Datasheet

XTR108
8
SBOS187C
www.ti.com
OVERVIEW
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. The digital data for adjustment are stored in an
external EEPROM device.
The analog signal path is composed of a compound multi-
plexer (MUX), programmable gain instrumentation amplifier
(PGA), and an output current amplifier. Analog support func-
tions include digitally controlled current sources for sensor
excitation, PGA offset control, linearization, voltage refer-
ence, and voltage regulator.
The digital interface communicates with external devices for
calibration and to store the resultant data in an SPI compat-
ible EEPROM. A complete system is shown in Figure 1. The
XTR108 serial interface is SPI compatible and only requires
four connections to the calibration controller: a serial clock
(SCLK), a serial data line (SDIO), a chip select line (CS1),
and a ground sense line. All logic signals to the XTR108
must be referenced to the potential of the ground sense line
(I
RET
pin on the XTR108).
Within this entire system there may exist three different
“GND” voltage levels. In addition, the voltage difference
between the I
RET
and I
O
potential will depend on the output
current level. It is not certain that the “GND” potential of the
calibration system will be at the same potential of either the
I
RET
or I
O
potential, and therefore the isolation couplers are
shown in Figure 1. All voltages specified for the XTR108
are with reference to the I
RET
pin.
TYPICAL CHARACTERISTICS (Cont.)
At T
A
= +25°C, V+ = 24V, unless otherwise noted. R
VI
= 6.34kΩ.
INPUT NOISE POWER DENSITY
10 100 1k 10k1
Frequency (Hz)
Noise Density (nV√Hz)
1000
100
10
C
FILT
= 0.01
PGA PEAK-TO-PEAK NOISE (RTI)
5s/div
1.25µV/div
BW: 0.1Hz to 10Hz
The XTR108 also needs to communicate with the external
EEPROM device independently from the calibration control-
ler to retrieve the calibration constants during normal opera-
tion. The XTR108 provides a second chip select function
(CS2) for the EEPROM device to facilitate this communica-
tion.
EE PROM
SDIO
CS2
SCLK
CS1
Isolation
Couplers
I
O
I
RET
XTR108
R
V
R
X
GND
PS
Calibration
System
Calib
GND
T
X
GND
FIGURE 1. Complete System Level Configuration with
Three Unique Ground Voltage Levels.