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XTR108 Schematic ( PDF Datasheet ) - Burr-Brown Corporation

Teilenummer XTR108
Beschreibung TWO-WIRE TRANSMITTER
Hersteller Burr-Brown Corporation
Logo Burr-Brown Corporation Logo 




Gesamt 30 Seiten
XTR108 Datasheet, Funktion
XTR108
XTR108
SBOS187C – OCTOBER 2001 – REVISED JULY 2005
4-20mA, TWO-WIRE TRANSMITTER
“Smart” Programmable with Signal Conditioning
FEATURES
q COMPLETE TRANSMITTER + RTD
LINEARIZATION
q TWO-WIRE, 4-20mA OUTPUT
q VOLTAGE OUTPUT (0.5V to 4.5V)
q ELIMINATES POTENTIOMETERS AND
TRIMMING
q DIGITALLY CALIBRATED
q 5V SUB-REGULATOR OUTPUT
q SERIAL SPIBUS INTERFACE
q SSOP-24 PACKAGE
APPLICATIONS
q REMOTE RTD TRANSMITTERS
q PRESSURE BRIDGE TRANSMITTERS
q STRAIN GAGE TRANSMITTERS
q SCADA REMOTE DATA ACQUISITION
q WEIGHING SYSTEMS
q 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.
EEPROM
RTD
V/I-0
V/I-1
V/I-2
V/I-3
V/I-4
V/I-5
R1 R2 R3 R4 R5
Excitation
SPI and
Control Circuits
Linearization
PGA
IO
V/I
XTR108
IRet
VPS
4-20mA
RLOAD
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.
SPI is a trademark of Motorola, Inc. All other trademarks are the property of their respective owners.
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.
www.ti.com
Copyright © 2001-2005, Texas Instruments Incorporated






XTR108 Datasheet, Funktion
TYPICAL CHARACTERISTICS
At TA = +25°C, V+ = 24V, unless otherwise noted. RVI = 6.34k.
70
60
50
40
30
20
10
0
10
20
30
100
TRANSCONDUCTANCE vs FREQUENCY
G = 50
G = 400
G = 200
G = 6.25
1k 10k
Frequency (Hz)
100k 200k
90
80
70
60
50
40
30
20
10
10
COMMON-MODE REJECTION vs FREQUENCY
G = 6.25
G = 100
G = 400
100 1k 10k
Frequency (Hz)
100k
IZERO VLOOP REJECTION RATIO vs FREQUENCY
100
90
80
70
60
50
40
30
20
10
100 1k
Frequency (Hz)
10k
IREF VLOOP REJECTION RATIO vs FREQUENCY
120
110
100
90
80
70
60
10
100 1k
Frequency (Hz)
10k
IREF vs TEMPERATURE
492
490
488
486
75 50 25
0 25 50
Temperature (°C)
75 100 125
20%
18%
16%
14%
12%
10%
8%
6%
4%
2%
0%
IOUT DRIFT AVERAGE
IOUT Drift (µA/°C)
6 XTR108
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SBOS187C

6 Page









XTR108 pdf, datenblatt
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 second-
order sensor nonlinearity. A current proportional to the
voltage at the input of the PGA is added to the sensor
excitation. The RLIN resistor is used to convert this voltage
into current. By appropriately scaling this current using the
linearization DAC, parabolic sensor nonlinearity can be
improved by up to a 40:1 ratio, as shown in Figure 3. The
linearization coefficient (ratio of the reference current change
to the input voltage) is expressed in µA/mV as follows:
G LIN
=
I REF
V IN
N14
16 RLIN
where N14 is the decimal value from register 14.
The recommended value of the resistor is 15.8k, for use
with 100RTD sensors. This value produces a full-scale
linearization coefficient of about 1mA/V. Please see the
section below on using the XTR108 with an RTD tempera-
ture sensor. If the sensor excitation is scaled down by
increasing the value of RSET, the value of RLIN should be
scaled proportionally.
5
4
3
2
1
0
1
200°C
Uncorrected
RTD Nonlinearity
Corrected
Nonlinearity
Process Temperature (°C)
+850°C
FIGURE 3. Pt100 Nonlinearity Correction Using the XTR108.
SUB-REGULATOR WITH EXTERNAL MOSFET
The XTR108 is manufactured using a low-voltage CMOS
process with maximum supply voltage limited to 5.5V. For
applications in a 4-20mA current loop, a special sub-regulator
circuit is incorporated in the device that requires an external
n-channel depletion-mode MOS transistor and three capaci-
tors, see Figure 2.
A number of third-party suppliers make n-channel deple-
tion-mode MOSFETs. A list of devices tested by Texas
Instruments, Inc. is shown in Table IV with the capacitor
values recommended for those devices.
MANUFACTURER
Supertex
Siliconix
Infineon
MOSFET MODEL
DN2535, DN2540
DN3535, DN3525
ND2012, ND2020
BSP149
CGATE VALUE
220pF
1000pF
220pF
1000pF
TABLE IV. Recommended Gate Capacitor Values For Se-
lected MOSFETs.
The capacitors CLOOP (0.01µF), CREG (2.2µF), and CGATE
are required for the regulator loop stability and supply
bypass. They should be placed in close proximity to the
XTR108 on the PCB. An additional 1µF capacitor may be
used to bypass the supply of an EEPROM chip.
If a MOSFET other than those listed in Table IV is used, the
value of CGATE should be adjusted such that there is no
overshoot of VS during power-up and supply glitches. Any
VS overshoot above 7.5V may damage the XTR108 or
deteriorate its performance.
LOOP VOLTAGE
The XTR108 transmitter minimum loop voltage can some-
what be effected by the choice of the external MOSFET. The
devices are tested to 7.5V compliance with Supertex DN2540;
choosing other MOSFETs can change this value slightly.
The maximum loop voltage is limited by the power dissipa-
tion on the MOSFET as well as its breakdown voltage.
Possible ambient temperatures and the power dissipation
should be taken into account when selecting the MOSFET
package. The external MOSFET can dissipate a consider-
able amount of power when running at high loop supply. For
example, if VLOOP = 24V and IOUT = 20mA, the DC power
dissipated by the MOSFET is:
PMOSFET = IOUT (VLOOP – VS) = 380m
For a SOT-89 package soldered on an FR5 board, this will
cause a 30°C rise in the temperature. The power dissipation
gets significantly higher when the circuit is driven into an
over-scale condition. Therefore, special attention should be
paid to removing the heat from the MOSFET, especially
with small-footprint packages such as SOT-89 and TO-92.
Please follow manufacturer’s recommendations about the
package thermal characteristics and board mounting.
UNCOMMITTED OP AMP
For added flexibility in various applications, the XTR108
has an on-chip uncommitted operational amplifier. The op
amp has rail-to-rail output range. The input range extends to
IRET potential.
12 XTR108
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SBOS187C

12 Page





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