108EA/2K5 Package Package Pins Type Drawing SSOP DBQ 24 SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) 2500 330.0 16.4 Pack Materials-Page 1 6.5 B0 (mm) K0 (mm) P1 (mm) 9.0 2.1 8.0 W Pin1 (mm) Quadrant 16.
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PACKAGE MATERIALS INFORMATION www.ti.com 16-Aug-2012 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) XTR108EA/2K5 SSOP DBQ 24 2500 367.0 367.0 38.
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IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, enhancements, improvements and other changes to its semiconductor products and services per JESD46C and to discontinue any product or service per JESD48B. Buyers should obtain the latest relevant information before placing orders and should verify that such information is current and complete.